MILK AND MILK PRODUCTS

Milk is the secretion of the mammary gland of female mammals (over 4000 species), and it is often the sole source of food for the very young mammal. The role of milk is to nourish and provide immunological protection. The milks produced by cows, buffaloes, sheep, goats, and camels are used in various parts of the world for human consumption. For much of the world’s population, cow’s milk accounts for the large majority of the milk processed for human consumption.

Milk is a complex biological fluid probably containing about 100,000 different molecular species, However, most of the major components proteins, lactose, fat, and minerals-can be separated and isolated from milk relatively easily.

Milk in its natural state is a highly perishable material because it is susceptible to rapid spoilage by the action of naturally occurring enzymes and contaminating microorganisms.

Many processes have been developed over the years-in particular, during the last century for preserving milk for long periods and to enhance its utilization and safety. Milk is converted into a wide variety of milk products using a range of advanced processing technologies. These include the traditional products, such as the variety of cheeses, yogurts, butters and spreads. Ice cream, and dairy desserts, but also new dairy products containing reduced fat content and health-promoting components.

MILK COMPOSITION

The major component of milk is water; the remainder consists of fat, lactose, and protein (casein and whey proteins) (Table 1.1). Milk also contains smaller quantities of minerals, specific blood proteins, enzymes, and small intermediates of mammary synthesis. The structures and properties of these components profoundly influence the characteristics of milk and have important consequences for milk processing. The composition of milk varies with the dairy breeds. The most commonly found breeds-Friesian, Jersey, Guernsey, Ayrshire, Brown Swiss, and Holstein-have fairly similar lactose levels, but milk fat and protein vary considerably. These differences are partly genetic in origin and partly the results of environmental and physiological factors. Within a herd of cows of a single breed, there are considerable variations in milk composition between individual cows.

For example, the milk fat content in Jerseys can range from 4% to 7%, with an average of about 5.0%. The composition of milk changes considerably with the progress of lactation. The first secretion collected from the udder at the beginning of lactation, known as colostrum, has a high concentration of fat and protein, particularly immunoglobulin’s, and a low content of lactose. The composition of the secretion gradually changes to that of mature milk within 2-4 weeks.

However, milk fat content is much more variable; the more frequent the milking, the greater is the variation. Generally, during milking, the fat content increases. Morning milk is usually richer in fat than evening milk. Other factors, such as mastitis, extreme weather conditions, stress, and exhaustion, can also exert an influence on milk composition. The milks of the sheep and the buffalo have much higher fat and casein contents than those of the other species. The most obvious characteristic of human milk, compared with other milks, is its low casein and ash contents.

TABLE: Typical Composition of Milks of Some Breeds of Cow (g/lOOg)

Breed	Protein	Fat	Lactose	Ash

Jersey	4.0	5.2	4.9	0.77
Friesian	3.4	4.2	4.7	0.75
Brown Swiss	3.5	4.0	4.9	0.74
Guernsey	3.7	3.7	4.7	0.76
Holstein	3.3	3.5	4.7	0.72
Ayrshire	3.5	3.9	4.6	0.72

Preservation

1) Pasteurization, thermalization, and sterilization, utilizing heat to kill microorganisms. These processes all involve the transfer of heat into the product in order to raise the temperature to achieve a closely controlled time-temperature process (e.g., 72"C, 15 s) for pasteurization. An example of the equipment used is a plate heat exchanger, shown in Figure 1.7

2) Chilling and freezing, to slow microbial growth and chemical change. This is widely used both during or prior to processing or for final product storage.

3) Reduction of pH, to inhibit microbial growth. This may be achieved by addition of acids or by bacterial fermentation of the lactose. An example of the equipment used is the cheese vat shown in Figure 1.3.

4) Dehydration (drying), to inhibit microbial growth and chemical change. The equipment used for spray drying.

Figure 1.6 Spray drier with fluid bed attachment (two-stage drying). 1, Indirect heater; 2, drying chamber; 3, vibrating fluid bed: 4, heater for fluid bed air; 5, ambient cooling air For fluid bed; 6, dehumidified cooling air for fluid bed; 7 , sieve. (Courtesy of Tetrapak)

5) Salting, to reduce water activity and inhibit microbial growth. Salt may be added as dry granular salt or by means of a brine solution, with the product being immersed for a period in a tank of concentrated brine.

6) Packaging, to contain the product, protects it, and reduces microbiological and chemical change. Examples of commonly used packaging are the cartons and plastic bottles for liquid products, the bulk 25-kg gas-flushed bags for whole milk powder, and the form/fill/seal packages for cheese.

MICROFLORA IN MILK

Contamination from the udder

Although milk produced from the mammary glands of healthy animals is initially sterile, microorganisms are able to enter the udder through the teat duct opening.

Gram-positive cocci, streptococci, staphylococci and micrococci; lactic acid bacteria (LAB), Pseudomonas spp. and yeast are most frequently found in milk drawn aseptically from the udder; corynebacteria are also common.

Where the mammary tissue becomes infected and inflamed; a condition known as mastitis, large numbers of microorganisms and somatic cells are usually shed into the milk. Mastitis is a very common disease in dairy cows, and may be present in a subclinical form, which can only be diagnosed by examining the milk for raised somatic cell counts. Many bacterial species are able to cause mastitis infection, but the most common are Staphylococcus aureus, Streptococcus agalactiae, Streptococcus uberis and Escherichia coli. These bacteria enter the udder by the teat duct, and Staph. aureus is able to colonise the duct itself.

Although the organisms involved in mastitis are not usually able to grow in refrigerated milk, they are likely to survive, and their presence may be a cause of concern for health.

Diseased cows may also shed other human pathogens in their milk, including

Mycobacterium bovis, Brucella abortus, Coxiella burnetii, Listeria monocytogenes and salmonellae. Recently, concerns have also been raised over the presence of Mycobacterium avium var. paratuberculosis (MAP) (the causative organism of Johne's disease in cattle) in milk from infected animals. The outer surface of the udder is also a major source of microbial contamination in milk. The surface is likely to be contaminated with a variety of materials, including soil, bedding, faeces and residues of silage and other feeds.

Many different microorganisms can be introduced by this means, notably salmonellae, Campylobacter spp., L. monocytogenes, psychrotrophic sporeformers, clostridia, and Enterobacteriaceae. Good animal husbandry and effective cleaning and disinfection of udders prior to milking are important in minimizing contamination.

Other sources of contamination

(1) Milking equipment and bulk storage tanks have been shown to make a significant contribution to the psychrotrophic microflora of raw milk if not adequately sanitised.

(2)Exposure to inadequately cleaned equipment and contaminated air are also sources of contamination.

(3) Milk residues on surfaces, and in joints and rubber seals can support the growth of psychrotrophic Gram-negative organisms such as Pseudomonas, Flavobacterium, Enterobacter, Cronobacter, Klebsiella, Acinetobacter, Aeromonas, Achromobacter and Alcaligenes, and Gram-positive organisms such as Corynebacterium, Microbacterium, Micrococcus and sporeforming Bacillus and Clostridium.

(4) These organisms are readily removed by effective cleaning and disinfection, but they may build up as biofilms in poorly cleaned equipment. Milk-stone, a mineral deposit, may also accumulate on inadequately cleaned surfaces, especially in hard water areas. Gram-positive cocci, some lactobacilli, and Bacillus spores can colonise this material and are then protected from cleaning and disinfection. Some of these organisms may survive pasteurisation and eventually cause spoilage. Other, less significant, sources of contamination include farm water supplies, farm workers and airborne microorganisms.

Natural antimicrobial factors in milk

Raw milk contains a number of compounds that have some antimicrobial activity. Their purpose is to protect the udder from infection and also to protect neonates, but they may also have a role in the preservation of raw milk during storage and transport. Lactoperoxidase is an enzyme found in milk. It has no inherent antimicrobial activity, but, in the presence of hydrogen peroxide (usually of microbial origin), it oxidises thiocyanate to produce inhibitors such as hypothiocyanite. This is referred to as the lactoperoxidase system, and it has bactericidal activity against many Gram-negative spoilage organisms, and some bacteriostatic action against many pathogens. For this reason it has been investigated as a possible means of extending the life of stored milk (5) Lactoferrin is also found in milk and is a glycoprotein that binds iron so that it is not available to bacteria. The chelation of iron in the milk inhibits the growth of many bacteria. In addition to producing an iron-deficient environment, lactoferrin is thought to cause the release of anionic polysaccharide from the outer membrane of Gram-negative bacteria, thereby destabilising the membrane.

Lysozyme acts on components of the bacterial cell wall, causing cell lysis. Gram-positive organisms are much more susceptible to lysozyme than Gramnegatives, although bacterial spores are generally resistant. Immunoglobulin’s of maternal origin are often present in milk, and colostrums is a particularly rich source. These proteins may inactivate pathogens in milk, but their significance in preservation is uncertain.

Pasteurization

Some form of heat process is commonly applied to milk to ensure microbiological safety, and to extend shelf life. In the UK, the most commonly used process is Pasteurization. Time-temperature requirements for pasteurization vary, both low-temperature, long time (LTLT, 63 - 65 °C for 30 minutes), and high-temperature, short time (HTST, 71.7 - 72 °C for at least 15 seconds) minimum processes are permitted. However, in practice, the HTST process is now generally used. Recent concern about the possible survival of MAP in pasteurized milk (discussed further in section 1.7.2.8: MAP) has seen many dairies increase the length of the HTST process to 25 seconds. Higher processes (such as ultra-pasteurization at 138 °C for at least 2 seconds) may also be applied to products with high fat and solids content. Plate heat exchangers are the most common method for milk pasteurization, but it is essential that they are designed, constructed and operated in such a way as to minimize the possibility of recontamination of the pasteurized milk by raw milk. Most commercial pasteurisers are fitted with sensors that continuously monitor the pasteurisation temperature, and are linked to automatic divert valves. If the pasteurisation temperature falls below a specified value, the valve opens and diverts the under-processed milk away from the post pasteurisation section of the plant and the filling line, into a divert tank. The correct operation of these monitoring systems is critical and should be regularly checked. It is also essential that there are no cross-connections between the raw and pasteurised sides of the process, and this should include separate clean-inplace (CIP) systems. It is also usual to maintain a higher pressure in the pasteurised milk to minimise the risk of cross contamination in the heat exchanger.

Recontamination of this kind may have serious public health consequences. Accepted pasteurisation processes are designed to reduce the numbers of vegetative microbial pathogens to levels that are considered acceptable, although bacterial spores are not destroyed. Most of the potential psychrotrophic spoilage bacteria are also eliminated. However, certain heat-resistant mesophilic organisms, referred to as thermoduric, are able to survive pasteurisation. Thermoduric species commonly isolated from pasteurised milk include Micrococcus spp., Enterococcus faecium and Enterococcus faecalis, Bacillus subtilis, Bacillus cereus, and certain lactobacilli. Psychrotrophic strains of these organisms may be able to grow slowly in the pasteurised milk at 5 °C, and, if present initially in high numbers, could eventually cause spoilage. Effective cleaning of the cooling sections of pasteurisers is important to ensure that these organisms do not build up on surfaces.

UHT or sterilization processes

Milk may also be subjected to more severe heat processes sufficient to achieve "commercial sterility". This may be done by batch heating in closed containers, or continuously with aseptic filling into sterile containers. Both conventional retort sterilisation and UHT processes must achieve a minimum Fo of 3 minutes to ensure product safety. These processes destroy all vegetative cells in the milk, and the majority of spores, although certain very heat-resistant spores may survive. This results in a long shelf life without the need for refrigeration, but also causes organoleptic changes in the milk, such as browning. Conventional sterilisation processes involve heating the milk in thick-walled glass bottles, closed with a crimped metal cap, at about 120 °C for approximately 30 minutes. However, modern large-scale production methods often use an initial UHT treatment prior to filling the container, followed by retorting for a reduced time (10 - 12 minutes), and then a rapid cooling process. This is said to give a product with improved organoleptic properties.

UHT processes may be direct or indirect. Direct systems inject high-pressure steam directly into the milk to obtain the desired temperature, and then employ flash cooling under vacuum to remove the resulting excess water. Indirect systems utilise heat exchangers and holding tubes. Direct systems are said to give better organoleptic properties, as the heating and cooling processes are very rapid, but they are more complex and expensive to install. UHT processed milk involves preserving milk by holding at a temperature of 140 - 150 °C for 1 - 2 seconds (minimum treatment is 130 °C for 1 sec) (3, 10). Heat treatment is usually followed by aseptic filling into sterile cartons or other containers. The maintenance of sterility in filling is vital to prevent recontamination of the treated milk. As with pasteurised milk, it is also vital to ensure that raw milk cannot recontaminate the UHT-treated milk. Certain very heat-resistant spores of mesophilic bacilli, classified as Bacillus sporothermodurans (11) are able to survive UHT processes and may subsequently grow in the final product. However, this organism has been shown not to be pathogenic (12) and does not seem to cause any detectable changes to the product. Thermoduric Bacillus stearothermophilus are able to survive UHT processes and cause flat-sour spoilage (3).

Other Methods of Treating Milk

1) Microfiltration, usually using ceramic membrane filters, can be used in combination with a minimum HTST pasteurisation process to remove significant numbers of bacteria from milk, and give a substantial extension to shelf life over conventional pasteurised milk (13). The fat is separated from the milk before filtration and is heat treated separately before being added back to the milk after processing. Milk produced by this method is on sale in several countries, and is said to have a shelf life of at least 20 days

2) Bactofugation is a centrifugation process that is also able to remove bacteria (including endospores) from milk. It has been used in the cheese industry for some years to minimize contamination with the spores of lactate-fermenting clostridia that cause 'late blowing'. The centrifugate produced by the process contains most of the microbial cells present initially in the milk, and this can be sterilized separately and then recombined with the treated milk, which is conventionally pasteurized, to restore its composition. A shelf life of 30 days or more is claimed for milk treated in this way.

3) Microwaving refers to dielectric heating due to polarization effects at a selected frequency band (300 MHz to 300 GHz) in a nonconductor. It has been in commercial practice for milk pasteurization for quite a long time as it provides the desired degree of safety with minimum quality degradation. Plate counts of raw milk undergoing continuous-flow microwave pasteurization, at 2450 MHz, were negative while the temperature reached was 82.2 °C (14).

4) Other methods that have been applied to milk processing include irradiation, high-pressure processing, ultra sound treatment, ultra high-pressure homogenization (UHPH), and pulsed-electric field (PEF).

BACTERIAL COUNT IN MILK

Dairy product quality assurance begins at the farm and ends in the hands of the consumer. In this regard, raw milk quality is essential and is closely monitored. Regulations require that bacteria and somatic cell counts of Grade “A” raw milk not exceed 100,000 Standard Plate Count (SPC) and 750,000 Somatic Cell Count (SCC), respectively. Raw milk must also meet other quality standards; It should be free of drug residues, free of added water and free of sediment, contaminants and other abnormalities. The overall condition and cleanliness of the dairy farm, as determined by routine inspections, are also considered.

Although regulatory requirements have been instrumental in ensuring the quality of raw milk, most segments of the dairy industry feel that more stringent standards (e.g. SPC < 10,000, SCC < 200,000) would result in an even higher quality milk supply. Depending on the purchaser of the milk, dairy farmers can also receive substantial monetary premiums by producing quality milk that meets standards far more demanding than regulatory requirements.

The SPC, which measures the overall bacterial quality of a sample, is used extensively in both regulatory and premium testing programs. In addition to the SPC, producer raw milk is often subjected to a number of other bacteriological tests used as indicators of how that milk was produced. These tests may be included in determining eligibility for premium payments or they may be used simply as an added quality assurance tool. The procedures most often used in addition to the SPC are the Preliminary Incubation

Count (PIC), the Lab Pasteurization Count (LPC) and/or the Coliform Bacteria Count (Richardson,

1985). While the SPC gives an estimated count of the total bacteria in a sample that can be detected with the method, the PIC, LPC and Coliform Count select for specific groups of bacteria that are often associated with poor dairy practices. Results of these testing procedures are used to help identify potential problems that may not be evident or detected with the SPC alone. This document provides an overview of these bacteriological procedures and provides a general discussion on the causes of high bacteria counts in raw milk.

Standard Plate Count

The Standard Plate Count (SPC) of a producer raw milk samples gives an indication of the total number of aerobic bacteria present in the milk at the time of pickup. Milk samples are plated in a semi-solid nutrient media and then incubated for 48 hours at 32°C (90°F) to encourage bacterial growth. Single bacteria or tight clusters (e.g. chains or clumps) grow to become visible colonies that are then counted. All bacterial plate counts are expressed as the number of colony forming units (CFU) per milliliter (ml).

Aseptically collected milk from clean, healthy cows generally has SPC values of less than 1,000. Higher counts suggest that contaminating bacteria are entering the milk from a variety of possible sources. Although it’s impossible to eliminate all sources of contamination, counts of less than 5,000 or even 1000 are possible; counts of 10,000 or less should be achievable by most farms. One of the most frequent causes of high SPCs is poor cleaning of the milking system. Milk residues on equipment surfaces provide nutrients for growth and multiplication of bacteria that can then contaminate the milk of subsequent milking. Other practices that might contribute to increased bulk-tank SPCs are milking soiled cows, maintaining an unclean milking and housing environment, and failing to rapidly cool the milk to or maintain it at less than 4.4°C (40°F). On rare occasions, mastitic cows that shed infectious bacteria can also contribute to or cause high SPCs.

Preliminary Incubation Count

The Preliminary Incubation Count (PIC) reflects milk production practices. This procedure involves holding the milk at 12.8°C (55°F) for 18 hours prior to plating. This step encourages the growth of groups of bacteria that grow well at cool temperatures. Bacteria in the incubated sample are counted with the SPC procedure and compared to the SPC of the un-incubated sample to determine if a significant increase has occurred. PICs are generally higher than SPCs, although in some cases no growth occurs or counts may even be lower. Counts 3-4 fold higher are often considered significant, but this depends on the initial SPC. Some consider counts of > 50,000 to be of concern regardless of the SPC, though in some cases the counts may be equal and in rare cases the PIC may be lower. It should be noted that contaminating bacteria from the same source can vary in their growth rates in this procedure, which can result in very different PICs at the same level of contamination.

High PICs are most often associated with a failure to thoroughly clean and sanitize either the milking system or, in some cases, the cows. Bacteria considered to be natural flora of the cow, including those that cause mastitis, are not thought to grow significantly at the PI temperature, although there may be a few exceptions. If a PIC is approximately equal to or slightly higher, or even lower, than a high SPC (e.g., > 50,000), it may suggest that the high SPC is possibly due to mastitis. Marginal cooling (e.g., milk is held over 40°F) or prolonged storage times may also result in unacceptable PIC levels by allowing organisms that grow at refrigeration temperatures to multiply.

Bacteria that grow well at refrigeration temperatures (psychrotrophic bacteria), are most frequently associated with high PICs. Although the same types of bacteria that cause high PICs can also cause defects in pasteurized milk products, the PIC of a raw milk supply does not indicate the potential quality or shelf-life of a pasteurized product made from that milk. These types of bacteria are mostly destroyed by pasteurization, but can occur as post-pasteurization contaminants in pasteurized milk.

Lab Pasteurized Count

Although most bacteria are destroyed by pasteurization, there are certain types that are not. The Lab Pasteurized Count (LPC) estimates the number of bacteria in a sample that can survive the pasteurization process. Milk samples are heated to 62.8°C (145°F) for 30 minutes, which simulates batch pasteurization. Bacteria that survive the heat treatment (thermoduric bacteria) are enumerated using the SPC procedure. LPCs are generally much lower than SPCs, with counts > 200-300 deemed high. The natural bacterial flora of the cow, as well as those associated with mastitis, are generally not thermoduric, although there may be a few exceptions. High LPCs are most often associated with a chronic or persistent cleaning failure in some area of the system or significant levels of contamination from soiled cows. Other common causes of high LPCs are leaky pumps, old pipe-line gaskets, inflations and other rubber parts, and milkstone deposits.

Coliform Count

The Coliform Count procedure selects for bacteria that are most commonly associated with manure or environmental contamination. Milk samples are plated on a selective bacterial media that encourages the growth of coliform bacteria, while preventing the growth of others. Though coliforms are often used as indicators of fecal contamination, there are strains that commonly exist in the environment. Coliforms may enter the milk supply as a consequence of milking soiled cows or dropping the milking claw into manure during milking. Generally, counts >50 would indicate poor milking hygiene or other sources of contamination. Higher coliform counts more often result from dirty equipment and in rare cases result from milking cows with environmental coliform mastitis.Quality Standards - SPC, LPC, PIC and Coliform Count

Suggested standards used for quality premiums or for trouble-shooting purposes are listed in table 1. The tests included in a quality program, as well as the limits used, will vary depending on the philosophy and requirements of the processor or cooperative. Generally, standards used to determine premium eligibility are based on values established for well managed farms. Although some of the test methods are not considered “official,” it is important that they be carried out in a professional manner using standardized procedures. It is not recommended that these tests be used to penalize dairy farms.

UTILIZATION OF PROCESSES TO MANUFACTURE PRODUCTS FROM MILK

Figure 1.8 illustrates the processes that are involved in the manufacture of the large range of products that can be produced from the very versatile raw material, whole milk. The products fall into two broad categories: those for immediate availability to the consumer (consumer products) and those that are ingredients that will subsequently be utilized to produce consumer dairy products or other foods.

Fermented Milk Products

There are two groups in this family: (a) cheese products in which part of the original liquid is removed during manufacture as whey and (b) products in which there is no whey drainage, such as yogurts. All or some of the standard food preservation tools of moisture removal, acid development, salt addition, and temperature adjustment may be used.

Cheese manufacture is a highly complex process. The composition of the initial milk is adjusted or standardized (fractionation) by centrifugal separation and possibly also ultra filtration. For most cheese types, the milk will then be pasteurized (72"C/15s) to reduce the risk from pathogenic organisms, adjusted to the desired fermentation temperature, and then pumped into a cheese vat. Starter culture consisting of a carefully selected species of lactic acid bacteria and a coagulant

(e.g., calf rennet) are then added and the milk is allowed to coagulate. This is by destabilization of the casein micelle. This permits the beginning of the fractionation and selective concentration processes that form the basis of cheese-making, Once the coagulum is of sufficien tstrength, it is cut into small particles and, by a process of controlled heating and fermentation, syneresis or expulsion of moisture and minerals (whey) occurs. Separation of the curd from the whey over a screen (filtration) follows. Depending on the cheese type, the curd may be allowed to fuse together (e.g., Cheddar) or may be kept in granules (e.g., Colby). Salt may then be incorporated into the curd for preservation, as dry granules or by immersion in brine. The curd is pressed into blocks by either gravity or mechanical compression, and the cheese then goes into controlled storage conditions for final fermentation and maturation.

For fermented milk products, the process of manufacture is somewhat simpler. For example, in yogurt manufacture, the milk to be used is fortified with additional protein (skim milk powder or concentrated milk), severely heated (e.g.? 95"C/5 min) to reduce the microbial load and to encourage whey protein/casein interaction, cooled to fermentation temperature (e.g., 36"C), and transferred to a fermentation vessel. Selected cultures are then added, and fermentation is continued until the desired pH of around 4.5 is reached. This causes the coagulation of the vessel contents, and the plain yogurt is then cooled prior to possible incorporation of fruit and flavoring, followed by packaging.

STARTER CULTURE

The major functions of microbial starter cultures in food and dairy products may be summarized as follows: To biopreserve the product due to a fermentation that results in an extended shelf life and enhanced safety; the production of bacteriocins may also have potential uses as food preservatives.

* To enhance the perceived sensory properties of the product due, for example, to the production of organic acids, carbonyl compounds and partial hydrolysis of the proteins and/or fats.

To improve the rheological properties (i.e., viscosity and firmness) of the product, and in some instances encourage gas production (i.e., eye formation in cheese) or color (white and blue mold or red smear). - To contribute dietetidfunctional properties to food, such as occurs with the use of probiotic microfloras. Table 7.1(277 OR 262) illustrates a possible scheme of classification of fermented milks and cheeses. Such fermentation processes are the result of the presence of microorganisms (bacteria, molds, yeasts, or combinations of these) and their enzymes in milk. In the dairy industry, these organisms are known as starter cultures; however, in some cheese varieties other microfloras could be present, and these are referred to as secondary cultures.

The essential roles of starter cultures are summarized as follows:

First, the production of lactic acid as a result of lactose fermentation; the lactic acid helps to form the gel and imparts a distinctive and fresh, acidic flavor during the manufacture of fermented milks; however, in cheesemaking, lactic acid is important during the coagulation and texturizing of the curd. Second, the production of volatile compounds (e.g., diacetyl and acetaldehyde) that contribute toward the flavor of these dairy products. Third, the starter cultures may possess a proteolytic or lipolytic activity that may be desirable, especially during the maturation of some types of cheese. Fourth, other compounds may be produced- for example, alcohol, which is essential during the manufacture of Kefir and Koumiss. Fifth, the acidic condition in these dairy products, and in some instances the production of bacteriocins, prevents the growth of pathogens, as well as many spoilage organisms

Fermented food products of milk

Product	Alternative names	Typical milk fat content	Typical shelf life at 4°C	Fermentation agent	Description
Cheese		1-75%	varies	a variety of bacteria and/or mold	Any number of solid fermented milk products.
Crème fraîche	creme fraiche	30-40%	10 days	naturally occurring lactic acid bacteria in cream	Mesophilic fermented cream, originally from France; higher-fat variant of sour cream
Cultured sour cream	sour cream	14–18%	4 weeks	Lactococcus lactis subsp. lactis*	Mesophilic fermented pasteurized cream with an acidity of at least 0.5%. Rennet extract may be added to make a thicker product. Lower fat variant of crème fraîche
Filmjölk	fil	0.1-4.5%	10–14 days	Lactococcus lactis* and Leuconostoc	Mesophilic fermented milk, originally from Scandinavia
Yogurt	yoghurt, yogourt, yoghourt	0.5–4%	35–40 days	Lactobacillus bulgaricus and Streptococcus thermophilus	Thermophilic fermented milk, cultured with Lactobacillus bulgaricus and Streptococcus thermophilus
Kefir	kephir, kewra, talai, mudu kekiya, milkkefir, búlgaros	0-4%	10–14 days	Kefir grains, a mixture of bacteria and yeasts	A fermented beverage, originally from the Caucasusregion, made with kefir grains; can be made with any sugary liquid, such as milk from mammals, soy milk, or fruit juices
Kumis	koumiss, kumiss, kymys, kymyz, airag, chigee	4%?	10–14 days	Lactobacilli and yeasts	A carbonated fermented milk beverage traditionally made from horse milk
Viili	filbunke	0.1-3.5%	14 days	Lactococcus lactis subsp. cremoris, Lactococcus lactis* biovar. diacetylactis, Leuconostoc mesenteroides subsp. cremoris and Geotrichum candidum	Mesophilic fermented milk that may or may not contain fungus on the surface; originally from Sweden; a Finnish specialty
Cultured buttermilk		1–2%	10 days	Lactococcus lactis* (Lactococcus lactis subsp. lactis*, Lactococcus lactis subsp. cremoris, Lactococcus lactis biovar. diacetylactis and Leuconostoc mesenteroides subsp. cremoris)	Mesophilic fermented pasteurized milk
Acidophilus milk	acidophilus cultured milk	0.5-2%	2 weeks	Lactobacillus acidophilus	Thermophilic fermented milk, often lowfat (2%, 1.5%) or nonfat (0.5%), cultured with Lactobacillus acidophilus

Streptococcus lactis has been renamed to Lactococcus lactis subsp. Lactis

CHEESE PRODUCTION

INTRODUCTION

Cheese manufacture is one of the classical examples of food preservation, dating from 6000- 7000 BC. Preservation of the most important constituents of milk (i.e. fat and protein) as cheese exploits two of the classical principles of food preservation, i.e. lactic acid fermentation and reduction of water activity through removal of water and addition of salt (NaCl). The establishment of a low redox potential as a result of bacterial growth contributes to the storage stability of cheese.

CHEESE VARIETIES

There are more than 400 varieties of cheese produced throughout the world, created by differences in milk source (geographic district or mammalian species), fermentation and ripening conditions as well as pressing, size and shape. Most of the cheese types that are produced today originated many centuries ago within smaller communities and are thus named, for example, Camembert and Brie from France, Gouda and Edam from the Netherlands, Cheddar and Cheshire from England, Emmentaler and Gruyère from Switzerland, Parmesan and Gorgonzola from Italy, and Colby from the USA; others are named for some aspect of their manufacture, e.g. Feta from Greece, processed cheese (best known as the cheese slices that go on hamburgers) from the USA, and Mozzarella from Italy; other names are more generic, e.g. cottage cheese from the USA

THE PRINCIPLES OF CHEESE MAKING

The basic principles of cheese making are the same for nearly all varieties of cheese. The manufacture involves the removal of water from milk with a consequent six- to tenfold concentration of the protein, fat, minerals and vitamins by the formation of a protein coagulum that then shrinks to expel "whey". The processes involved are: acidification, coagulation, cooking, salting, dehydration or syneresis, moulding (or shaping) and pressing, packaging and maturation or storage.

CHEESE MANUFACTURE

The general manufacturing protocol for most cheese varieties is outlined in Figure 1.

The manufacture of most varieties of cheese involves the following.

1 Pasteurisation of the milk kills nearly all the microorganisms present, including the harmful pathogenic bacteria that cause diseases, such as tuberculosis and leptospirosis, and other undesirable microorganisms, such yeasts and coliforms, that may alter the cheese characteristics by producing carbon dioxide and undesirable proteolysis.

2 Acidification of the milk is important for the proper release of whey from the cheese curd and to control the growth of many undesirable bacteria. It is usually accomplished by the addition of lactic acid bacteria that convert lactose to lactic acid. Most varieties of cheese cannot be made without the addition of a "starter" which is a culture of carefully selected lactic acid-producing bacteria. The special starter cultures are identified and distributed in deep frozen form by the New Zealand Dairy Research Institute to the different cheese plants. The large volumes of starter required for cheesemaking are made in special bulk starter fermentation pots in which the milk is heat treated to destroy unwanted bacteria, spores and phages and cooled to about 22°C, a temperature suitable for starter growth. The frozen starter is mixed in and fermentation continues for about 6 to 16 hours. (This process is similar to that used for yoghurt manufacture.) The amount of starter required varies for the different cheese varieties but, for Cheddar, this is normally between 1.25 and 2.0% of the cheese milk. The amount of lactic acid produced and the moisture in the finished cheese regulate and control the subsequent rate of the biochemical changes that take place during the ripening or maturation of the cheese.

3. Coagulation of the casein fraction of the milk to form a gel can be achieved by lowering the milk pH and the addition of "rennet", a mixture containing a specific proteolytic enzyme. The most commonly used rennet contains the enzyme chymosin, either as an extract of calf abomasum or as the recombinant product. Other types of rennet are derived from other animal sources, microorganisms or plants.

The four main groups of caseins in milk are the αs1-, αs2-, β- and κ-caseins. These phosphoproteins are held together by microclusters of calcium and phosphate and exist in milk as micelles of about 100 nm in diameter containing hundreds of molecules of each type of casein.

The more hydrophobic regions of these phosphoproteins are believed to be located inside the micelle with the more hydrophilic regions of κ-casein on the outside. The negatively charged carboxy-terminal of the κ-casein molecules is thought to protrude ’hair-like’ from the micelle and repel other casein micelles (charge stabilisation). In addition to this, the hair-like macropeptide portions of κ-casein are unable to interpenetrate (steric stabilisation). These two mechanisms are thought to enable the micelles to stay in solution as colloidal particles.

The addition of rennet (includes any of a range of acid proteinases) leads to the partial proteolysis of κ-casein by cleavage at the Phe105-Met106 bond. The release of the hydrophilic carboxy-terminal peptide (glycomacropeptide) results in destabilisation of the micelles which become less negatively charged and more hydrophobic. These micelles then aggregate (in the presence of calcium and at a temperature above 15_C) to form a coagulum.

A rennet coagulum consists of a continuous matrix of strands of casein micelles, which incorporate fat globules, water, minerals and lactose and in which microorganisms are entrapped (Figure 2).

4. Syneresis, or shrinking, of the coagulum is largely the result of continuing rennet action. It causes loss of whey, and is accelerated by cutting, stirring, cooking, salting or pressing the curd, as well as the increasing amount of acid produced by the starter, and gradually increases during cheesemaking. As a result, the cheese curd contracts and moisture is continuously expelled during the cooking stages.

5. Salt is added to cheese as a preservative and because it affects the texture and flavor of the final cheese by controlling microbial growth and enzyme activity. The salt can be added either directly to the curd after the whey is run off and before moulding or pressing into shape, or by immersing the shaped cheese block in salt brine for several days following manufacture. Addition of salt to the cut curd draws more whey from the cheese curd and some of the salt diffuses into the curd. The pH of the curd, the contact time and the salt particle size and structure are all important in determining how much salt is absorbed by the curd. Salt is also involved in physical changes in cheese protein solubility and conformation, which influence cheese rheology and texture. Another important function of salt in cheese is as a flavour or a flavour enhancer.

6. Curd manipulations:

(i) Heat treatments. The application of heat to cheese curd at any of several different times during the manufacture of particular cheese varieties, such as Cheddar, Mozzarella or Emmentaler, is to selectively stop the growth of certain types of bacteria and consequently influence the maturation pathway of the cheeses. It also alters the composition and texture of the cheese by increasing the syneresis without increasing the acidity.

(ii) Stretching the curd is an important operation for several kinds of cheese, in particular the pasta filata style, Mozzarella being the best known. Traditionally the curd was immersed in hot (about 80_C) water, and the fluid mass of cheese was pulled into strands to align the protein fibres and then poured into a container to cool. It was then immersed in brine. Large scale production means that special machines (Figure 3) are used for stretching.

Figure 2 Diagram showing the action of rennet on the casein micelle. The enzyme in rennet cleaves the κ-casein releasing a large peptide. The surface of the micelle changes from being hydrophilic and negatively charged to hydrophobic and neutral. As a consequence the micelles aggregate to trap fat globules and microorganisms in the developing curd

(iii) Cheddaring is a mild form of stretching in which the cheese curd is piled up and held warm so that it flows under the force of gravity. It is periodically turned to flow again. The pH of the curd falls during this process and whey continues to exude. Again, in large scale manufacture, this is done in large machines.

(iv) Washing the curd either in the cheese vat or after dewheying helps remove more lactose which changes the pH of the cheese. It also reduces syneresis and is important in the manufacture of cheeses such as Colby, Gouda and Egmont.

7. Moulding. The formation of the final cheese shape into spheres, flattened spheres, discs, cylinders or rectangular blocks is traditional but for some varieties, e.g. Camembert, it affects the maturation pathway. Some cheeses are pressed in moulds (nowadays made of plastic or stainless steel) under the whey for a short time whereas others are compressed at high pressures for several hours.

8. Maturation or ripening. The ripening of cheese involves three major biochemical events.

(i) Glycolysis: Lactose is metabolised to lactic acid, which may then be catabolised (broken down into smaller molecules) to form acetic and propionic acids, carbon dioxide, esters and alcohol by the enzymes of the microorganisms in the milk, including the added starter.

(ii) Lipolysis: The lipids are broken down to form free fatty acids, that may then be catabolised to form ketones, lactones and esters by natural milk enzymes and those that are added to create the flavor in particular cheese varieties, e.g. Romano, Blue Vein and Feta cheese.

(iii) Proteolysis: Proteins (caseins) are gradually broken down to form peptides and amino acids by the enzymes of the coagulant, the natural milk enzymes and the enzymes of the starter bacteria and other added microorganisms, e.g. moulds such as Penicillium camemberti used in the manufacture of Camembert and Penicillium roqueforti used in the manufacture of blue-veined cheeses such as Roquefort and Stilton. The enzymes of these mould species typically result in a high level of proteolysis in these cheese types. The amount of acid present has a marked effect on the level of proteolysis seen in the resultant cheese. The activity of the coagulant enzyme, the amount of enzyme

remaining in the curd and, as a consequence, the amount of proteolysis is dependent on the amount of acid produced in the initial stages of cheesemaking. The pH also controls the level of moisture, which in turn affects proteolysis in the cheese. The final pH of the curd and the rate of pH decline determine the extent of dissolution of colloidal calcium phosphate from the curd. This modifies the susceptibility of the caseins to proteolysis during manufacture and influences the rheological properties (such as texture) of the cheese. The breakdown of the proteins to peptides (proteolysis) transforms the rubbery and flavourless cheese curd into a cheese that has a desirable texture and flavour. Further proteolysis produces amino acids and the further biochemical glycolysis and hydrolysis result in the formation of amines, aldehydes, alcohols and sulphur compounds that add to the flavour of the cheese.

9. Packaging - Many cheeses are made and matured in large blocks (e.g. 20 kg) and they are exported as such. When they are to be sold in supermarkets, they are usually cut into appropriate size blocks and either shrink wrapped in an atmosphere of carbon dioxide, which dissolves into the body of the cheese, or vacuum sealed in a special "top-and-bottom" "webbed" package. The subsequent anaerobic environment prevents mould growth on the cheese surface. Many cheeses, such as the Brie and Camembert, are ready for sale at maturation and are packaged in special aerating wrapping and in porous boxes.

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