These are colostral milks or pathological milks which normally should not be collected or marketed. These milks have an abnormal mineral and protein composition (high chloride content - low casein content) which gives them a characteristic salty taste. Their presence in low concentrations in the collected milk does not change the organoleptic characteristics of the mixture and therefore cannot be detected by sensory analysis. The adulteration can only be detected by specific instrument methods which are chemically or physically based.
The main alterations in milk taste are due to bacteriological, chemical and physical changes in the milk constituents. These three aspects will be studied in this chapter.
2.2.1 Defects of microbial origin
Milk is a highly favourable culture medium for microbial life because of its chemical composition, which is rich in various nutrients, and because of its water content. Thus it is highly vulnerable to any contamination that can occur in production, processing and marketing.
Serious taste and odour defects can appear due to an accumulation of products resulting either from cell metabolism or from the effect of complex enzyme systems on the milk constituents. Many undesirable changes in organoleptic quality are possible when environmental conditions are conducive to microbial proliferation and enzyme activity. The following list of adverse changes is not restrictive. Most frequently one speaks of milk that is sour, bitter, fruity, rancid, malty, with an off-flavour tast, and also of dirty milk, etc. These forms of spoilage are associated with the growth of yeasts, moulds and bacteria. In view of its ecological characteristics, bacterial contamination is the most frequent and the greatest, and its potential development should be feared most of all. This contamination is responsible for two main types of defects: souring and lipolysis.
The defects due to acidification are the most frequently encountered since lactic flora is one of the main natural contaminants of milk, being predominantly mesophilic in character. Presence of this flora is demonstrated by a greater or lesser rapidity of acidification depending on temperature conditions at the production, processing and distribution stages. This form of spoilage is apparent in the sour taste, due not to lactic acid (which is barely volatile because of its low vapour pressure), but to the presence in smaller quantities of organic acids (acetic acid, propionic acid, carbon dioxide).
Furthermore, the heterofermentative character of lactic flora is demonstrated by a more or less marked release into the environment of numerous different volatile compounds (aldehydes, ketones and alcohols) besides the organic acids mentioned above. These compounds, when they develop in limited quantities, are sometimes sought after since they help to form the typical aroma of many milk products. On the other hand, when they are present in high concentrations they produce an unpleasant taste and odour. A classic example is diacetyl, which in a highly diluted state is responsible for a nutty taste and in a more concentrated state results in decided bitterness. Likewise, different tastes designated as malty, grape or caramel, have been attributed to the proliferation of Streptococcus lactis maltigenes.
Lactic bacteria generally are not heat resistant and most of them are destroyed by low temperature pasteurization. However, the survivors or recontaminating bacteria can be responsible for further souring if temperature conditions are favourable to their development. The predominant lactic flora in milk is mesophilic and the natural environmental conditions in warm and hot regions are often favourable to its proliferation. The use of cold treatment at different stages of production, processing and marketing makes it possible in practice to decrease considerably the dangers of spoilage due to uncontrolled proliferation of these lactic bacteria. However, it should be noted that in these conditions even with cold techniques the multiplication of microorganisms and enzyme changes are not wholly blocked and that irreversible changes in the organoleptic quality of the product can still occur.
The defects due to uncontrolled proteolysis have their origin in bacterial flora which contaminates milk on the farm and can induce proteolysis of varying intensity depending on the species concerned, time periods and temperatures. In practice, mesophiles in the case of chilled milk and psychrotrophic bacteria in the case of refrigerated milk can be the cause of milk protein solubility of varying intensity because of their extra-cellular proteases. The protein fractions released (peptides, aminoacids and amino-ammonia) can be responsible for the development of abnormal bitter, ammonia or putrid tastes. Moreover, although most proteases are heat-sensitive, some enzymes coming from psychrotrophic bacteria are thermostable and their harmful effects can appear even in standard sterilized products. Therefore milk products, although sterile, can spoil during storage due to residual enzymes that have not been rendered inactive.
Many findings have shown that susceptibility to lipolysis is not the same for all milks. Two types of factors determine the frequency and intensity of defects, viz. enzyme activity as such and the condition of the fat substratum.
The enzymes responsible for lipolysis have two different origins. Fresh cow's milk contains several lipolytic enzymes, membrane lipase and plasma lipase. Although their isolation and characteristics have not yet been totally elucidated, it is generally recognized that these enzymes have little effect on raw milk when milk is in the resting stage and the fat globules are intact. In milk products which have been subjected to heat treatment for pasteurization (72°C for 15 seconds) or sterilization (classic or UHT), their activity is believed to be nil because they have been rendered inactive by heat treatment. However, recent studies have revealed cases of enzyme reactivation causing defects.
The second origin of lipases is associated with unsatisfactory hygienic conditions of the milk on the farm. Very many bacteria species can produce lipases. Psychrotrophic bacteria are the greatest threat because of their ability to develop at low temperature and their high extra-cellular activity.
The most active and most frequently encountered microorganisms and bacteria species have a relatively short multiplication period (4 to 24 h) at commonly applied temperatures (0 – 49C); when the temperature rises the growth rate is multiplied by 5.8 for every 10°C rise.
Correlations between bacterial density and the appearance of defects have been established. In milk, when germ concentration exceeds 106/ml off-tastes can be detected. When the fat content is partly concentrated (cream, butter, cheese and powders) defects can emerge at lower thresholds, especially during prolonged storage.
Many extra-cellular lipases of psychrotrophic bacteria are also characterized by extremely high heat resistance; they are only partly inactivated by the heat treatment used in UHT pasteurization and sterilization, although the bacteria themselves may be destroyed. Organoleptic defects may therefore appear even in properly heat-treated microfree media.
As mentioned previously, the susceptibility of milk to lipolysis is dependent on lipolytic enzymes and also on fat condition.
Frequency and intensity of lipolysis increase when the initial globular state of the fat is modified. The influence of endogenous factors traceable to the cow (effect of feeding - effect of lactation stage) which determine the synthesis of the globular membrane and the stage of lactation has been demonstrated. In most cases, however, factors extraneous to the animal have a dominant role in the development of lipolysis. Excessive shaking, addition of air, repeated thermal shocks, and homogenization, all of which can occur at different stages of production and processing adversely affect the integrity of the fat globule, modify the interfaces between the fat and non-fat phase and lead to an increase of lipolysis. In the light of many recent studies it appears that the development of milk production systems - characterized by the expansion of collection areas, increased mechanization of milking, collection and transport - leads to longer processing times. These factors have overall led to a more marked degree of spoilage of the raw material of milk than in the past and to an increase in organoleptic defects in the finished product.
To benefit fully from cold treatment, this must only be applied in environments where few psychrotrophic bacteria are present. Quality improvement of milk at the production stage, where the aim is to inhibit contaminant germs by observing strict rules of hygiene, is therefore inseparable from the rational utilization of cold treatment.
2.2.2 Defects due to fat oxidation
Oxidized flavour is a result of the effect of oxygen on the milk phospho-lipids which causes the development of various flavours - cardboard, metallic, fish or oily.
Oxidation of polyunsaturated fatty acids of the phosphatides of the globular membrane induces the formation of hydrogen-peroxide. The results of the reaction are the formation of alcohols, acids, aldehydes and ketones producing the above-mentioned defects.
The presence of contaminant metal traces (iron and copper) and also energy radiation exposure (light or radioactive) catalyzes oxidative reaction.
In practice, in order to reduce these oxidative defects it is advisable to decrease metal contamination by using appropriate materials made of stainless steel or glass and handling the milk in recipients providing effective protection against sunlight or artificial light. The replacement of trapped air in the product and in the space at the top of the container by an inert atmosphere (CO2 and nitrogen) or by a vacuum is also highly recommended in the case of food products with a high fat content which must be stored for lengthy periods at ambient temperatures.
In the specific case of recombination of milk products the use of iron- and coppercontaminated water should be forbidden and the addition of air during mixing and transfer operations should be prevented. Low pasteurization can intensify the emergence of defects, but stronger heat treatment has the opposite effect; it causes the release of sulfydryl (-SH) groups originating from lactoserum proteins which have an anti-oxidant effect and prevent the formation of off-taste. Chemical antioxidants have the same positive effect; however, their addition is generally forbidden by law. In some countries ascorbic acid can be added to supplement the milk which in low concentration and in the presence of copper stimulates oxidation, but at higher concentration prevents the development of oxide flavours.
Homogenization provides effective protection against oxidation. Significant changes in the globule surface and in the composition and structure of membrane proteins are probably responsible for this improvement in flavour stability. In the specific case of milk products made with recombined milk, it is essential to obtain a stable emulsion of the fatty matter in order to prevent rapid spoilage. The observance of optimum homogenization conditions is of primary importance in this case.
It should also be noted that the defects resulting from oxidation of fatty acids can develop in non-fat products when they contain trace phospho-lipids.
In all cases low-temperature storage of products containing milk fat provides an additional guarantee against oxidation spoilage.
2.2.3 Defects due to heat treatment
Most milk constituents can be physically and chemically altered by heat treatment used for preservation. The extent of the resulting changes will increase with the duration and temperature of heat treatment, but it also depends on the specific sensitivity (which can vary) to heat of the respective milk components; these can be listed, in decreasing order of importance, as free enzymes, serum lactoproteins, linked enzymes, phospho- and caseinocalcium complexes, lactose and lipids.
Heat treatment completely inactivates enzymes and destroys the most heat-resistant microorganisms but causes major changes in product characteristics, which generally decrease acceptability. For example, alteration will be slight for heat-treated and pasteurized products, slightly intensified for UHT sterilized products, and more marked for products that have been subjected to the conventional lengthy autoclave sterilization.
As a rule inactivation of most enzymes is obtained through normal heat treatment and pasteurization (60–100°C) which give a better stability to heat-treated products, and eliminate in particular organoleptic defects due to the development of uncontrolled proteolysis and lipolysis in UHT-treated long-life products. However, the inactivation of particularly heat-resistant enzymes may not be complete. This is the case, for example, of lipases and of thermolabile phosphatases when protected by the fat phase and/or the protein phase during heating which are later reactivated during storage. Freak organoleptic properties due to this type of lipolysis and proteolysis have been reported quite frequently, especially for milk products derived from milks that are initially highly contaminated with psychrotrophic bacteria.
Many changes in heated milk characteristics are the result of direct spoilage of lactoserum proteins or interactions involving soluble milk proteins following heat treatment.
Direct modifications are the development of sulfhydryl (-SH) and hydrogen sulfide (H2S) groups from β - lactoglobulin, and also the formation of small quantities of free sulfides and mercaptans. These impart a burned taste, but it should be noted that these free groups also decrease the oxidation-reduction potential and give a better protection of lipids against oxidation.
Recombined milk products frequently exhibit this burned taste, which is usually considered unpleasant. Its intensity increases with the heat-induced denaturation that the milk undergoes during its transformation into powder, and also with the additional heat treatment to preserve it following recombination. The development of this taste is therefore in direct ratio with the solubility value of the powder selected and the time-temperature relation of the heat treatment applied to the recombined product to ensure its stability for the desired length of time.
Thus the taste of a pasteurized product prepared from a “low temperature” powder will be little changed. On the other hand, the change will be more marked for a milk sterilized by the autoclave process and obtained through recombination with a “high temperature” powder.
The main interaction between lactoserum proteins and casein is the formation of a complex between the β -lactoglobulin and Kappa-casein. This occurs when more than 50 percent of the lactoserum proteins are denatured. It has many consequences, but where organoleptic quality is concerned, it retards the release of the R-SH and H2S groups when additional heat treatment is applied.
Heating changes the salt balance toward insoluble forms. Changes in the composition of micella surfaces and phosphate precipitation decrease the stability of the colloidal phase but these consequences have little influence on organoleptic quality.
Lactose is generally not affected significantly by UHT pasteurization and sterilization, but the damage is greater when more drastic heat treatment is used. In this case a decomposition of the lactose occurs and acids form, in particular formic (50 to 75 percent of the newly formed acids), lactic, acetic, pyruvic, propionic and butyric acids; hydroxymethylfurfural and furfuraldehyde also develop. All these components help in forming the taste, odour and colour of milk but their respective roles have not been clearly determined.
The main change occurring from heating of milk products còntaining lactose corresponds to a non-enzymatic browning reaction or Maillard reaction. This reaction, which requires little activation energy, is self-catalyzing; in its initial phase condensation occurs between a free aminic group of the casein, especially the lysine group and the lactosereducing aldehyde functions. It leads to the formation of a Schiff base followed by the Amadori rearrangement and degradation into a brown pigment (melanoidins). The browning does not always appear during heat treatment since the Schiff bases so formed can become degraded slowly in dry, liquid or medium-moist milk products during storage. A caramel-like taste develops.
In practical terms, in order to minimize the browning reaction the amount of heat treatment must be limited and storage times and temperatures reduced. The use of certain additives like sodiumbisulfite, sulfur dioxide, and formaldehyde, or the presence of (-SH) groups in the environment can effectively help to prevent these defects.
Under the influence of intense heating the fat produces lactone and methylketone compounds which if highly concentrated can cause undesirable flavours (coconut flavour) in concentrated and dried milk. but at lower levels they are responsible for the characteristic and much sought-after flavour of food cooked in butter. In moderately heat-treated liquid milk products their contribution to the development of freak organoleptic effects appears to be less.
Because of its high water and liquid content, milk can be a major vector of fat-soluble and water-soluble foreign substances which cause off-tastes. These substances can come from the feed given to the cow, as well as from the environment before and after the milking process. They can be transmitted either indirectly in the full udder through the respiratory and/or digestive system, or directly by contact with the product after milking.
Transmission through the respiratory system is due mainly to volatile compounds contained in the atmosphere of badly ventilated premises (gases from feed or manure; gases eructed by healthy or diseased animals and the presence of foreign chemical substances, e.g. fossil fuels, disinfectants). It has thus been possible to prove the origin of various odours or tastes of cabbage, ensilage, cow, and farm. When the source of the flavour disappears and the animals once again breathe wholesome air the foreign substances accumulated in the udder are rediffused in the bloodstream and are expelled via the lungs.
The feed consumed by the animal is another known cause of unpleasant tastes and odours. When the cow consumes fresh or preserved strongly flavoured forage (ensilage, cabbage, wild tuber plants, etc.) two to four hours before milking, their characteristic taste and odour appear in the milk and can persist until elimination or metabolization of the feed, or sometimes up to about 12 hours after ingestion. It should also be noted that the problem of undesirable flavours from feeding is often associated with sudden changes of diet -for example, the transition from dry winter feeds to rich green summer forage.
Direct absorption of substances by the milk following milking is less than formerly believed since rather more volatile compounds pass through the animal. Recent studies have shown, however, that with whole milk highly volatile and fat-soluble organic solvents were directly and primarily retained by the fat phase, since skimmed milk does not show this defect. The presence of these substances in premises for the collection, processing and storage of milk or milk products has often been the source of this occurrence and therefore their presence should be prohibited.
Another class of defects designated as “chemical flavours” can be caused by the contamination of milk by chemical agents included in the formulation of detergents and disinfectants used for cleaning recipients and equipment. For instance, alterations due to the presence of chlorinated and iodized compounds have been the most frequently reported. Less frequently, phenol compounds from disinfectants and weed-killer have been considered responsible. This applies also to the development of foreign tastes and odours of the chlorophenol type transmitted by water disinfected with chlorine. It should also be mentioned that milk receptacles that have been used occasionally to contain or transfer other products (petroleum products, pesticides, non-food disinfectants and alcohol) can remain highly impregnated and, due to the residues present when reutilized normally, cause marked adverse changes in the flavour of the milk which can persist even after it has been greatly diluted.
Most technical treatments have little effect on the elimination of transmitted tastes and odours. Only vacuum treatment (degasing and evaporation) can decrease the extent of these defects, but their effectiveness is closely linked to the type of volatility of the offending substances in the physical conditions of the treatment applied.