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In early times, people kept animals and cultivated vegetables to provide for their own needs. Animals were utilized not only for heavy work, but also as a source of food; cows were used for the production of milk and meat.
Families in these early times were almost completely self-sufficient. However, during industrialization and profession specialization, farmers became suppliers for consumers, and the process began whereby farms grew in size, scaling up all the time. Less farms with more animals is a trend that continues today.
The distance between the farm, the dairy and the consumer became greater, as did the time lapse between milking and the drinking of milk. Milk storage on the farm, and the time taken to bridge the gap between producer and consumer gave bacteria the chance to acclimatize and grow in this nutritious liquid. It became a problem to keep milk quality at the same level as just after milking. If you lower the temperature of stored milk, chemical processes and microbiological growth will slow down, delaying the reduction in quality. This knowledge enabled farmers, transporters, and dairy organizations to provide milk to consumers after a time delay, without an unacceptable impact on quality. Cooling is a very good method to keep the quality of milk at a high level.
Refrigerating milk on the farm has two main aims:
- to inhibit bacterial spoilage
- to extend storage on the farm so as to decrease milk transport costs.
Full hygiene in all aspects of milk production is essential in the production of quality milk. A critical aspect is to ensure that the growth of bacteria during the storage interval must also be curtailed. At body temperature, bacteria in milk will multiply very quickly and even milk with a low initial bacteria count will sour rapidly.
Milk produced under hygienic conditions will retain good quality for a period of up to 15 to 20 hours. However, it is not only the storage temperature that is important; the cooling time to reach storage temperature, normally 4 °C, is also critical. Bulk milk coolers have been specially designed to cool the milk to 4 °C within a specified time period.
One general definition of quality could be: "the consumer gets what he or she expects". Quality is extremely important, and milk producers are increasingly being expected to show that everything has been done to meet quality standards. If the producer succeeds in doing so, the consumer will have faith in the quality of the product, creating all-round benefits.
The quality of milk involves many different aspects. In this chapter we will discuss the main influences on the quality of raw milk:
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- physical hygiene
- chemical hygiene
- microbiological hygiene.
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Hygiene influences on milk quality.
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Density, freezing point, osmotic pressure and acidity are examples of physical hygiene. The density of normal milk varies between 1.028 and 1.038 g/cm3 depending on the milk composition. The freezing point of milk is the only reliable parameter to check milk for dilution with water. Between individual cows, the freezing point has been found to vary from -0.54 to -0.59 °C. The acidity of a solution depends on the concentration of hydronium ions [H+] in it. When the concentrations of hydronium [H+] and hydroxyl [OH-] ions are equal, the solution is neutral (pH = 7).
The different components of milk, especially fat and protein, may undergo chemical changes during storage. These changes are normally of two kinds, oxidation and lipolysis. The products of these reactions can cause off-flavouring in milk and butter.
OXIDATION. The oxidation of fat gives milk a metallic flavour, whilst it gives butter an oily, tallowy taste. The presence of iron and copper salts accelerates the start of auto-oxidation and the development of metallic flavor, which is also caused by the presence of dissolved oxygen and exposure to light, especially direct sunlight or light from fluorescent tubes.
When exposed to light, the amino acid methionine is degraded to methional. This is the principal contributor to the sour ‘sunlight flavour’. Since methionine does not exist separately in milk, but is one of the components of milk proteins, fragmentation of the proteins must occur incidentally for the development of the sour flavor.
To avoid the oxidation of fat and protein in milk, the most important issue is to control contact with oxygen and direct sunlight. When the milk is awaiting for transport, it must be protected from direct sun light.
LIPOLYSIS. The break down of fat into glycerol and free fatty acids is called lipolysis. Lipolysed fat has a rancid taste and smell. High storage temperatures encourage lipolysis, but the responsible lipase cannot act unless the fat globules have been damaged. In normal farming and dairying routines there are many opportunities for fat globules to be damaged, for example by pumping, stirring and splashing the milk. In addition, sharp edges and curves in milk tubes can damage the fat globules. These details must not be overlooked when installing a milking system.
Food poisoning and food infections can be the result of poor microbiological milk hygiene. These dangerous microbiological aspects can be reduced by milk cooling and it is important to study them.
‘Micro-organisms’ is the collective term for ‘all small living organisms which are not visible to the eye and occupy an intermediate position between the vegetable and animal kingdoms’. They are found everywhere; in the atmosphere, in the water and in the soil. Since they break down organic material, micro-organisms play a very important role in the natural cycle.
There are thousands of micro-organic species which are important to the existence and economic structure of human society. For example, during the breakdown of dead organic material certain species form simple chemical elements that plants can then re-use. Micro-organisms increase soil fertility and crop production, which result in more food harvested. Certain species are present in animal intestines and are essential for food digestion.
Micro-organisms play a very important role in nature (Tetra Pak 1995).
Some micro-organisms are used in food processes, for example, cheese, yoghurt, pickles, beer and wine production, as well as in acid production for food preservation.
Other micro-organisms produce toxic substances that kill other organisms. One example is the mould penicillum, which produces the substance penicillin. Other micro-organisms cause diseases in animals and plants, reducing a nation's food supply, whereas others cause food deterioration such as mould, discolouring, etc.
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Bacteria are single-celled organisms that multiply mostly by binary fission, i.e. by splitting into two. The simplest method of classifying bacteria is according to their appearance, yet to be able to see bacteria they must first be stained, then studied under the microscope at a magnification of about 1 000. The most widely used method of staining bacteria is called Gram dyeing, and bacteria are divided into two main groups according to their Gram stain characteristics: (i) red gram negative, and (ii) blue gram positive.
In the word morphology, ‘morph’ stands for form and ‘ology’ for the study of. Morphology of bacteria therefore means the study of the form of bacteria. Morphological features include:
- Shape
- Size
- Cell structure
- Mobility, i.e. the ability to move in a liquid spore and capsule formation
SHAPE OF BACTERIA. Bacteria shapes can be divided into three categories: spherical, rod-shaped and spirals. The relative position of bacteria to each other is another important distinguishing characteristic. Figure 3.4 shows how spherical bacteria (cocci) occur in different formations. Diplococci arrange themselves in pairs; Staphylococci form clusters (Greek ‘staphylon’ = ‘bunch of grapes’); while streptococci form chains (Greek ‘streptos’ = ‘chain’).
Spherical bacteria occur in different formations (adapted from Tetra Pak).
The figure below shows rod and spiral-shaped bacteria respectively. The rod bacteria (bacilli) vary in both length and thickness, and they also form chains. Spiral bacteria (spirillum) are also of varying lengths and thickness, and have different numbers of turns.
Rod and spiral shaped bacteria (adapted from Tetra Pak).
SIZE OF BACTERIA. Cocci vary in size between 0.4 and 1.5 micrometres (1 micrometre = 0.001 mm). The length of bacilli can vary between 2 and 10 micrometres, although some species are larger and some are smaller.
CELL STRUCTURE OF BACTERIA. Like all other cells, bacteria contain a semi-liquid, proteinous substance called cytoplasm. Cytoplasm also contains starch, fat and enzymes that are involved in the metabolism of the cell. Each cell has nuclear material (DNA), the genetic information that controls the cell’s life and reproduction. In the cells of higher animals and botanical species, the nucleus, contrary to the basic substance of the cell, also contains the substance protoplasm.
Schematic view of bacterial cell.
The above figure shows a schematic view of the structure of a bacterium. The nuclear material is suspended freely in the basic substance of the bacteria cell (cytoplasm). The cytoplasm is surrounded by a cytoplasmic membrane that performs many vital functions, including regulation of the exchange of salts, nutrients and metabolic products between the cell and its environment. The cytoplasmic membrane is in turn enclosed in a further envelope, the actual wall of the cell. This serves as the ‘skeleton’ of the bacterium, giving it a definite shape. Some bacteria have the ability to form a protecting capsule (see Figure III.10).
| MOBILITY OF BACTERIA. Some cocci and many bacilli are capable of moving in a liquid nutrient medium. They propel themselves with the help of flagella, which are similar to long hairs growing out of the cytoplasmic membrane (see Figure III.8). |
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Rod- and spiral-shaped bacteria. |
The length and number of the flagella vary from one type of bacteria to another. Bacteria generally move at speeds of between 1 and 10 times their own length per second, with the cholera bacterium, as one of the fastest, is able to travel 30 times its length per second.
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The spore is a form of protection against adverse conditions, including:
- Heat and cold
- Lack of moisture
- Presence of disinfectants
- Lack of nutrients.
There are various types of endospore formation in bacteria
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Spore and capsule formation (adapted from Tetra Pak 1995).
Only a few types of genera of bacteria form spores. Of these, bacillus and clostridium are the best known. Under adverse conditions, these organisms gather nuclear material and some food reserves in one area of the cell. During spore formation, the vegetative part of the bacteria cell dies. The spore then germinates back into a vegetative cell and, if conditions become favourable again, starts reproduction.
The cell eventually dissolves and the spore is released. Spores have no metabolism. They can survive for years in dry air, and they are more resistant than bacteria to chemical sterilants, antibiotics, drying and ultraviolet light. They are also resistant to heat. For example, it takes 20 minutes at 120 °C to kill them with 100 % certainly. However, spore-forming bacteria in the vegetative state, like all other bacteria, are killed in a few minutes by boiling them at 100 °
Growing conditions for bacteria.
Temperature is the greatest single factor affecting bacteria growth, reproduction and food deterioration. Bacteria can only develop within certain temperature limits, and these limits vary from one species to another.
Temperature conditions and classification of bacteria by temperature preference.
There are enormous differences between the various species of bacteria. Some species grow at temperatures close to freezing point, in exceptional cases even a few degrees Celsius below, whereas others need considerably higher temperatures.
In general, growth of bacteria in milk and milk products is considerably reduced by cooling to below 10 °C , while temperatures as low as 4 or 3 °C are required to almost completely stop almost all activity. Storage of milk at low temperatures will, however, not destroy bacteria. Freezing may lead to a slow destruction of the product as ice crystals rupture cell walls.
Maximum temperature is the temperature above which bacteria will cease to develop, while optimum temperature is the temperature at which bacteria develop best. If the temperature is increased above the maximum, bacteria are quickly killed by heat. It takes much more heat to kill bacterial spores.
Bacteria are classified into the following temperature categories:
| Category |
Minimum °C |
Optimum °C |
Maximum °C |
| Psychrophilic |
-10 |
-5 |
25 |
| Psychrotrophic |
0 |
20 |
40 |
| Mesophilic |
10 |
30 |
45 |
| Thermotrophic |
25 |
45 |
75 |
| Thermophilic |
30 |
50 |
80 |
PSYCHROPHILIC are the cold-loving bacteria. They are frequently found in raw milk and usually originate from contaminated water. For this reason they are sometimes called water bacteria. In many cases, the adulteration of milk with water actually means an inoculation of the milk with this kind of bacteria.
PSYCHROTROPHIC are cold-tolerant bacteria are also found in dust from barns, feed and from other sources. If unpasturized milk is stored for long periods on the farm or at the milk plant, psychrotrophics may well spoil it. The majority of psychrotrophic bacteria are actually mesophilic, having an optimum temperature in the same range as normal mesophilic bacteria (see below).
MESOPHILIC bacteria differ from psychotrophic bacteria by being able to grow at very low temperatures. Under normal conditions they are destroyed by pasteurization, but may be found in pasteurized milk as a result of recontamination.
THERMOPHILIC. bacteria from soil, hay or other dry and dusty feeds may contaminate raw milk on the farm. Milk solids that accumulate in improperly sanitized milking utensils are also a common source of contamination. Enormous populations of thermophilic bacteria may build up in dairy plants if milk is kept at high temperatures over long periods, or in dairy equipment that is used continuously for extended periods and not sanitized properly.
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Light is not essential to bacteria because they do not contain chlorophyll and synthesise food in the same way as plants do. Instead, light tends to kill bacteria as it contains ultraviolet light, a chemical activating ray that causes changes in the cell protein. In nature, the bacteria-killing effect of sunlight plays an important role, especially concerning bacteria-filled dust in the air. It is the primary reason why sunny streets and light rooms are much poorer in bacteria than dark and stuffy places.
A suitable acidity level is very important for the proper development of microorganisms. In milk it is the pH that is decisive and not the titratable acidity. At the normal pH of milk, many micro-organisms are able to develop, but some, like mould and yeast, prefer a more acidic environment. Others, like most of the protein-fermenting bacteria, stop reproduction at increased acidity. The acid produced by lactic acid bacteria will prevent the development of certain putrefying bacteria and actually preserve milk, although it becomes sour. Lactic acid bacteria themselves can also only tolerate a certain acidity, although not all types are equally sensitive. This means that during the process of acidification of milk, various species of lactic acid bacteria may succeed each other. Normally, acid production stops in milk at pH 4.2.
While all higher level organisms require free oxygen (O≈) to live, this is not always the case for micro-organisms. Moulds require oxygen because of their way of reproduction, and the same goes for many types of yeast and bacteria. However, other yeasts and bacteria, however, are not dependent on the presence of free oxygen, and some do not tolerate oxygen at all.
Micro-organisms can be classified into groups according to their oxygen requirements:
AEROBIC. Most yeasts, all moulds and a large number of bacteria belong here. These require free molecular oxygen for their development.
ANAEROBIC. Includes most of those bacteria that flourish in the absence of oxygen.
FACULTATIVE AEROBIC/ANAEROBIC. These organisms can grow in aerobic as well as anaerobic conditions, although often exhibit a preference for one or the other. A typical example of this group are the ordinary lactic acid bacteria, which develop more quickly at the bottom of a can or bottle than at the top. As a result, the milk at the bottom of containers starts to acidify first. Sometimes, the top layer of the milk seems sufficiently ‘fresh’ whilst the milk at the bottom is already sour.
MICRO-AEROPHILIC. These only grow in areas with a low oxygen concentration.
Water is the major component of the bacteria cells, and considerable quantities are required for the production of new cells. Dried products, such as milk powder, are protected from bacteriological deterioration because they lack water. The drying process itself does not destroy all micro-organisms. Many survive long storage periods in dry products. Immediately after drying, the bacterial count of milk powder decreases only slowly, and it may take years before the product becomes more or less sterile. High storage temperatures will help promote the destruction of the bacteria. In addition to the water content of the product, the osmotic pressure of the water is important.
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Nutrients are required for the development of micro-organisms because they supply the ‘building materials’ for new cells. Furthermore, the breakdown of complex compounds into simpler compounds delivers the energy required for the cells to function. The breaking down of compounds in combination with the production of other compound is named fermentation.
Milk is rich in nutrients and is as such an excellent nutrient for many micro-organisms. However, since the requirements of the various organisms vary, not all micro-organisms find all the nutrients they need in milk, and so not all are able to grow.
Bacteria normally reproduce asexually by fission. First, the size of the cell increases. The clear material then gathers in one area of the cell and divides into two identical parts. The parts that move away from each other result in two organisms that may break away or remain together, in turn resulting in different but characteristic arrangements.
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| Time(min) |
Bacteria (#) |
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0
20
40
60
80
100
120
180
240
300
360
420
460 |
1
2
4
8
16
32
64
512
4096
32768
262144
2000000
16000000 | |
Development of bacteria with a generation time of 20 minutes.
The concept ‘generation time’ was introduced to indicate the rate of growth of micro-organisms. It is the time a certain species or strain requires to double in number during the exponential phase of the growth curve.
The picture below shows the growth curve of bacteria transferred to a substrate by inoculation. Development phase (a) is called the ‘lag phase’, and is the delay before the bacteria start to reproduce, as they must first acclimatize to the new environment. The lag phase may also be observed in a culture that has been dormant, for example, one that has been stored at a low temperature prior to inoculation. The length of this first phase varies according to how many of the bacteria were inhibited at the moment of inoculation. If viable, growing bacteria are used and there is no period of incubation; reproduction then begins at once.
After the lag phase, the bacteria begin to reproduce quickly for the first few hours. Development phase (b) is called the ‘log phase’, because reproduction proceeds logarithmically.
Growth curve of bacteria (adapted from Tetra Pak 1995).
During phase (b), toxic metabolic waste products accumulate in the culture. The rate of reproduction therefore eventually slows down, and as bacteria are constantly dying so a state of equilibrium is reached between the death of old cells and the formation of new ones. This next phase (c) is called the ‘stationary phase’. In the following phase (d), the formation of new cells ceases completely and the existing cells gradually die off. At the end of phase (d) the culture is extinct, hence the ‘mortality phase’.
The shape of the curve, i.e. the length of the various phases and the gradient of the curve in each phase, varies with temperature, food supply and other growth parameters.
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When milk is secreted in the udder it is virtually sterile. But before the milk leaves the udder, bacteria manage to enter through the teat channel and infect it. These bacteria are normally harmless and few in number, only a few tens or hundreds per ml. However, in cases of bacterial udder inflammation (mastitis), milk can be heavily contaminated with bacteria and may even be unfit for consumption, not to mention the suffering it causes the cow. There are always concentrations of bacteria in the teat channel, but most of them are flushed out at the beginning of milking. It is therefore advisable to collect the first bacteria rich jets of milk from diseased animals.
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Bacteria enter through the
teat channel. |
During udder inflammation the milk is heavily infected
by bacteria. |
In the course of handling on the farm, milk is liable to be infected by various micro-organisms, mainly bacteria. The degree of infection and composition of bacterial population depend on the cleanliness of the Cows' environment and those surfaces with which the milk comes into contact, for example, the pail/milking machine, strainer, transport churn or tank and agitator. Milk-covered surfaces are usually much greater sources of infection than the udder.
When cows are milked by hand, bacteria can get into the milk via the milker, the cow, the litter and/or the ambient air. The magnitude of the influx depends largely on the skill and the hygiene-consciousness of the milker. Certain dangers are eliminated in machine milking, but another one is added, namely the milking machine itself. A very large number of bacteria can enter the milk if the milking equipment is not cleaned properly.
Due to its very specific composition, milk is susceptible to contamination by a wide variety of bacteria. Farm milk may contain anything from a few thousand bacteria per ml, from a farm with good hygiene practices, to several million if the standard of cleaning, disinfection and cooling is poor. Daily cleaning and disinfection of all milking equipment is therefore the most decisive factor for the bacteriological quality of milk. For milk to be classed as top quality, the bacteria count (Colony Forming Units/CFU), should normally be less than 100 000 per ml. In some countries, 10 000 per ml can be reached easily.
Rapid cooling to below 4 °C greatly contributes to the quality of the milk on the farm. This treatment slows down the growth of the bacteria in the milk, thereby greatly improving its keeping qualities. The influence of temperature on bacterial development in raw milk is shown in Figure 3.14 . Starting from 300 000 CFU/ml, we can see the speed of development at higher temperatures and the effect of cooling to 4 °C.
Bacterial development in raw milk (adapted from Tetra Pak 1995).
Cooling to 4 °C, or even 2 °C, in conjunction with milking makes it possible to deliver milk at two- or three-day intervals, provided that the milk container/tank is well insulated.
In situations of non-hygienic farming and infection, the initial bacteria count rises sharply and bacterial reproduction starts at an already high level. Combined with am optimum temperature, bacterial growth is enormous. To avoid development of bacteria it is important to keep the number of bacteria as small as possible, partly by directly cooling the milk to around 4 °C.
However, it is vital to recognize that cooling is a compliment, not a substitute, for hygienic working conditions. Avoiding infections through good hygiene practices, and cooling the milk as soon as possible after milking, combine to ensure high milk quality. Cooling is a good expedient, and with efficient cooling you can help win the battle against micro-organisms.
Bacteria development by different start colony count and two different temperatures.
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Many of the bacteria in milk are casual visitors. They can live, and possibly reproduce. Milk is, however, often an unsuitable growth medium for them. Some of these bacteria die when competing with species which find the environment more congenial. Groups of bacteria that occur in milk can be divided into:
- Lactic acid
- Butyric acid
- Putrefaction
- Coliform
- Propionic
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If you wish to find out more about the positive and negative aspects of bacteria, the dairy microbiological handbooks are good reference works (e.g. R. K. Robinson 1983).
Among mammals, milk is the last nutritional link between mother and offspring among mammals. Besides being a complete, well-balanced diet for the newborn, milk also contains anti-microbial agents that protect the suckling young from various infectious diseases.
| The knowledge that milk, and in particular colostrum (the first milk after parturition), contains immune factors essential for the survival of offspring is very old. Thousands of years ago, herdsmen recognised that newborn lambs, kids and calves must obtain the first milk (colostrum) if they want to survive. |
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Calves must obtain colostrum if they are to survive. |
Today, it is well documented that milk contains several antibacterial factors. The best known of these are the immunoglobulins, which can be found in high concentrations in colostrum and which provide an immediate immunisation of the newborn.
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Lacto-peroxide
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Xanathine-oxidase
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Lactoferrine
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Lysosym
Milk also contains non-specific factors as lysozyme, lactorferrin and peroxidase. This type of peroxidase, which is called lactoperoxidase, is identical to the peroxidase present in salvia and gastric juice.
Fungi are a group of micro-organisms that are frequently found in nature among plants, animals and human beings. Different species of fungi vary a great deal in structure and method of reproduction. Fungi may be round, oval or threadlike. The threads may form a network, visible to the naked eye. Fungi are divided into yeasts and moulds.
YEASTS
| Yeasts are single-cell organisms of spherical or cylindrical shape and the size of yeast cells varies considerably. For example, brewer’s yeast, saccharomyces cerevisiae, has a diameter in the order of 2 – 8 mm, and a length of 3 – 15 mm. Yeast cells of certain other species may be as large as 100 mm. |
Structure of the yeast cell (adapted from Tetra Pak 1995)
Yeast cells normally reproduce by budding, though there are other methods. Budding is an asexual process. A small bud develops on the cell wall of the parent cell. The cytoplasm is shared for a while by parent and offspring, but eventually the bud is sealed off from the parent cell by a double wall. The new cell does not always separate from its parent, but may remain attached to it while the latter continues to form new buds. The offspring cell also form fresh buds of its own, which can result in large clusters of cells attached to each other. Some types of yeast reproduce by forming spores (these are quite different from bacterial spores).
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Yeast has the same need for nutrients as other living organisms, such as bacteria. |
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As for bacteria, although yeast needs less water; some can grow with very little water. |
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Yeast can grow in a PH value range of between 3 and 7 (optimum is between 4.5 and 5). |
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The optimum temperature is normally between 20 and 30º C. |
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Yeast can grow both with and without the presence of atmospheric oxygen. Yeast cells are facultatively anaerobic, which means that in the present of oxygen they grow better. |
Conditions for the growth of yeast.
Yeasts are usually undesirable in dairy products because they often ruin them. However, Russian ‘Kefir’ and Finnish ‘Viile’ are examples from a small product group where yeasts are necessary to give the correct quality. In the brewing, wine, baking and distilling industries, yeast organisms are valuable coworkers.
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MOULDS
| Moulds belong to quite different groups of fungi. They consist of thread-like strands of cells called mycelium.
The mould fungi has a many-branched body called the mycelium, which may be microscopically small, or large enough to be seen with the naked eye. |
Penicillum with conidiophores producing chains of conidia (adapted from Tetra Pak 1995). |
| The mycelium consists of individual threads called hyphae. These hyphae constitute the vegetative part of the fungus. The part responsible reproduction consists of hyphae that often grow straight up and carry spores. |
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Moulds can grow on materials with a very low water content and can extract water from air. |
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Moulds can grow in a pH value range of between 3 and 8.5. |
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The optimum temperature is normally between 20 and 30ºC. |
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Moulds usually grow in aerobic conditions. |
Conditions for the growth of moulds.
There are many different families of moulds. Groups that are of importance in the dairy industry include penicillium and milk mould, geotrichum candidum.
Bacteriophages are viruses, i.e. bacterial parasites. By themselves they can survive, but they can only grow or replicate within bacterial cells. They have very specific hosts, e.g. single species of strains of bacteria. Bacteriophages, or phages, can only be seen by means of an electron microscope.
The micro-organisms used in the dairy industry are called ‘starter cultures’. A starter culture is a mixture of organisms. The quality of the starter culture is preserved until after arrival at the dairy by maintaining high standards of hygiene in all steps of the processing chain.
As milk is usually contaminated with bacteriophages, it is important that the milk used for starter cultures, usually skimmed milk, is heated to inactivate the phages. Fig 3.21 shows what happens if this is not done, or if the milk is recontaminated by phages at a later time.
Growth of starter bacteria and phages and influence on infected starter culture (adapted from Tetra Pak 1995).
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Structure pf bacteriophages (Tetra Pak 1995).
Phages only attack bacteria, usually young actively growing ones, within which they can reproduce. The bacteria subsequently disintegrate, releasing a crowd of 10 to 200 phages per bacterium that then attack new victims.
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The phage attaches to the surface of its host and the DNA is injected into the cell.
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The cellular machinery then produces new phage DNA and phage proteins.
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The new phages are assembled inside the bacterial cell, which is then lysed.
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The mature phages are released.
The great variety of bacteria, yeasts and moulds, and their widely varied activities, are of the utmost importance for life on earth in general, and humanity in particular. Micro-organisms in soil and water are responsible for degrading available sources of organic nourishment into forms that plants can assimilate. By doing so they also perform an indirect service to the animal kingdom.
Human beings also benefit more directly from micro organisms. Lactic-acid forming micro-organisms, for example, can be used to preserve fodder (silage for livestock). The same principle is applied to the preparation of certain foods such as sauerkraut, green olives and cucumbers.
Micro-organisms are of paramount importance in the manufacture of dairy products such as yoghurt, cheese and cultured butter. Choice of the right types of micro-organisms is an essential factor for maximising the quality of such products.
It should be mentioned here that milk may contain residues of antibiotics emanating from treatment of cows suffering from mastitis; the most commonly occurring being penicillin. This is in spite of regulations saying that milk from cows treated with antibiotics must not be sent to the dairy.
It would be a false idealisation of micro-organisms not to mention that some of them, the pathogenic micro-organisms, are regarded as mankind’s worst enemies. Although it is true that pathogens are far outnumbered by the harmless or useful ones, their effects are much more obvious.
Almost all over the world, governments have passed laws requiring pasteurization of milk that is produced at a dairy and intended for consumption. A typical temperature/time combination for pasteurisation is 72 °C /15 - 20 seconds, which kills all pathogens.
It is important is to know that cooling is a compliment and not a replacement for hygienic working practices, and that prevention is better than cure. Avoiding infections is the first priority.
Cooling is the weapon against growth, and with efficient cooling and good care the battle against micro-organisms can be won. Milk quality rises, as does the quality of all milk products. This leaves only one winner, human health. (For even more detailed coverage of the micro-organisms present in milk, see Tetra Pak 1995).
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