Traditional Cheesemaking/Part 2: From milk to cheese

Chapter 1: Milk[edit | edit source]

Definition and composition

Fresh milk is a white liquid, with an agreeable odour and a faintly sweet taste, produced by female mammals to feed their offspring. Humans use cows' milk as well as buffaloes', goats', sheep and mares' milk, either to drink directly or to make into different foods, such as cheese, butter, cream and yoghurt.

Seven-eighths of milk is water, with solids the nutritious part forming the remaining one-eighth. The solids can be broken down approximately as follows (see also Table):

Lactose		8%
Fat		0%
Protein	Casein 2.8%	35%
	Whey proteins	7%
Mineral salts		7%
		13.0%

One hundred kg of milk contains approximately 87 kg or litres of pure water and 13 kg of solids.

Milk production

Several factors affect the quality of milk, and to obtain the high-quality clean milk necessary for cheesemaking particular attention must be paid to the animals' diet and health and to the standard of hygiene in the milking parlour.

Diet

The following are all suitable as animal feed but may affect the colour, taste or smell of the milk in different ways:

• Natural or artificial pasture (gives a yellow butterfat due to the
  presence of Beta Carotene Vitamin A)
• Hay (gives a white butterfat and a rather colourless butter)
• Silage (which is not suitable for hard cheese)
• Fresh meal, especially if mixed with molasses.

Milk: a balanced food.

		Half a litre of milk contains	Percentage of daily daily requirement
Protein	90 g	17 g	19
Fat	90	18 g	20
Carbohydrate	325 g	23.5 g	7
Calories	2,550 Kcal	340 Kcal	13
Minerals
Potassium	95 g	81 g	85
Calcium	75 g	60 g	34
Phosphorus	50 g	41 g	27
Trace elements
Iodine	15 g	02 mg	13
Zinc	0 mg	9 mg	32
Vitamins
B2	95 mg	9 m	46
B12	005 mg	003 mg	60
C	75.00 mg	5 mg	11

Health

Milk must not be used for cheesemaking if the animal is diseased or sick in any way and care must be taken to prevent the following:

• Aftosis (foot-and-mouth disease)
• Brucellosis or undulant fever: (brucella abortus in bovines and brucella melitensis in goats)
• Mastitis
• Tuberculosis.

Milk must also be excluded from cheesemaking if it shows any chemical or physical abnormality, for example:

• If the milk contains colostrum (until the sixth day after the birth)
• If the milk contains antibiotics (until five full days after the last injection)
• If the animal has been vaccinated within the previous 24 hours
• If the milk has an abnormal flavour or aroma
• If the milk has an abnormal colour, for example bloody.
• If the milk is contaminated (with, for instance, dung, cattle feed or sediment).

The milking procedure

Before milking

• Clean and disinfect the milking parlour (this should have a hard floor that can be cleaned after each milking).
• Brush off the animal's flanks if they are dirty.
• Wash all utensils with a good dairy disinfectant and rinse with clean water.
• Wash the udder with lukewarm water and disinfectant with a cloth used for this and no other purpose.
• Wash and disinfect the milker's hands.
• Control mastitis. This disease will destroy the udder and produces an abnormal milk.
• Ensure that all milkers and cheesemakers are clean and healthy.

After milking

• Wash all utensils immediately after use and keep them in a clean place.
• Cool the milk to at least 15 C.

It is also important to remember the following:

• In hand milking do not give the last milk to calves because it contains the highest proportion of fat.

Instead, milk three teats completely and leave the fourth for the calf.

• Never mix new milk with old.
• Don't use cloth filters because they are very difficult to clean and may contaminate the milk.
• Don't use milk buckets for other purposes.
• Don't use rusty or oxidized churns or equipment.
• Don't leave milk churns or receptacles full of water.
• Send milk destined for cheesemaking to the cheese factory or dairy immediately after milking to prevent bacterial spoilage.
• Wash, scrub well and rinse all equipment daily with hot water, detergents and a dairy disinfectant.

Mastitis detection (Califomia mastitis test)

This test determines the presence or absence of leucocytes (pus indicating infection) in milk. There is a close relation between the number of leucocytes present in the sample and the degree of reaction.

Equipment

A pallet with four small numbered receiving dishes (Figure 5).

A narrow-necked plastic measuring bottle containing reagent. (A suitable reagent is one resazurin tablet and one rennet tablet dissolved in 50 ml distilled water.)

Method

1. Extract a few squirts of milk from each teat into the corresponding dish in the pallet (Figure 5).
   2. Tilt the pallet to one side to equalize the amount of milk in each dish, leaving about 2 ml in each.
   3. Add the same amount of reagent to each dish according to instructions.
   4. Gently agitate the pallet to mix the milk and reagent. Wait for 30 to 60 seconds and observe the results.

File:SK17016A.GIF

Figure 5. Testing for mastitis

Results

Normal milk: The milk does not congeal and its colour does not change.

Slightly infected milk: Small clots will form, the milk will thicken very slightly and the colour will darken. (The colour produced will vary according to the type of reagent used.)

Heavily infected milk: The milk will become very thick and dark.

Recommendations

Slightly infected milk should be separated from healthy milk and pasteurized before use. Antibiotics are not necessarily recommended for light cases of mastitis; improved dairy hygiene and frequent strip milking should be sufficient to cure these cases.

Heavily infected milk should never be used for cheesemaking and must be kept completely separate from good milk. It should be boiled and used as calf feed. In serious cases of mastitis it is advisable to consult a vet.

Precautions should be taken in all cases of mastitis not to spread the infection from affected animals to healthy ones through hands, towels and milking machines. Apart from special attention to hygiene, affected cows should be milked last. If a milking machine is used, a re-usable, cheap, in-line filter can be used to detect mastitis, which shows up on a fine screen as small lumps or particles.

Reception

Milk should arrive at the cheese factory as soon as possible after milking to prevent excessive acid development; too high a level of acidity will damage the cheese. The milk should be received and weighed outside the plant to avoid suppliers having to enter the building and so prevent possible contamination. As all equipment for transporting and holding milk must be washed with hot water and a good dairy detergent and rinsed immediately, facilities for this should be provided in the reception area.

Analysis

A series of tests determine the milk quality and ensure that the milk is pure, clean and suitable for cheesemaking. The main tests are:

Sensory analysis

• Odour free from acidity and containing no foreign substances.
• Taste normal or strange?
• Appearance colour and consistency.

Laboratory tests

• Bacteriological
• Physio-chemical
• Titration of acidity
• Percentage of fat
• Density of milk
• Control of impurities

Reductase Test

This test, based on the speed with which milk changes colour as a result of the reaction between methylene blue (methylthionine chloride) and bacteria, indicates the level of bacteria in the milk.
Equipment and reagents

Reductase tube approximately 25 ml capacity, ringed at 10 ml
A jug for the sample
A 37 C incubator, with test tube
A 1 ml pipette Methylene blue All equipment must be sterilized.

Method

1. Cool 200 ml of distilled water to 40 C.
2. Add one tablet of methylene blue and let it dissolve completely. (Keep this solution in a dark bottle and do not expose it to light.) Note: Since methods differ from country to country, it is advisable to follow the instructions of the producer of methylene blue solution.
3. Pour 10 ml of milk into each test tube.
4. Add 1 ml of methylene blue solution to each test tube.
5. Agitate the tubes to mix the solution with the milk.
6. Place the test tubes in the incubator at 37 to 38 C and check them every hour.

Results

If two-thirds or more of the milk in a test tube becomes discoloured in less than the acceptable time given below, it should be rejected.

Time	Level of bacteria	Quality of milk
after 5 hours	very low	very good
3 to 5 hours	low	good
2 to 3 hours	medium	acceptable
1 to 2 hours	high	bad
less than 1 hour	very high	unacceptable

Recommendations

Milk which discolours before two hours should not be used and indicates a lack of hygiene in its production: badly washed churns and milking equipment, improper cooling temperatures or impure water favouring the growth of undesirable micro-organisms.

Acidity determination

The level of acidity in milk relates to its microbiological content and therefore can indicate its purity and freshness. It can also be used to calculate the effectiveness of the lactic cultures being used in the cheese plant and the time required to reach desired levels of acidification.

Equipment and reagents

A clear flask
An eye-dropper
An acidimeter
A 10 ml milk pipette
A one-tenth solution of sodium hydroxide (NaOH)
An indicator solution of phenolphthalein in 2 per cent alcohol

Method

1. Add 9 ml of the milk to the flask.
   2. Add three or four drops of phenolphthalein.
   3. Fill the burette of the acidimeter with the solution of NaOH and begin to titrate the milk in the flask. The titration is complete when the milk turns pink and remains so for at least 10 seconds.

Result

The acidity in Dornic is equal to the number of 0.10 ml of NaOH used.

Recommendations

Fresh milk should normally have a reading of between 16 and 18 Dornic. In remote mountainous areas, however, readings of up to 20 Dornic, resulting from long trips over mountain passes often on mule or horseback, are acceptable. If the milk has an acid level of more than 20 to 21 Dornic, 6 to 10 per cent of clean water may be added, as soon as the milk arrives at the cheese factory, in order to reduce the acidity.

File:SK17019A.GIF

Figure 6. An acidimeter

File:SK17019B.GIF

Figure 7. A pH meter

Chapter 2: Curd[edit | edit source]

Cutting

There are two phases of the cutting of the curd. The first phase consists of inserting the cutting harp into the vat just along its inner edge and beginning to cut the curd in one direction. Each time the opposite end of the vat is reached, a 180 degree turn is made, lifting the harp a bit but not totally removing it from the vat in order not to damage the curd. Upon arriving at the other edge of the vat, the curd is then cut crosswise, that is, at right angles to the previous cutting direction. The same cutting procedure is followed, such that a cries-cross pattern appears in the curd and vertical strips are formed. At this point there is a pause in the cutting and the sectioned curd is left to rest for five minutes, during which time the whey begins to separate from the solids.

Next the second stage of the cutting begins. The vertical strips of curd are turned with the help of plastic plates that are moved by a second worker, and are then cut with the harp which is passed through them in a perpendicular manner. Grains or cubes of curd are formed in this way. The number of passes made depends upon the size of grain desired. In principle, in order to obtain a semi-hard cheese, an attempt is made to cut the curd in grains of 6 to 7 mm in diameter. However in practice, the grain size is between 5 and 10 mm, a variation that is perhaps due to the difficulty of the operation and the inexperience of the cheesemakers. As a general rule, the grains of curd should have a size similar to a mediumsized kernel of corn.

The entire curd cutting process lasts about 10 or 15 minutes. The cutting of the curd must be done with much care, since if it is not cut correctly there will be many losses due to the pulverization of the grains (grains cut too small) and to the separation of fat, which, upon mixing with the whey, changes the latter from an almost transparent yellowgreen to a whitish colour. These problems will reduce the yield of the conversion of milk to cheese.

An easy way to gauge the size of the curds.

Type of cheese	Size of curd required
Fresh	up to 2 cm in diameter
Andean (soft)	lima bean (1.5 cm)
Tilsit (semi-hard)	maize grain (1 cm)
Gruyere (hard)	wheat/rice grain(0.5 cm)

Stirring

Stirring, or agitating the curds (Photo 3), separates them from the warm whey and causes them to shrink and increase in density as a result of the whey loss. For hard cheeses, the major part of the whey must be removed from within the grains of curd or the cheese will be too moist and, as excess moisture and excess lactose favour the multiplication of microbes, it will not have a long shelf life.

Stirring should gradually quicken so that grains of curd move swiftly along the surface of the whey. The time required for stirring varies according to the type of cheese being made: curds for soft cheese, which requires large grains with a high moisture content, should not be stirred for long, while those for semi-hard and hard cheeses should be stirred for longer, to produce small grains with a low moisture content.

Increased levels of acidity and temperatures facilitate the construction of the curd grains. If the milk is very acid, or stirred at too high a temperature, then the curd will be too hard and the stirring will have to be accelerated.

Removal of whey

After stirring has stopped, the grains of curd sink to the bottom of the vat because of their greater density and the whey can be removed. Occasionally, however, curds will float, usually due to the presence of carbon dioxide from coliform contaminations. The whey is skimmed from the vat with a plastic or stainless bucket, changing the position of the bucket so that the mat of curd at the bottom of the vat is not pressed in the same spot. Normally about 30 per cent of whey is removed; more, if acidity is more than 13 Dornic. Whey can be removed from the base of a vat if a suitable drain-lock and sieve are used.

Washing and salting

Washing the curds by adding and stirring warm water enables the removal of the remaining lactose and lactic acid, and adds water. This is suitable only for the washed-curd types of cheese, such as Swiss and Dutch cheeses. The addition of salt during washing helps prevent the growth of microbes and gives the cheese a longer shelf life.

Certain cheeses Tilsit and Danbo, for example need hot water (65 to 75 C). In these cases it is very important that the water is added slowly and steadily while the curds are being stirred. This process should take five to ten minutes. If not done correctly, the curds will form hard crusts and retain moisture instead of expelling it.

While the water is being added, the curds must be stirred continually. This stirring continues for a specific period of time, depending on the type of cheese. Very soft cheeses, for example, need only 10 minutes from the moment the water is first added. A semi-hard cheese (Tilsit, Danbo) needs about 30 minutes of stirring in all. A very hard cheese (Gruyere, Parmesan) needs 60 to 80 minutes. During this process, the whey is expelled from the curds. Most of the whey is then removed with plastic dippers to facilitate gathering the curds for the subsequent moulding.

Moulding and Pressing

To shape the cheeses, the curds are packed into moulds. The moulding table is covered with a thick nylon mesh (2 mm squares). The moulds are placed on top of this mesh so that the cheeses may acquire a pleasing grid pattern on the outside. While one person stirs the curds, another scoops them out and pours them into the moulds, until they are full (Photos 4 and 5). The whey drains out through the holes in the sides of the moulds and base. Drainage can be speeded up by lightly pressing down on the curds with the hands. In about five minutes, all the visible whey will have been drained and a compact mass will have formed. The cheeses are then ready to be turned over for the first time. The subsequent pressing process depends on the type of cheese.

Soft cheeses, made from large curds, are not pressed because they would lose too much humidity. After the curds have been placed in the moulds, their own weight provides sufficient pressure to form the cheeses. The correct room temperature is absolutely essential for this process: it must be warm, with 20 C being the optimal temperature to allow the specific bacteria to grow and produce the correct acidity. This acidity in turn allows the whey to drain off, without any extra pressure. To maintain the 20 C temperature, the doors of the moulding room must be kept closed. The run-off beneath the moulds should be rinsed away with the warm whey (never with cold water) to avoid an abrupt drop in temperature. In high altitudes and colder climates, the moulding cheeses must be covered with a large piece of plastic to keep them at 20 C.

Hard and semi-hard cheeses, made from small-and medium-sized curds, are pressed for specific lengths of time depending on the type of cheese. Each cheese in its mould is wrapped in a cloth, with the edges folded up smoothly over the top. A piece of wood, shaped to fit into the mould, is placed on top of the cloth. A round piece of cement, made to fit into the mould, is placed on top of the wood. Generally, 4 kg of weight are needed for Andean cheese, 6 kg for Tilsit, and 10 for Danbo. Danbo cheese is made in a rectangular mould with rectangular weights.

The cheeses are removed from the moulds after a half an hour. The cloths are wrung out to remove the whey and any dry crusts that have formed around the edges of the cheeses are cut off. The cheeses are turned upside down and placed back into the moulds. They are then rewrapped (called 'dressing') in the cloth and weighed down again for another hour, then unwrapped, unmoulded, and turned over again. Next, the cheeses are returned to the moulds and placed on a dry cloth, unwrapped. They are left sitting until the next day (approximately 12 to 14 hours). During this entire process, an effort must be made to keep the room temperature as close to 20 C as possible. An experienced cheesemaker will, at this point, have a good idea how successful the fermentation process has been. The cheese should be yellow, with a firm texture, and the top edges should have pulled away slightly from the mould. A poorly processed cheese will be pale, with a mounded top and its edges touching the mould all the way around. This can result from the presence of coliforms or any gas-producing micro-organism; even reactivation of the starter culture can cause gas formation. These cheeses will swell to form bloated balls and the only way to save them is to process them immediately into Mozzarella or Provolone (see p.44). It is very important for the cheesemaker to realize that coliform gassy fermentation is a serious problem and s/he must recheck the entire process used, including milk quality, culture and general hygiene procedures.

The moulding of Gruyere cheese is a difficult operation. A thin steel strip is fitted on the edge of a coarse-woven dipping cloth, overlapping slightly. The steel-edged cloth is passed down and under the curd bed in one smooth swoop. The four corners of the dipping cloth are brought together to form a bag, and the filled bag of curd is pulled out of the whey, then deposited directly into a round, wooden Gruyere cheese hoop. Remove the small amount of curd left in the bottom of the vat and quickly return it to the curd bed. Fold over the heavy cloth and knead the cheese lightly with the palms of the hands.

A Gruyere cheese, which is very large, needs a great deal of pressure about 10 times its own weight. The pressing should be gradual: a little pressure at first, then slowly increase the weight. If the pressure is too heavy at the beginning, while the cheese still has a lot of whey, a thick crust will form around the entire cheese, preventing the rest of the whey from draining out. The end result will be a cheese with a hard, dry outer layer and a white spongy acidic inner layer, with drops of whey. As it continues to mature, the flavour will become bitter.

To facilitate whey separation from curd:

· The temperature should be 31 to 33 C

· Add more rennet to reduce coagulation time

· Prevent too rapid acidification, avoid milk that is overripe or too much culture

· Cut the curd earlier

· Cut the curd into finer (rice-sized) grains

· Increase stirring to two hours

· Increase temperature during stirring to 55 C (for Parmesan)

· Turn the cheese more frequently in the moulds.

To slow down the whey separation from curd:

· Use pasteurized milk

· Raise temperatures for coagulation (35 C in Andean or lower in Camembert, 26 to 29 C)

· Cut the curd after it has become well stiffened

· Cut the grains larger

· Stir slowly for a shorter time

· Increase the temperature of the hot water used during the second stirring to form a hard surface that will prevent the escape of whey

· Salt the curd.

Weighing

After 24 hours remove the cheese from the mould, weigh it to calculate the conversion from milk into cheese and identify each cheese clearly with the date of its pressing.

Conversion

The conversion or yield from milk to cheese varies considerably, but depends to a large degree on the fat and protein content of the milk, the quantity of fat lost during cheesemaking and the amount of water absorbed during the stiffening process (see Figure 12 and Table). Other factors also play a part; in Europe, for example, the yield from autumn milk is higher than from spring milk due to the higher protein content of the former and its mineral balance (see also p.14). A high conversion cheese one which requires less milk for the same quantity of cheese will be cheaper to produce and more cost-effective than a low conversion cheese. Conversion may be expressed either as the quantity of milk required to produce 1 kg of cheese or by the number of kilograms of cheese obtained from 100 litres of milk:

Yield of cheese from milk.

	No. of litres of milk required for 1 kg cheese	No. of kg of cheese produced from 100 litres milk
Fresh cheese	7.5	13.3
Andean cheese	8.5	11.8
Tilsit	9.5	10.5
Danbo	9.5	10.5
Gruyere	11	9.1
Provolone	11.5	8.7
Parmesan	12	8.3
Mozzarella (fresh Provolone)	10.5	9.5

File:SK17039A.GIF

Figure 12. Relative composition of milk and cheese

Relative composition of milk, soft cheese and hard cheese.

	Milk	Soft cheese	Hard cheese
Fats	40 g	240 g	315 g
Protein	35 g	205 g	275 g
Carbohydrate	48 g	25 g	25 g
Mineral Salts	7 g	20 g	25 g
Water	870 g	500 g	350 g
Salt	10 g	10 g
Vitamins	ABDEK	ABDEK	ABDEK

Brining

Brine, a known solution of salt in water, forms the cheese rind. The salt solution hardens the outer layer of the cheese by drawing off the moisture from the surface of the cheese. The final texture of the rind depends on the salinity, acidity and temperature of the brine: too little salt will not draw off enough moisture and the rind will not be properly hardened. Too much acidity will damage both the rind and the cheese during ageing, and if the brine is too cold then there will be insufficient exchange of whey and salt and the rind will be soft.

Method

Boil 30 litres of water and, while it is still hot, dissolve in it 10 kg of salt. This gives a salinity of about 20 to 22 C Baume. Cool the brine to 12 C before submerging the cheese. Salt may be sprinkled on the upper side of the cheese to obtain uniformity.

Cheese is left in the brine (Photo 6) according to its size, for example: Andean cheese (1.2 kg) requires 6 to 8 hours, Tilsit (3 kg) 20 to 24 hours and large Gruyere (40 kg) 48 hours.

The salinity of the brine gradually decreases and its acidity increases. When the acidity rises above 40 D the brine should be discarded. When the salinity falls below 18 Baume, salt should be added until it reaches its original salinity. The temperature of the brine must also be checked from time to time and should vary between 8 and 12 C. After brining, remove the cheese, allow it to dry a little and place it on the lower shelves in the maturing room so that it does not drip on to older cheeses.

Chapter 3: Cheese[edit | edit source]

Maturation

Maturation, also known as ripening or curing, results in some instances from the growth and multiplication of aerobic microbes on the rind, progressing inward after a few weeks, and produces a well-ripened cheese with a pleasant aroma, flavour and texture. Enzymes, produced by B. Liners bacteria in particular, pass into the cheese mass, contributing to the aging process, and are especially important in the formation of taste and aroma. Not all cheeses rely on microbes entering from the rind the starter culture microbes or their enzymes will be present throughout the cheese.

Changes in cheese during ripening

Growth of bacteria. Rapid growth takes place during the first few days of the ripening period. One or two grams of three-day-old cheese may contain several hundred million bacteria.

Change in the types of bacteria. During the first few days the streptococcus organisms, chiefly from the starter, are in the majority. Later, lactobacilli and others, like propionic bacteria (in Swiss cheese), predominate. (See Figure 13).

File:SK17042A.GIF

Figure 13. Normal bacterial contamination

Decrease in sugar content. The remaining lactose present usually disappears in the cheese within a few days.

Decrease in moisture. There is a slow decrease in the percentage of moisture, even in paraffined cheese.

Decrease in acidity. There is a decrease in total acidity as the ripening progresses.

Change in pH. The pH is lowest (most acid) in the cheese about the third or fourth day after moulding. It then increases slowly during the ripening period. A pH level of 5 to 5.2 of cheese after pressing will rise to 5.4 to 6 after four weeks and 5.8 to 6 after eight weeks. If, however, the pH level drops, the cheese will have a sour or bitter flavour.

Change in the flavour. Good quality cheese kept at 15 C should develop a pleasant, full flavour in four to eight weeks. Certain volatile flavouring compounds are formed.

Change in the body of the cheese. The chemical and physical properties of the casein change as the ripening progresses. Increasing amounts of casein are changed to a more soluble form by the action of bacteria and enzymes. The change is accelerated at higher ripening temperatures. The body of a good cheese changes from a tough corkylike texture to a smooth, waxy consistency. In acid cheese a mealy, pasty body results.

Production of gas. Gas production is normal in some cheese. It is, of course, abundant if the cheese contains the typical gas-producing bacteria, as with Swiss cheeses.

Conditions

The temperature, relative humidity and ventilation in the maturing rooms where the cheeses remain until their sale all affect the maturation of the cheese. Ideally, the room should be ventilated, but the air should have a relative humidity of about 80 to 90 per cent and the temperature should be between 13 and 15 C. If too low a temperature is used, the cheeses will mature very slowly and may have an acid flavour and a crumbly texture; if too warm, the cheeses will become soft and break down. If the humidity is too low the cheeses dry out, become very hard and will crack; too high and they may become covered with a foulsmelling yellow-white scum. A thermo-hygrometer measures both temperature and relative humidity accurately, but it is sufficient to measure only the temperature daily. The approximate relative humidity can be estimated by inspecting the cheeses, an ability that comes with experience. Dry, cracked cheese indicates a low humidity and yellow surface scum indicates high humidity.

Temperature and humidity can be manipulated in various ways. The room temperature can be raised by opening the door to the production room where the vat is being heated and can be cooled by adding a ceiling or a second roof to prevent direct sunlight or by opening windows at night. Cover the window opening with a fine mesh to prevent insects and other pests from entering. A dry curing room can be made more humid by spraying the walls and floor with water daily, taking care not to wet the cheeses, and by keeping doors and vents closed to prevent the escape of moist air. A more expensive solution is to add pools of water or pipes which wet the walls when necessary. Opening windows and doors might dry out a damp curing room, but in very damp climates it is sometimes necessary to put in moisture-absorbing substances such as sawdust or sand.

Wiping

As the cheese is initially very acid, the lactose having been transformed into lactic acid, bacteria will preferentially grow on the rind. A cheese left unattended in the maturing room will soon become covered with a layer of mould and decay. Wiping the cheeses gently with a moist cloth not only reduces mould formation but can also inoculate a new rind with bacteria from old rind, thereby helping the rind to develop and encouraging the maturation from the surface to the inside of the mass. Smearing a liquid bacterial culture on the surface also protects against mould.

Wiping is done in two stages:

1. Wipe the sides and upper face of the cheese with a moist cloth, keeping the shelf dry.
2. Two days later turn the cheese over and wipe the other face and the sides.

Provolone

Smoked Provolone

Smoked Provolone belongs to the 'pasta filata' or stretched curd category of cheeses traditionally produced in Italy, Bulgaria, Rumania and Turkey. It is related to Caciocavallo and Mozzarella. It is made from fresh milk, raw or pasteurized, though slightly sour milk can be used. Bacterial starters are added to achieve a specific level of acidity and curds are formed by the coagulating action of rennet. Andean cheese, Tilsit and Danbo are all made in a similar way. However, for Provolone, instead of moulding, the curds are piled onto a table and left to ferment. After fermentation, the curds are cut and heated in water and then stretched and rolled into round, pear-like or sausage shapes. The weight can vary from 450 to 2,270 g. The cheeses are hung in plastic or string nets during the subsequent drying and ripening. The outside has a shiny surface, smooth and well-sealed, without cracks or holes. After hanging the cheese turns yellow and is then ready to be smoked, if required, which imparts the characteristic pleasant smokey aroma of this popular cheese.

Raw material and acidity test

Smoked Provolone is made from curd which has fermented over a 15-to 30-hour period at 20 C. The acidity of the curd is the decisive factor in forming smooth and well-shaped cheeses. In hot weather, 15 hours of fermentation is enough, whereas in cooler periods, up to 30 hours are needed.

A simple test after fermentation determines the correct acidity and therefore the right moment to begin stretching and moulding: take a one cm strip of curd and submerge it in water (65 C) for two minutes. Remove it and stretch it. If it stretches smoothly to approximately double its length, it is ready. If not, or if it breaks, it needs to ferment longer to increase the acidity. If the curds are not acid enough the curd will be lumpy. On the other hand, if the curd stretches very quickly and is very soft it has overfermented and the strings will break and not hold together during shaping.

In rural cheese factories, Provolone has been successfully produced from curds which had been intended for Danbo/Tilsit or Andean cheese, but which had suffered from coliform gas fermentation and were therefore disqualified from the ripening process. These cheeses have to be processed within a maximum of 40 hours after moulding.

Hot-water processing

When the curds are ready to be stretched, cut them with a large knife into narrow strips, not more than one cm wide.

Heat a large (50 litre) pan with 20 litres of water to 80 C. Add about 20 kg of the strips of curd to the water, making sure that all the curd is covered by water. Leave the curds submerged for 5 to 10 minutes, allowing the water temperature to drop to 65 C. Using a large wooden paddle, begin stretching the curds with a pulling movement. Continue until a smooth, uniform, white plastic mass forms. At this point, place the paddle under the curds and lift them out, letting the mass fall on either side of the paddle. Pull out a segment of the mass with both hands, pulling, stretching and squeezing it, and smoothing out holes, lumps, rough parts, drops of water or whey (Photos 7 and 8). Then let it fall back into the water and begin again with another segment. If the curds have not been well worked, they retain a great deal of whey, which is discarded during the ripening process, leaving folds in the surface of the cheese.

During this entire process, the temperature of the water and the curds must remain at between 60 and 65 C.

Moulding

When the mass of curd is shiny and of a uniform consistency, pull a segment and begin to roll the curds up into a very tight shape, rotating the ball until it reaches the desired size. This must be done by pulling firmly and steadily on the curds so that each thin layer will join the one beneath. If this is not done properly, air pockets and whey will be trapped between the layers of the cheese and create an imperfect product. When fully formed, the hot ball of cheese is placed in the middle of a piece of thin cloth, 50 cm square. The cloth is folded lengthwise over the cheese and its ends are twisted in opposite directions, compressing the cheese and removing the last drops of whey.

Cooling and brine solution

The cheese is removed from the cloth and tossed into a cold water-bath to cool. The bath must be kept constantly cold by a flow of running water. After approximately two to four hours the cheeses are removed from the cold water-bath and placed in a brine solution (20 to 22 Baume) for four to six hours, according to their size. The brine solution is prepared by adding 10 kg of industrial salt to 30 litres of boiled water. Since the cheeses float, extra salt must be sprinkled on top to obtain uniform salinity.

Drying and weighing

After the salt bath, the cheeses are placed in a net bag made from either plastic or string. They are tied together in pairs and hung over a stick for three to five days to dry (Photo 9). Once dry, all the cheeses from the same batch are weighed together to determine the yield produced from the fresh milk.

Smoking

Smoking gives the cheese its characteristic golden colour and appealing aroma and flavour. It also acts as a germicide on the surface of the cheese. It is important that the smoke does not heat the cheese directly. An effective design, therefore, is to produce the smoke away from the smoking room, so that only the cool smoke reaches the cheeses (Photo 10). When the fire is burning well, sawdust is added to produce copious amounts of smoke. Smoking takes four to eight hours, depending on the intensity of the smoke, until the cheeses are shiny and golden coloured. Care must be taken to select wood from a safe source; industrial cuts are unsuitable, for instance.

Yield

The yield of Provolone cheese is less than other types of cheese, since part of the milk fat is not incorporated into the curds, but is lost in the water bath. Ten and a half to eleven litres of milk are needed to obtain 1 kg of Provolone cheese.

The smoking process further if educes the weight of the cheese by 10 per cent. If this yield is not obtained, too much milk fat has been lost in the water bath, either because of low acidification of the curds or overlong submersion in the hot-water bath.

Ripening and preservation

Smoked Provolone cheese can be consumed immediately after smoking, when the smoky flavour is most prominent. It can also be preserved for several weeks, which hardens and ripens the cheese. The ideal conditions for ripening consist of a 14 to 16 C temperature and an 80 per cent humidity. However, in the tropics, Provolone has been successfully ripened at 20 C and 95 per cent humidity.

Undesirable bacterial growth seldom presents problems during the ripening process, due to the high acidification and temperature of the curds during stretching. This process destroys the majority of microorganisms and inhibits the development of surviving bacteria.

The main problems are external, such as fly larvae on the surface of the cheese. Ants and cockroaches have also been known to descend the strips. Moulds can grow, too, when there is excessive humidity. In this case, the cheeses must be cleaned with a dry cloth.

To avoid these problems, and to conserve the moisture in the cheese, the cheeses can be wiped with vegetable oil or with a plastic solution (Mowilith or Foodplast) which also improves their external appearance. As the cheese ripens, the acidic taste is replaced by a sharp, highly aromatic flavour and the texture becomes firmer and drier. Occasionally 'eyes' will appear in the body of the cheese, due to gaseous fermentations.

Use of the by-products

The whey or water mixture left after the stretching of the cheese is rich in fat. It can be passed through a cream separator, or just left to set overnight and the resulting fat can be skimmed off and made into butter.

Disadvantages of Provolone cheese

• The processing is slightly difficult at first, until one becomes accustomed to judging the correct moment to begin the stretching and pulling.
• The temperature of the curds is quite high and takes getting used to.
• The yield is low, which causes the price of the cheese to be higher than other cheeses.

Mozzarella cheese

Mozzarella is a soft, white cheese, similar to Provolone, but with a higher whey content and is sold fresh. It is packed in plastic bags and needs to be refrigerated for not more than about 10 days. Consumers prefer lowfat Mozzarella (30 per cent fat) for dietetic purposes. It is often used as a pizza topping, since it forms the characteristic long strands when heated.

The production of Mozzarella is the same as that for Provolone until the curds are about to be stretched. Then they are worked only half as long as the time taken for the Provolone curds, leaving more whey in the curds. They are stretched and rolled into firm balls, as with Provolone, but instead of being squeezed in the cloths and put into cold water they are immediately put into plastic bags and sealed. They are put next to one another on shelves, in a cold room (4 C), to be delivered as soon as they are cool, (normally the next day). Mozzarella can also be dried in a cold room without the plastic bags, producing a firm texture which is easier to slice, often preferred by Pizzerias.

The yield of wet Mozzarella is 1 kg per 10.5 litres of milk with three per cent butterfat.

Preserving and record keeping

Preserving cheese

A well-made cheese will last for a long time. Nevertheless, some factories add sodium or potassium nitrate to the milk to prevent the swelling up of the cheese caused by cold or butyric bacteria. These additives are not always successful however: active aerobic cold bacteria in milk, for example, prevent potassium nitrate from working and can produce a bitter taste in the cheese, lowering its quality. It is better to eliminate the need for preservatives by ensuring that good hygiene prevails at all times.

Record keeping

A record (see Figure 14) of daily procedures can help analyse and understood successful and unsuccessful batches. The record should include all the items given in the box.

File:SK17052A.GIF

Figure 14. Example of record keeping

Common problems and their causes

Cheese with cracks

Over-acidified milk or carelessness with starters and production can result in too much whey remaining in the cheese. During secondary maturation, the necessary enzymes will be unable to penetrate the cheese from the rind to the centre, often producing a cheese with two colours, which is ripe on the outside but bitter and crumbly within. Specific causes can be one or more of the following:

• Undue care at milking
• Milk exposed to sunlight (that is, heat) resulting in high acidity
• Dirty milk churns
• Negligence and delays in manufacture

Daily technical report 1. date quantity of milk in vat 3. type of cheese to be made 4. quantity of fat in the milk 5. acid level of milk 6. type and quantity of culture and amount of rennet used 7. temperature of coagulation 8. time in minutes of cutting and stirring 9. size of curds 10. acidity of whey 11. amount of whey extracted 12. amount of water added 13. amount of salt added 14. time in minutes for washing and stirring 15. temperature of whey after final watering 16. total time of preparation from rennet to pressing 17. number of moulds 18. weight of new cheese before brine bath 19. weight of cheese after maturation 20. conversion of milk to cheese 21. observations

Early blow-ups (Pressler defect)

Blow-ups, where the cheese becomes full of small or large bubbles of gas, can occur during pressing (called the Pressler defect). They are normally caused by Aerobacter aerogenes or Escherichia cold entering the milk during milking or transport or through cloths or cloth filters and are invariably due to a lack of hygiene, such as:

• Unhygienic milking or dirty cloths
• Dirty churns
• Milk containing antibiotics or other inhibiting substances (mastitis, for example) which sets back growth of the desired bacteria.
• Degenerated cultures
• Dirty water
• Unhygienic workers.

Blow-ups can be prevented if the level of milk contamination is slight with an active culture of streptococcus, but nothing can prevent the growth of coliforms if milk is contaminated by antibiotics.

Late blow-ups

Clostridia are mobile, anaerobic and spore-forming microbes. They are present in large quantities in faeces and produce large quantities of carbon dioxide and hydrogen gas and cannot be eliminated by pasteurization. These bacteria enter the milk from the hands or clothing of personnel or when tiny bits of excrement fall into churns and milking equipment. Care must be taken to avoid, in particular:

• Unhygienic milking near manure and muck
• Rotten silage for feed.

The only defence is to prevent the bacteria entering the milk and the importance of good hygiene cannot be overemphasized.

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Figure 15. Late blow ups

pH meters

Some cheesemakers prefer pH meters (Figure 7) to the more traditional acidimeters, as they give faster and more accurate results and are much easier to use.

Readings should be taken of the pH of the milk before the starter is added, of the starter itself and throughout the cheesemaking process since this can help maintain consistency and prevent errors.

Readings vary according to the type of cheese being made and local conditions but, once requirements are established, the pH meter is an excellent instrument for helping to produce a reasonably standard product. pH meters are available in a wide variety of sizes: a stick pH meter would be adequate for a small plant but a built-in system is advisable in a larger factory.

Some meters come with a digital read-out and special buffer solutions of known pH value to calibrate them; however, pH electrodes are very fragile.

Test for antibiotics

Method

Inoculate 10 ml of milk suspected of containing antibiotics with one tenth ml of starter culture and incubate at 32 C for five hours. Measure the level of acidity.

Result

There will be little or no increase in acidity if the milk contains antibiotics (or other inhibitory substances).

Equipment and reagents

Gerber butyrometers with tops

Gerber centrifuge (manual or electric) of 1,000 to 1,200 rpm

Special milk pipettes of 11 ml capacity

10 ml unbreakable pipettes (for sulphuric acid)

Water bath (65 C)

Sulphuric acid (H2SO4) at a relative density of 1.820 to 1.825

Amyl alcohol

Method

It must be emphasized that great care must be taken with this test, as it involves the use of sulphuric acid.

1. Put 10 ml of sulphuric acid in the butyrometer.
2. Add 11 ml of milk, taking care that it runs down the side of the butyrometer so as not to mix too quickly with the acid and burn the milk solids.
3. Add 1 ml of amyl alcohol to the mixture.
4. Put the top tightly on the butyrometer and agitate until the solution is well mixed.
5. Place the butyrometer in the centrifuge and leave it in motion for approximately five minutes.
6. Remove the butyrometers and place them in a water bath at 65 C for three or four minutes. Read the level of fat in the butyrometer.

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Figure 8. Equipment for testing fat content

Recommendations

The normal fat level of milk depends on several different factors but should in any case not be below three per cent.

Factors affecting fat content		Fat content
Breed	Brown Swiss	4%
	Jersey	5%
	Holstein	3%
Lactation	first months	low
	last months	high
Milking	first milking	2%
	last milking	4.5%*
	average	3.25%

*For this reason the calf should not be given the last milk.

Density determination

This simple test to determine milk density reveals whether or not the milk has been diluted. Although density varies considerably in milk from different animals and breeds, the following table gives an indication of relative densities:

Relative density and composition in normal, watered and skimmed milk.

	Relative density	Fat	Non-fatsolids	Total solids
Normal milk	1.028-1.033	3% min	8.5% min	11.5% min
Watered milk	1.025	lower	lower	lower
Skimmed milk	1.035	lower	same	lower
Double fraud*	1.030 much	lower	lower	lower

*Milk which has been both skimmed and watered. As the density is the same as that for normal milk the fraud can only be detected with a fat test.

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Figure 9. Measuring the density of milk

Equipment

Jug

Thermometer

500 cc measuring cylinder

Milk density meter, calibrated at 15 C (Scale: 22 to 36 = 1.022-1.036)

Method

1. Pour 500 cc of milk sample down the inside of the measuring cylinder

without making it bubble.

1. Gently place the milk density meter in the cylinder and let it float. It

will rise to give a reading at the surface of the sample.

Temperature correction

For each degree above 15 C add 0.2 to the relative density. For each degree below 15 C subtract 0.2. For example, if the milk density meter shows a reading of 28 at a temperature of 25 C, then the corrected density reading is:

28 + (10 x 0.2) = 30

Calculation of non-fat solids (Richmond's formula)

1. 22 x percentage fat + 0.25 x specific gravity (corrected to 20 C) + 0.72 = percentage non-fat solids

Non-fat solids in normal milk fluctuate between 8.5 and 9 per cent. If the results are lower the milk is probably diluted.

Pasteurization

Even when hygienically produced and stored, milk can contain high levels of micro-organisms. Drinking untreated milk can cause gastric distress and more serious ailments such as brucellosis, tuberculosis and typhoid. If the animal is also dirty or unhealthy then the health risk is even greater.

Heat destroys the bacteria in milk and many people boil milk before drinking it to avoid illness. But boiling milk adversely affects proteins: some proteins may be denatured, and some lactose may be converted into caramel and burned milk has an unpleasant flavour and smell.

Pasteurizing milk avoids these problems. The milk is heated to a temperature well below boiling point for a prescribed period sufficient to eliminate harmful bacteria, but not enough to destroy the flavour or nutritive value.

Method

Filter the milk and run it into the cheese vat. Heat it either to 63 C for 30 minutes, to 68 C for 15 minutes (as practised in Ecuador) or 73 C for 15 seconds. Using cold water in the double-walled vat, cool the milk to the temperature needed for its coagulation (see p.73).

Cheese from unpasteurized milk

Although most factories in America and Europe now use pasteurized milk for drinking and cheesemaking, there are still several types of cheese made from unpasteurized milk. Gruyere and Emmental, for example, are made in Switzerland, France, Germany and Austria from traditional formulae using unpasteurized milk. Other cheeses are also made from unpasteurized milk, not to keep costs down but to obtain a distinctive flavour. Cheese made from unpasteurized milk clearly demands a far higher level of hygiene than cheese made from pasteurized milk. The strictest standards, from the condition of the dairy herd to the final steps in the cheesemaking process, must be rigidly adhered to when unpasteurized milk is used.

Cultures

The dairy culture contains micro-organisms useful for the manufacture of cheese and butter. Generally there are two types of coexisting microorganisms. One type produce lactic acid from lactose and for that reason are called acidifiers, while the other type make substances with aroma and flavour and have been named aromatizers. The first type of microorganisms ensure the presence of acid in the cheese and butter, thus prolonging the conservation time for these products since high acidity does not permit putrefying micro-organisms to live. The second type of micro-organisms produce a pleasant odour and flavour in both products, improving their quality and therefore increasing their sale price. The most commonly used culture in rural cheese factories is known as lactic culture, since its principle function is to form lactic acid from the lactose of the milk.

Mother-and-daughter solutions

A lactic culture can be initially propagated from a liophilized (freeze dried) powder form of the culture. When added to sterilized milk the acidifying and flavour-producing bacteria in the powder (see Figure 11) begin to multiply, producing an acid flavour and a pleasant odour. The milk coagulates, ideally forming a smooth white, gelatinous mass without cracks, grains or bubbles and with very little whey, containing an acidity level of 70 to 80 Dornic. This first propagation is known as the mother solution. Second and subsequent propagations are known as daughter solutions. The first daughter solution is made by mixing a small amount (2 per cent) of the mother solution with sterilized milk, and the second daughter solution is formed from the first daughter solution and so on successively.

A fresh solution should be made every day, or at least every other day, and in favourable conditions a lactic culture should last for a month (see Figure 10).

It is essential to observe careful hygiene with each propagation, both to eliminate any bacteria that the milk might contain and to prevent contamination. It is also important to use antibiotic-free milk.

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Figure 10. Mother and daughter solutions

Propagating the mother solution

1. Put a heat-resistant funnel and bottle into a suitable volume of clean water in an appropriate container. (Glass may be used but care must be taken to cool and heat them slowly to prevent shattering.) Fill the bottle completely with water and ensure that air bubbles, which may contain bacteria that would survive the boiling, are excluded. Boil the water for 30 minutes, making sure that the bottle is completely submerged throughout the sterilizing procedure.
2. Put one or two litres of fresh antibiotic-free milk in a small, clean, three-litre pan. Cover the pan well and bring the milk to the boil, stirring occasionally with a pre-sterilized spoon to prevent milk solids sticking and burning. As soon as the milk boils fully, lower the heat to prevent it from boiling over. Never blow on the pan to prevent the milk from boiling over as this will contaminate it. Boil the milk over a low heat for 15 minutes. NB: It is better to sterilize the bottle, funnel and milk at the same time, using two pans, but if only one pan is used, boil the water first.
3. Carefully fill the bottle with the boiled milk, using the funnel. Seal the bottle with a hermetic top immediately and let it cool for a few minutes before submerging it in cold water. Keep the boiled water for the incubation tank.
4. Cool the milk to between 25 C and 30 C in a bath of stirred water. Do not open the bottle to take the temperature, as this will expose the milk to potential contamination. Instead, take the temperature of the water immediately surrounding the bottle.
5. Open the bottle, vial or packet of powdered culture near an open flame to help eliminate airborne microbes (some skill in microbiology is required). Open the bottle of sterilized milk, add the powdered culture and agitate the bottle so that the milk and powder are thoroughly mixed. Plug the bottle with sterilized cotton wool.
6. Put the bottle of inoculated milk in a water-bath at 20 C to 23 C (not less than 20 C) and place it in an incubator. Ensure that the temperature of the water in the water-bath remains constant throughout the fermentation process and agitate the bottle occasionally for the first few hours to prevent the powder settling at the bottom of the bottle.
7. The bacteria in the mother solution take several hours to adapt to their new environment and incubation can take about 16 to 18 hours. After this time the population is high and in its final growth phase. If the solution is left in the incubator for longer it can become too acid and will coagulate, with the whey separating out. Some coagulation is normal: 0.6 or 0.7 per cent lactic acid is sufficient for milk to begin to coagulate. If carried too far the bacteria will die or degenerate and the culture will be useless (see Figure 11). It is important, therefore, to stop the incubation before the whey begins to separate. Skim the solution as it contains a very few lactic bacteria.
8. The solution should be used quickly or it will lose some strength. If it is not to be used immediately then it must be covered to prevent contamination by moulds, and cooled in a refrigerator to slow down bacterial growth. Cultures must not be kept in the curing room where there is a risk they will be contaminated by the many microbes present.

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Figure 11. Development of lactic bacteria in milk

Propagating the daughter solution

Propagating the daughter solution is an almost identical process, but as the bacteria are already accustomed to the milk there is no phase of adaptation and they begin to multiply more quickly. Put some milk at a temperature of 20 to 23 C in the bottle, add some of the mother solution (1 to 2 per cent of the total milk), agitate them near an open flame and then fill the bottle with the remaining sterilized milk. A daughter solution needs to be incubated for only 12 to 16 hours but incubation conditions should otherwise be the same as for the mother solution.

Points to remember

• Keep the culture (1 litre) separated from the large starter container (5 litres).
• Whenever the mother culture shows signs of deterioration or insufficient growth it must be discarded and a new mother culture established from the dry starter culture powder.
• Propagate the mother culture and the starter every day, or at least every other day.
• Renew the culture at regular intervals. Sometimes a mother culture loses its desirable properties after two weeks, but will usually last for a month. It is always advisable to have a fresh dry culture in the refrigerator for speedy use.
• Starter cultures are available, in a concentrated form, which require no preparation at all and which can be added directly to the cheese vat. These are available in deep-frozen or freeze-dried forms; the latter is ideal for a small-scale operation, or for use in an emergency.

A The bacteria adapt to the new environment, as when a new culture is prepared.

B The bacteria, having adapted to the milk, use the lactose and produce lactic acid. Their population increases rapidly and the milk thickens.

C As the amount of lactose available decreases and the milk becomes acidified bacterial growth diminishes and bacteria lack the ability to reproduce.

D The lack of lactose and oxygen and the excessive acidity produced by the bacteria themselves cause their degeneration and death.

Maturation of the milk

When the milk in the vat has reached the coagulation temperature, the lactic culture is added in the proportion of one litre per 100 litres of milk. The purpose for this procedure is to allow for the production of lactic acid from the lactose of the milk due to the action of the microorganisms of the lactic culture. It is necessary for the milk to reach an optimum acidity in order to achieve proper separation of the whey from the curds. The milk maturation time is very variable since it depends on the acidity of the milk upon arrival at the cheese factory. In locations where the milking is done very early before the sun gets hot, and where the cheese factory is near the place of milking, it is possible for the milk to arrive very fresh, with 16 or 17 degrees of acidity. In this case, it will be necessary to leave the milk with the lactic culture for at least an hour before curdling, in order for the acidity of the rnilk to reach 18 or 19 Dornic degrees. In other locations, in spite of early milking, the great distance from the cheese factory means that the milk arrives after two or three hours when its acidity is between 18 and 19 Dornic degrees. The maturation or acidification time of the milk in this case should not exceed half an hour. Finally, it is possible that the milking is done at noon, that is, at the hottest time of day, and the milk reaches the cheese factory at the end of the day after 3 or 4 hours in transit. Under these circumstances it is likely that the milk already has too much acidity and therefore its maturation time should be nil, in other words the rennet should be added immediately after adding the lactic culture to the milk. In fact, it may be necessary to add some water if the acidity is greater than 21 Dornic degrees, since excess acidity as well as the lack of acidity results in imperfections in the cheese.

Coagulation

Coagulation is judged subjectively by the cheesemaker, by carefully examining the coagulum on the vat side, using a finger or probe.

The degree or quality of coagulation determines the final moisture content of the cheese and, since the amount of water present affects the fermentation process and therefore the final texture of the product, coagulation is a crucial step in the cheesemaking process.

The process is based on rennet (or other coagulating enzymes), an enzyme which coagulates milk by bonding the principal protein, casein, into a network which entraps the fat. There are three distinct phases:

1. Casein and calcium (Ca2+) are found in a free state in milk.
2. The enzyme, rennin, attacks the micelles of casein and breaks them down, facilitating the union with calcium.
3. The calcium forms bridges between the micelles of casein to form a smooth, white, gelatinous coagulum incorporating a large part of the fat.

Rennet

The enzyme rennin is found in both natural and substitute rennets. Natural rennet is a substance produced in the stomach walls of calves, lambs and kids and used to coagulate their mothers' milk. Substitute rennets, in tablet, powdered or liquid form are generally processed and purified in commercial laboratories using substances made from moulds, namely mucormieli or mucopusillus. These are similar to pepsin and cymacin, which are the enzymes taken from the stomach of animals to make natural rennet. It can be produced in large quantities, making it cheaper than natural rennet.

Factors affecting the action of rennin

The enzymatic action of rennin is affected by temperature, acidity, salt, light, air, age, and the source and method of preparation:

· Rennin is very active and coagulation is rapid at high temperatures: 34 to 36 C. Curd formed at this temperature will be firm and can be cut into large pieces (see p.33) suitable for soft cheese. Within limits, the higher the temperature the more moisture will remain and the softer the cheese. Hard cheese, made from small curds requires a relatively low temperature: 31 to 33 C. At lower temperatures, say 20 to 30 C, milk coagulates very slowly and the subsequent curds will be weak with a lot of fat lost to the whey. In any case, the temperature must remain constant: milk that is allowed to cool during the process will produce irregular curds, have unevenly distributed moisture and lose some casein into the whey.

If the milk is either too cold or too hot the rennin will not be active at all.

• Milk with a high level of acidity will coagulate quickly (and vice versa); this also depends on acidity.
• Standard preparations of rennet will already contain salt, which would have been added to preserve the rennet.
• As soon as rennet is exposed to light, air and age it begins to lose strength and, as it weakens, increasingly larger quantities must be used until eventually it ceases to be active altogether. Rennet must be stored in a cool, dry, dark place, measured with clean, dry utensils and handled with clean, dry hands. It should last for a number of months in the refrigerator.

Method of coagulation

Ensure milk is at the appropriate temperature (34 to 36 C for soft cheese; 31 to 33 C for hard cheese). Use 2.5 g rennet to 150 litres of milk (or according to instructions). It is normal practice to dilute the liquid rennet (or dissolved powder) with water simply to facilitate mixing a small quantity in a large volume of milk. Add to the milk. Stir in well for two to five minutes, then leave undisturbed for 30 to 40 minutes. The curd will be ready when it separates easily from the wall of the vat and splits, but does not stick or crack when penetrated with a spoon or finger.