Flavor in one of the most important qualities that determine the acceptability of milk. Even though milk is highly nutritious, people will not drink it if they do not like it. Hence, milk should be produced under conditions that give good flavor initially, and also be handled to protect its flavor at every step from the cow to the consumer.
As population densities increase around large metropolitan centers, milk is usually transported longer distances to supply the urban population, and the farms located near the cities tend to increase their scale of operations. Both changes may influence milk's initial flavor and particularly its flavor stability. Transporting milk longer distances usually increases the time between production and processing, which provides a greater opportunity for development of off-flavors. To maintain bacteriological quality during this extended storage period, greater emphasis is placed on improved sanitary practices and effective refrigeration. Defects of bacterial origin are generally kept under control, but the changes may result in an increased incidence of other defects. For example, as dairying became more intensive, bulk collection systems were introduced. They provided for cooling milk to lower storage temperatures, and when couple with longer storage times, oxidized and rancid flavors could develop.
When the average number of cows in herds increases, farmers have to provide a larger proportion of the cow's feed in dry form as hay and concentrate, instead of as pasture and silage. In general, dry feeds yield milk with greater susceptibility to oxidized and rancid flavors than do succulent feeds. Feeding high levels of concentrate, as practiced in intensive dairy areas, increases the concentration of unsaturated fatty acids in milk lipids, with an accompanying increase in liability of the fat to oxidation.
As distribution channels are equipped to provide more effective refrigeration of milk during delivery to stores and homes, and during storage in retail outlets, processors extend the expected shelf-life of their products. For example, a Californian company dates the products it packages in paper containers for expected 'shelf-life' of 12 days in distribution channels, and extends its tests in the quality control laboratory for an additional week. Thus, it is not sufficient to produce milk with good flavor initially. Increasing attention is being given to the stability of milk flavor during storage.
NORMAL MILK FLAVOR
Milk of good quality is a very bland food with a slightly sweet taste, very little odor, and a smooth, rich feel in the mouth. Because of its bland flavor, the presence of minute quantities of abnormal constituents frequently results in off-flavors. Dairy producers and processors have, of necessity, been so concerned with the control of off-flavors, that until recently little attention has been given lower temperatures than prevailed with can collection systems. Most people associate the palatability of milk with its 'richness'. It is generally assumed that milk fat is one of the most important constituents in contributing to the desirable flavor of milk. Although there is a close correlation between the concentration of fat and solids- not-fat in milk as produced by the cow, little attention has been given to the contribution of the solids-not- fat constituents to its flavor.
In studies conducted in the 1960s, tests were made to determine the minimum difference in the concentration of selected milk constituents that could be detected reasonably consistently (two times out of three) by a trained laboratory panel. Special care was taken in the preparation of the milk samples to avoid any abnormal flavors that might have influenced detection of differences. In homogenized milk of typical commercial composition (3.5 % fat and 8.0 % S.N.F.), the percentage of fat had to be increased by more than S.N.F (i.e. to over 9 %) before the panel could differentiate it from the control milk two times out of three, whereas the S.N.F. had to be increased by only O.6% for comparable differentiation. This result indicates that a small increase in S.N.F. would have a greater influence on milk flavor than a comparable increase in fat.
The experiments were extended to study the roles of individual constituents or fractions of S.N.F. An increase of O.33% lactose was detected two times out of three. A concentrate of non-dialysable constituents of skim milk (mainly protein) was prepared by dialyzing concentrated skim milk against normal skim milk, and this concentrate was used to increase the concentration of protein and other colloidal constituents in milk. An increase of 2.S% of non-dialysable constituents (2.2% of the increase as proteins) was needed for two-thirds correct detection. Thus, lactose appears to be important, and the proteins relatively unimportant, in contributing to the normal flavor of milk.
The increasing sales in the United States and Canada of milk products containing increased S.N.F. concentrations indicate that these products have acceptable flavor. Products containing 2% fat and 10% S.N.F. are widely distributed and are gaining in popularity. Skim milk with a normal S.N.F. concentration has never been popular, but non fat products with increased S.N.F. have developed into successful commercial products.
Cows' milk is a palatable beverage, but nature did not develop it to appeal to man's taste. We should not limit ourselves to using milk as nature produces it for the calf. To increase milk sales, we should take advantage of modern technology to modify milk to man's taste and nutritional requirements.
OFF-FLAVORS IN MILK AS PRODUCED
Feed and weed flavors
In many countries the most common flavor defect of milk is feed flavor. The incidence in different countries is hard to assess because evaluations are based only on subjective judgments, and opinions regarding the intensity of the off-flavor that constitutes a defect are very variable. Likewise, levels that are objectionable to consumers are equally variable. The presence of feed or weed flavors in a high proportion of milk samples in a number of surveys indicates that the off-flavors may be detected by, and are presumably objectionable to, many consumers.
Understanding the mode of transmission of flavor substances in the cow's body assists practical control of feed flavors. The respiratory system and the digestive tract are both important in transmitting flavor substances to the milk odors from some feeds pass into the cow's blood from air in the lungs, and are then carried to the udder and appears. in the milk. Some flavor-producing substances are absorbed by the blood from the digestive tract, and are then transmitted to the udder. For some feeds, both the respiratory and digestive tracts are involved in the transmission of flavor to milk. Some feeds, such as garlic and onion, release volatile flavors after partial digestion in the rumen. odors belched from the rumen are inhaled into the lungs and transferred to the blood. This pathway provides a more rapid transfer of feed flavors from ingested feed than direct absorption from the digestive tract.
Fortunately, blood provides a two-way street for transportation of feed flavors. When the concentration of the flavor substances is higher in the milk in the udder than in the blood, the substances transfer from the milk to the blood. If sufficient time is allowed after the feed is consumed, the flavor substances are eliminated from the blood, partly by transfer of volatile substances to the air in the lung, and partly by metabolism of the substances. In either case, they are eliminated from the cow's body.
The time interval between eating and milking is an important factor influencing the intensity of feed flavors. Whether the off-flavor resulted from only breathing the odor of silage, or from eating the silage, the feed flavor was most pronounced from 2 to 3 hr. later, but had been eliminated from the milk 5 hr. later. These results show why feeds commonly used in dairy rations will not cause objectionable flavors if they are fed after milking, and withheld from the cows during the 4-5 hr. before milking.
Not all feeds respond to flavor control. Some flavor substances accumulate in cow body tissues, particularly in the fat. They then transfer to the blood, and hence to the milk, over long periods of time. one of the most lingering flavor defects reported resulted from eating potatoes grown on a field treated with benzene hexachloride (for nematode control). A mothball-like flavor was still detectable in the milk three weeks after this feed was discontinued.
Flavors from some weeds persist for longer than 12 hr. after they are eaten, and therefore such weeds must be kept out of a cow's ration. In selecting feeds for a dairy ration, one criterion must be that either the feed does not impart an undesirable flavor to milk, or the flavor can be controlled by withholding the feed for a reasonable time before milking. It is impracticable to withhold feed for more than 5 hr., as prod production will suffer. Flavors from many feeds can be controlled by withholding the feed 2-4 hr. before milking.
Reports on some feeds and their effect on milk flavor are inconsistent. For example, sugar beets, beet pulp, and beet tops do not normally cause objectionable flavors, but are sometimes responsible for fishy flavor. The difficulty appears to be sporadic, varying with each individual cow and with different lots of feed. The quantity of feed consumed, its quality, and in some cases its maturity, may influence its flavor- producing characteristics.
Many weeds cause off-flavors in milk. Weeds are variable in their effects. Flavors caused by some can be controlled if the weeds are withheld from the cow 5 hr. before milking. The off-flavor from some weeds (onion, penny cress) appears principally in the milk fat, whereas that from others (bitterweed) is associated with the skim milk portion. The defect may be more pronounced with young plants (cocklebur) or with older plants or their seeds (penny cress). In most cases the off-flavor is present when milk is drawn from the cow, but this is not always true. For example, skunkweed flavor does not appear in fresh milk or cream, but shows up in butter or cheese. Wild carrot eaten alone sometimes produces a mild off- flavor, but when eaten along with mule's tail or mare's tail, the skunkweed defect becomes pronounced. The scorched flavor caused by swine cress is evident only after milk or cream is heated. on the other hand, pasteurization markedly reduces or changes the character of other feed flavors. The ensiling process destroys the substances causing ragweed and pepper grass flavors but does not affect the substances causing onion flavor. Such inconsistencies in the influence of different feeds and weeds on flavor indicate the danger of making general statements regarding their behavior.
Since odoriferous substances are volatile, attention has been given to developing practical procedures for removing them by vaporization. The value of using aerators or surface coolers as a means of reducing feed flavors is often overrated. When alfalfa-flavored milk cooled over a surface cooler was compared with the same milk cooled in a container, experienced judges had difficulty in detecting consistently the sample with supposedly less feed flavor. Although aeration may cause some reduction in the intensity of some feed flavors, it cannot be relied on to control the defect.
In recent years many liquid milk processors in the United States and Canada have installed equipment that subjects milk to a combined vacuum and heat treatment to remove volatile flavors. The effectiveness of such equipment depends not only on the intensity of the treatment, but also on the nature and concentration of the flavor substances. Some flavor treating machines are very effective, whereas others have little influence on milk. Use of flavor-removing equipment can provide the processor with an extra safeguard in protecting the flavor quality of the milk he sells, but present indications are that the producer will still have to take the major responsibility for control of feed flavors by sound management practices.
Cowy, barny and unclean favors
These terms describe flavors that are attributable to unsatisfactory production conditions, but there are several possible causes. one of the most common is inhalation by the cows of foul air in poorly-ventilated barns or corrals in which wet manure has accumulated. The barny, unclean odors are transferred to the milk through the cow's respiratory system in the same manner as for feed flavors. Although direct absorption of the odors by milk during and after milking is frequently mentioned as a possible cause, research has established that this source is insignificant compared with absorption through the cow's respiratory tract.
Metabolic disturbances of the cow may also result in cowy and other unclean flavors. Ketosis is frequently accompanied by cowy flavor. Cows with the disease have a high concentration of acetone bodies in their blood. These compounds appear in the milk and are at least partly responsible for the cowy flavor. Other conditions that upset the cow's d digestive processes have also been associated with cowy and other undesirable flavors.
Mastitis reduces the flavor quality of milk, but the effects are variable. Mild cases may result in a flat flavor, probably due to a lower concentration of the normal milk constituents. In more severe cases of mastitis, the milk is usually criticized for cowy, unclean, and salty flavors. In some cases cowy, barny and unclean flavors may be attributable to the cow's feed. Some weeds cause the appearance in milk of indole and skatole compounds that give the characteristic odor to faces. Abnormal silage fermentations may also result in unclean flavors in milk.
Although cowy flavor is listed as a defect present in milk as drawn from the cow, the flavor may intensify during storage. The increased intensity may be due to increased concentrations of acetone and butanone, but nothing has been reported regarding the source of and conditions influencing the appearance of these compounds during storage.
Foreign and chemical flavors
Some flavors that are present immediately after milking may be caused by chemical contamination. Treatment of teats with salve is a possible source of medicinal flavor. Equipment used in handling milk may also contribute off-flavors, such as the phenolic taste from certain plastics, or chemical tastes from sanitizing agents used to treat milking equipment. Combinations of chemical contaminants may be more troublesome than one alone, as illustrated by difficulties encountered by some dairy producers and processors with chlorophenol flavor. Phenolic and chlorine compounds in combination can, in certain circumstances, be detected at lower concentrations than either alone, because they may react to form chlorophenols.
Salty taste is caused by two conditions: mastitis and advanced lactation. The defect is rarely detected in pooled milks, because only a small proportion of cows would produce salty milk in a well-managed herd. Nevertheless, salty milk should be eliminated from the supply, because it is detrimental to milk quality.
Milk that has excellent flavor as produced may develop objectionable defects before it reaches the consumer. Recent changes in the dairy industry have caused changes in the flavor problems encountered. In the early days of the market milk industry, when sanitation practices were inadequate and relatively few refrigerators were available in homes, defects caused by bacteria, such as sour flavor, limited the time that milk remained usable. Today such bacterial defects are encountered less frequently because of modern sanitation practices, improved refrigeration and transportation facilities, and widespread adoption of pasteurization. Defects caused by chemical changes during storage such as oxidized and rancid flavors make up a larger percentage of observed defects than formerly. Milk as purchased is now more uniform in flavor and defective product is more likely to result in a complaint than in former years. Hence, processors must pay more attention to improving both the initial flavor and the flavor stability of milk during its expected usable life.
Oxidized flavor is a troublesome defect of non-homogenized milk, skim milk, cream, and certain other dairy products. The flavor is described by terms so various as metallic, papery, cardboardy, oily, and tallowy, indicating the great variability of the predominant flavor characteristic. The defect is caused by the oxidation of fatty constituents in the product. Knowledge of the milk constituents that are involved in the development of oxidized flavor, or influence susceptibility of milk to the defect, helps one to understand the factors that influence the flavor. The compounds responsible for the flavor are produced by oxidation of unsaturated fatty acids in the phospholipids in the membrane surrounding the fat globules. Conditions are more conducive to oxidation at the surface than inside the fat globules for at least four reasons: (1) The phospholipids, which are concentrated at the surface lace, contain a higher proportion of unsaturated fatty acids than the remainder of the fat globule; (2) The oxidation catalyst, copper, is also concentrated at the surface of the fat globule; (3) oxygen from air dissolves in the skim milk portion of milk, and thereby reaches the surface of the fat globules; (4) The ascorbic acid in milk, which is also dissolved in the skim milk portion, is an important pro-oxidant in normal milk, even though it can also act as an antioxidant. As a counter effect, at least one important natural anti-oxidant, tocopherol, is concentrated in the material at the surface of the fat globules. Thus, the susceptibility of milk to oxidized flavor depends on the balance between the pro-oxidant and anti-oxidant factors at the surface of the globule.
The milk processor can take advantage of two processes that prevent or delay development of oxidized flavor: homogenization and heat treatment. The oxidative stability of milk (its ability to resist oxidized flavor) is determined to a large extent at the farm. For this reason, in recent years the main efforts to control oxidized flavor have been concentrated on the point of production.
Milk varies greatly in its susceptibility to oxidized flavor. The variability is particularly evident when the milks of individual cows are compared. Some milks develop oxidized flavor without any treatment other than refrigerated storage for as little as one day. others are so resistant that even if 5 p.p.m. of copper are added they do not develop oxidized flavor in seven days of storage. Obviously, this extreme variability is important in determining whether oxidized flavor will develop in milk and what measures must be taken to control the defect.
One of the most important factors influencing the oxidative stability of milk is the cows' feed. Pasture and other succulent feeds generally yield milk that is very resistant to oxidized flavor. The defect is encountered most frequently with dry feeds. Changes in some feeding practices in order to increase production per cow appear to be increasing the susceptibility of milk to oxidized flavor. In the United States, during seasons when the milk is susceptible to oxidized flavor, some dairymen select feeds containing high concentrations of tocopherol, or supplement rations with tocopherol to increase the oxidative stability of the milk.
The oxidative stability of milk is also influenced by some other conditions related to the cow, such as heredity, number of lactations and stage of lactation. It is helpful to recognize that such variables influence oxidized flavor, even though they cannot be manipulated to control the defect.
The cow has a marked influence on the initial oxidative stability of milk, but virtually every treatment milk receives between the cow and the consumer influences, in one way or another, the development of oxidized flavor.
Low storage temperatures favor the development of oxidized flavor. This effect is contrary to that expected of chemical reactions. Two explanations of this behavior relate to retarding the growth of bacteria, and reducing the activity of the natural anti-oxidants in milk, but they have not been supported by experimental evidence. Storing milk at low temperatures is encouraged to maintain high bacterial quality, even though it may aggravate problems with oxidized flavor. It also frequently results in longer storage times. Thus, the adoption of mechanical refrigeration for cooling milk, particularly in bulk tanks, has sometimes been accompanied by an increased incidence of oxidized flavor in milk.
A common serious cause of oxidized flavor is contamination of milk with copper. The role of metals, especially copper, in catalyzing oxidized flavor has been recognized for many years. Tinned-copper equipment has been largely replaced by 18-8 type stainless steel for the manufacture of most dairy equipment. nickel-copper alloys, commonly called 'white metal' or 'stainless metal; have been substituted for stainless steel to reduce costs, but this is an unsatisfactory compromise. White metal introduces appreciable copper contamination to milk. Copper bearing alloys must be eliminated from the cleaning circuits, as well as from milk-contact surfaces of equipment, otherwise copper will dissolve from the copper-bearing metals during circulation cleaning, and deposit on stainless steel. This copper is then picked up by the first milk that passes through the equipment after cleaning.
Another serious cause of oxidized flavor in many countries is exposure of milk to light, particularly during distribution in clear glass bottles. This may induce either oxidized flavor, or a light-activated flavor, or both. The two defects differ chemically, but in many respects the effects are similar. They are discussed in greater detail in the section dealing with light activated flavor. Regardless whether the defect is predominantly oxidized or light-activated flavor, or both, the obvious control is to protect milk from exposure to light.
Pasteurization slightly increases the susceptibility of milk to oxidized flavor. Milk pasteurized at temperature-time combinations that do not seriously impair creaming properties usually develops oxidized flavor more rapidly than non-pasteurized milk. More severe heat treatments, however, reduce susceptibility, apparently both by producing anti-oxidants and by reducing the catalytic activity of copper. Treatments that produce a cooked or heated flavor usually delay development of oxidized flavor. Products such as the non-fat pasteurized milk enriched with solids-not-fat, which is common in the United States, are usually pasteurized at temperatures appreciably above the minimum required for pasteurization (e.g. at 180-185 F for 20 sec. or longer) in order to help control oxidized flavor.
Homogenization markedly increases the resistance of milks to oxidized flavor. Many explanations have been given for this effect, but none has been supported by convincing experimental evidence. The inhibiting effect is so pronounced that homogenized milk rarely develops oxidized flavor. The popularity of homogenized milk in the United States and Canada is attributed largely to its superior flavor because of the practical control of oxidized flavor provided by homogenization.
De-aeration has been suggested to prevent development of oxidized flavor. De-aerating equipment is available that effectively removes dissolved oxygen from milk and thereby delays or prevents the defect. Unfortunately, the process is of little value commercially, because most of the bottle and carton fillers in common use allow enough oxygen to redissolve in milk to produce oxidized flavor.
Several other methods of inhibiting oxidized flavor are not useful in control programs, either because they are not permitted by present regulations, or because they are detrimental to other quality characteristics of milk. Many anti-oxidants effectively prevent oxidized flavor when added to milk, but in most countries their use is not permitted. There are statements in the literature that bacteria growing in milk retard oxidized flavor, either by using up dissolved oxygen necessary for the oxidative reactions, or by producing anti-oxidants. Partially hydrolyzed proteins have anti-oxidant properties, so proteolytic activity of bacteria could also indirectly retard oxidized flavor. However, the numbers of bacteria necessary to retard development of oxidized flavor are very large (generally over 1 million/mL.). Hence, variations in the smaller numbers of bacteria normally found in milks that meet legal standards would not be of practical importance in determining whether milk develops oxidized flavor.
Light-activated flavor results from chemical changes in protein when milk is exposed to light. other names by which the defect has been identified include sunshine, sunlight, and solar activated. Terms used to describe the flavor are cabbage, burnt, burnt feather, burnt protein, and mushroom.
Because light may induce either oxidized or light-activated flavor, or both, some investigators have not differentiated clearly between the two defects. This has resulted in confusion regarding similarities and differences between them. Several characteristics of light-activated flavor aid in understanding conditions that influence its development and methods for its control.
Homogenized milk is susceptible to light-activated flavor, but resistant to oxidized flavor. Light-activated flavor increases with intensity and duration of exposure, and the predominant flavor characteristic changes during exposure and subsequent storage. The temperature of the milk during exposure and storage is also critical. Higher exposure temperatures (50-60 F) result in more pronounced flavor, but the flavor of samples exposed at low temperatures (35-40 F) may increase in intensity for several hours after exposure. If, after exposure, the milk is held at higher-than-normal refrigeration temperatures (e.g. 50-60 F), the flavor may decrease in intensity. The time interval between homogenization and exposure also has an influence. If milk is stored one or more days after homogenization, but before exposure, it is much less susceptible to the defect.
Light of the shorter wavelengths (blue and green) has greater effect than that of longer wavelengths (yellow and red). Fluorescent light is more troublesome than incandescent light, because it usually has a higher proportion of its energy in light of short wavelengths. Also, because fluorescent lamps are not hot, milk is frequently placed close to them in retail display cases.
Selecting fluorescent lamps that emit a high proportion of the 'warm' tones helps to minimize damage from fluorescent light in display cases. Likewise, use of colored bottles or paper containers that absorb much of the light of shorter wavelengths provides protection, but does not necessarily prevent activated flavor under very adverse conditions.
The cows' feed influences light activated flavor as it does oxidized flavor. Green feeds appear to yield milks with greatest resistance to the defect. Hence, as for oxidized flavor, the susceptibility of milk to light activated flavor varies seasonal]y and is greatest during the winter.
The term 'rancid', when applied to milk and other dairy foods, refers to a flavor defect caused by hydrolysis of fat, rather than by oxidation of fat. Milk always contains an enzyme (or a group of enzymes) known as lipase, which is able under certain conditions to hydrolyze fat, splitting off fatty acids that are responsible for the rancid flavor. Freshly drawn milk from healthy cows is never rancid. Depending on conditions, rancidity may develop on aging of raw milk. Pasteurization destroys lipase, so properly pasteurized milk will not go rancid.
In most milks, the so-called 'membrane' around the fat globules appears to protect the fat from attack by lipase. Certain treatments, known as activating treatments, change the fat globule surface sufficiently to permit the lipase to act on the milk lipids and produce rancid flavor. Three activating treatments that may be encountered in milk production and processing are (1) agitation of warm milk, particularly under conditions that produce foam; (2) homogenization of raw milk (or mixing raw and homogenized milk); and (3) temperature fluctuations such as cooling milk, warming it to about 86 F and then cooling it again.
When milk samples from individual cows are cooled and stored one or two days, a small proportion of the samples may develop rancid flavor. This rancidity is sometimes referred to as spontaneous, because no treatment other than cooling is required for its development. Some investigators believe spontaneous rancidity is caused by a lipase which differs from that present in normal milk, whereas others consider that the fat globules of 'spontaneous' milk do not have sufficient protective matter on their surfaces to prevent attack by lipase. Regardless of the explanation, spontaneous rancidity is not a serious source of off-flavor in commercial milk supplies. It occurs most commonly with milk from low-producing cows that are very advanced in lactation and should be dried off. Also, if milk from such cows is mixed with non- spontaneous milk, the mixture usually does not develop rancid flavor unless it receives an activating treatment. Hence, when rancidity is encountered in commercial milk, the immediate cause is usually an activating treatment.
Susceptibility of milk to induced rancidity varies greatly. Rancid flavor of differing intensity develops in milk samples taken from individual cows in a herd, and even from individual quarters of a cow's udder. Individual cows show marked daily variations in susceptibility of their milk. Susceptibility is often greater when cows are in advanced lactation. Green succulent feeds make milk more resistant than dry feeds. The numerous variables that influence susceptibility are at least partly responsible for sporadic and seasonal variations in incidence of rancidity.
To achieve labor efficiencies, the producer adopted the bulk collection system and changed from bucket milkers to pipeline milking systems. The average number of cows on dairy farms increased, and to increase milk yield per cow farmers depended more and more on feeding hay and concentrate mixtures. These progressive developments seriously aggravated rancid flavor problems. The change is not attributable to a single cause, but to a combination of changes that took place simultaneously.
When the bulk tank system was adopted, it was possible to cool milk more rapidly and store it at lower temperatures than with the can system. Rancid flavor induced by some activating treatments develops more rapidly as the storage temperature is decreased. In some instances, defective or inadequate refrigeration systems resulted in prolonged agitation of warm milk, or temperature fluctuations resulting from additions of warm milk to previously cooled milk. In some districts, milk was collected only on alternate days, resulting in longer storage which favored the development of rancidity.
When new installations were made, most were entirely of stainless steel in the milk-contact areas. This eliminated sources of contaminating metals. The lipase in milk is unstable in conditions that favor oxidation. Thus, eliminating copper contamination, added to the rapid cooling of milk, increased the stability of the lipase.
Many early installations of pipeline milkers provided conditions which subjected milk to activating treatments. Milk was commonly elevated in vertical sections of the vacuum line by bubbling air through it. Excessive amounts of air entered the systems at milker claws, fittings, etc. Pumps were operated continuously, but starved. Low producing cows in advanced lactation were not so readily detected and were retained in the milking strings too long.
As the number of cows per herd was increased, farmers could no longer supply enough pasture or silage for the cows. Hay and grain concentrate mixtures provided increasing proportions of the nutrients for milk production, and these dry feeds yielded milk with greater susceptibility to rancidity.
All of the above conditions probably contributed to rancid flavor problems. To control the defect, many unsatisfactory practices and equipment installations have been eliminated. Although most commercial milk is undoubtedly more susceptible to rancidity than that produced in pre-war years, the industry has learned to cope with the problem by minimizing activating conditions.
As noted above, pasteurization, by inactivating the lipase, provides the processor with a very effective method of controlling rancidity. High susceptibility of milk to rancidity may limit flexibility of operations in a processing plant by necessitating prompt pasteurization to prevent development of rancidity. In properly pasteurized milk, rancid flavor should not be a problem.
Processing plant employees must be aware of conditions that may induce rancidity. If cooled milk is warmed for separation, the cream should not be cooled again without being pasteurized. Homogenized milk must be pasteurized before, or immediately after homogenization Homogenized products should not be mixed with raw products. Prolonged agitation of warm raw milk (80-120 F) or pumping it through starved pumps may induce rancidity.
In commercial processing, milk is almost invariably subjected to heat treatments at least as severe as the minimum required for pasteurization, and frequently more severe. In general, the intensity of the heated flavor increases with that of the heat treatment.
Pasteurization produces a slight heated or cooked flavor in milk. This process is used so widely that most consumers consider a slight heated flavor to be typical of normal milk and they do not find it objectionable. A slight heated flavor contributes to the apparent sweetness and richness of milk masks some off-flavors of low intensity, and may make milk less susceptible to oxidized flavor. For these reasons, some processors deliberately use pasteurization temperatures appreciably in excess of the legal minimum to produce a slight heated flavor in the freshly pasteurized milk. This flavor decreases in intensity during storage. Hence, although uniformity is desirable, the intensity of the heated flavor does not remain constant during the time milk is in distribution channels.
Heat treatments at much higher temperatures and for longer times than required for pasteurization are used in the processing of many dairy products, and they produce pronounced heated flavors with differing predominant characteristics. The first that becomes evident results from liberation from the proteins of hydrogen sulfide and other sulfides. More severe heat treatments give scorched and burnt flavors caused by more extensive changes in the proteins; caramel flavors attributable to reactions involving lactose; and lactone or; coconut-like flavors resulting from changes in the milk fats. These more severe heat treatments are accompanied by changes in the tactile. or mouth-feel, characteristics of milk. Such flavors are typical of sterilized milks, and are objectionable to most consumers. Hence, extensive research has been directed toward sterilizing milk without producing the objectionable flavor caused by conventional sterilizing treatments.
Off-flavors caused by the growth of bacteria in milk are not detectable until large numbers of bacteria are present, usually millions per milliliter. Hence, the milk would not meet legal standards for bacterial quality. Nevertheless, defects caused by bacteria are encountered from time to time, and it is important to know their characteristics and conditions under which they develop.
Milk is such a good food for bacteria, as it is for man, that it is very subject to spoilage. It must be rigorously protected from bacterial contamination, and kept cold to minimize growth of bacteria that are present. If flavors of bacterial origin develop in raw milk, this indicates that sanitary practices have been inadequate, or that the milk has been held at too high a temperature, or too long.
A sour, high acid, or malty flavor may result if raw milk is held at temperatures above 45 F. At lower temperatures, the flavors that develop are usually caused by psychrophilic bacteria and are described as fruity, bitter, rancid or putrid.
Pasteurization destroys the milk souring bacteria so effectively that, when pasteurized milk spoils from microbial growth, the cause is usually bacteria that contaminate the milk after pasteurization. Thus, the keeping quality of pasteurized milk stored at 45 F or below can serve as an indication of sanitary practices in the plant. As for raw milk, the psychrophilic organisms that grow at these low temperatures cause fruity, bitter. rancid or putrid flavors. Properly pasteurized milk, adequately protected from post- pasteurization contamination and stored at 45 F, should resist psychrophilic spoilage for at least two weeks.
MILK FLAVOR QUALITY CONTROL
Flavor Quality Control
Some of the more common off-flavors that may be present in milk as it comes from the cow or may develop during processing and storage were discussed in the first two papers in this series. Understanding the nature of desirable and undesirable flavors in milk, and conditions that influence these flavors, provides a sound basis for an effective flavor control program. In addition, not only quality control personnel, but also milk producers and processors, can benefit materially by familiarizing themselves with techniques for flavor perception and evaluation, and by developing proficiency in identifying the common off-flavors of milk.
Preparing Milk Samples
The first important step in a flavor control program is the technique for collecting and preparing milk samples. Commercially processed milk in its consumer package usually serves as a satisfactory sample and container. The products must be protected from light, refrigerated during transport, and either examined promptly or stored at a standardized temperature before examination.
Milk samples that must be collected from bulk containers or individual cows pose special problems. Frequently, the method of treating the sample must be adapted to the flavor defect that is being studied.
For most purposes, milk bottles (usually a quart) are satisfactory and conveniently available. Bottles with rubber closures should be avoided as the rubber may impart foreign flavors to the milk. Polyethylene bottles with polyethylene screw caps are very satisfactory. They are light and durable, withstand heat shocks of laboratory pasteurization, and may be autoclaved if desired. For studies of oxidized flavor, they have an additional advantage over glass they adsorb less copper that would otherwise contaminate the samples.
Raw milk should be pasteurized before it is tasted, but the time and method depend on the defect. For many purposes, the milk should be pasteurized immediately and examined while fresh and again after a standardized storage treatment. Pasteurization, however, prevents the development of rancidity by inactivating the lipase. If rancidity is of concern, the milk should be stored raw for at least two days and then pasteurized before the flavor examination. on the other hand, if the greatest concern is with oxidized flavor, the milk should be pasteurized before storage to prevent the development of rancidity and bacterial flavors that might otherwise mask oxidized flavor.
Pasteurization of flavor samples in accordance with legal definitions is not practicable. Samples may be pasteurized in their containers by heating and cooling them in a water bath, with a thermometer in a reference bottle containing the same volume of milk as the sample bottles. Shaking the bottles provides adequate agitation. The containers should be closed to minimize volatilization of feed flavors. The temperature-time combination used should produce little or no heated flavor, yet provide protection from disease and be convenient for the equipment available. If a thermostatically controlled agitated water bath is used, heating to 145 F for 30 min. is satisfactory; otherwise a combination such as 150 F for 4 min. or 161 F with no holding period is more convenient.
It is difficult to recognize flavors from descriptions. Hence, it is important to practice identifying the most common defects by repeatedly tasting samples with known flavors. The procedures outlined below are suggested as convenient methods for preparing samples having typical flavor defects.
Feed or Weed flavor
Where facilities permit, it is best to feed an individual cow feeds or weeds known to cause characteristic flavors (e.g. silage) 1 to 2 hr. before milking, then collect the milk from this cow. In processing plants it is usually possible to select milk with feed flavor from individual producers during receiving operations. Samples prepared by soaking feeds in milk do not usually give typical defects.
Place a strip of bare sheet copper or 1 ft. of coiled copper wire in a bottle of pasteurized milk (not homogenized) and store in refrigerator one or two days. Unless the milk is unusually resistant, oxidized flavor will develop.
Light-activated flavor is usually evident in freshly processed homogenized milk exposed to sunlight for 2 hr. in quart glass bottles, and then held in a refrigerator for at least 4 hr. Homogenized milk is preferred because it is more susceptible]e to light activated flavor and less susceptible to oxidized flavor than non- homogenized milk. Adding ascorbic acid (50 mg. /quart) before exposure enhances light-activated flavor but inhibits oxidized flavor.
Mix equal volumes of raw milk and homogenized milk and store in refrigerator overnight. If rancidity is not evident, heat to 70-90 F and hold. Taste at hourly intervals until the desired intensity develops, then pasteurized rise.
The best way to obtain a typical salty flavor in milk is to examine samples from individual cows and select one with the defect. Salty flavored milk is produced most frequently in advanced lactation or when the cow has mastitis.
Heated flavorP> Heat milk to a temperature higher than is normally used for pasteurization, such as 170 F for two min., and cool.
EVALUATING MILK FLAVOR
Understanding how we perceive flavors aids in flavor evaluation. Flavor is a blend of sensations from several sensory organs. The most important sensations contributing to milk's flavor are the taste, smell and feel.
Taste is detected in the mouth, principally on the tongue. There are only four tastes—sweet, sour, salt and bitter. To produce a taste sensation, a substance must be in solution in order to diffuse into the receptor sites in the taste buds.
Odor is detected by olfactory nerves in the nasal passages. The number of odors is almost limitless, and many are difficult to describe. To have an odor a substance must be volatile. Most compounds with strong odors are more soluble in fat than in water, whereas the reverse is true of most substances with pronounced tastes.
Another sensation that contributes to flavor is the way the food feels in the mouth. Some tactile characteristics of milk are smoothness, chalkiness, and richness attributable to consistency.
Although most milk is drunk cold, warming milk enhances its odor and facilitates detection of off-flavors. For quality control evaluations, milk usually warmed to about 70 F. The bottle should be shaken, the cap removed and the sample smelled. A head space above the milk (e.g. bottle two-thirds full) aids in evaluating odors.
Next, the sample should be tasted. The milk should be moved around in the mouth to contact all portions, particularly of the tongue, and to promote volatilization of compounds that contribute to the odor. Particular attention should be given to the changing sensations with time. Sweet, sour and salty tastes are usually detected quickly, whereas bitter tastes become evident more slowly but are retained longer.
The most important step in learning to evaluate milk flavor is to gain proficiency in identifying defects. Correct identification provides helpful guidelines for control of defects. Evaluating the intensity of defects, however, is also of value in quality control programs.
Many different systems for designating intensity of flavors are in use. In some, intensity is indicated only by descriptive terms such as slight, distinct or strong, or by one, two or three plus signs. A simple numerical system applies scores for these descriptive terms: 0, no criticism; 1, slight; 2, distinct; and 3, strong. An inverse of this scale, or one giving a higher maximum score for flavor, such as 5 or 10 points, is preferable for quality scoring as it recognizes higher quality with a higher score. Any numerical system is arbitrary, but there is an advantage in selecting one that is in common use. In the United States a system that is widely used assigns scores within the range 10 for good milk to 0 for poor milk.
TROUBLE-SHOOTING FLAVOR DEFECTS
If an off-flavor is found in a milk sample, a systematic approach helps in identifying the defect and its cause. An experienced individual will identify the most common off-flavors of milk by taste, and will be able to proceed immediately to determine the most probable cause. A beginner in flavor quality control work will be guided in his identification of off-flavors by comparing defective samples with samples having known off-flavors prepared as described above. Changes in intensity of a defect during storage provide helpful evidence regarding its identity. Hence, samples should be tasted fresh and again after storage in a refrigerator for at least 48 hr.
To pin-point possible causes of off-flavors in raw milk, it is helpful to collect milk samples at different steps. Samples might be collected from individual cows, at the discharge from the pipeline milker, from individual cans or the bulk tank, from the tank after only one milking and later at the time the milk is collected, or from morning and evening milkings separately.
If an off-flavor is present in a fresh sample collected from pooled raw milk, it is usually attributable to feed. In some cases, there may be a marked difference between morning and evening milk in intensity of the off-flavor. Examining the feeds for known troublesome materials may substantiate suspicions regarding possible causes. Recommendations for corrective measures include:
- Use only feeds that cause little or no feed flavor. Eliminate weeds from pastures and from crops to be used for hay or concentrates.
- Arrange feeding schedule to
- Prevent the cows from eating feeds that may cause off-flavors during the 4-5 hr. before milking. Provide an environment where the cow can breathe fresh air free of feedy, cowy or barny odors.
Individual cows in a herd may produce milk with an off-flavor when it is drawn, such as a salty taste attributable to mastitis or advanced lactation, or a cowy flavor caused by ketosis. It is rare that such defects can be identified in the mixed milk from the entire herd, but they make the pooled supply less palatable. Therefore, milk from such cows should be withheld from the pooled milk.
In some instances, the cow may appear to be responsible for off-flavors that are actually caused by equipment, or treatment of the cow or equipment. Examples are a medicinal flavor from salves used on the teats or udder, or chlorophenol flavor from plastic or rubber parts of the milking equipment.
If an off-flavor is not present in fresh milk, but develops during storage, possible identities are light activated, oxidized, rancid and microbial flavors. Light-activated is the least likely in raw milk, and in any case its cause and correction would be immediately apparent. Comparison with reference samples aids in differentiating between oxidized and rancid flavors. Also, pasteurization of the fresh sample prevents development of rancid and bacterial flavors but not oxidized flavor. Some microbial flavors caused by psychrophilic bacteria are difficult to differentiate from rancid flavor because the bacteria produce lipolytic enzymes that release fatty acids, and also proteolytic enzymes that yield degradation products from protein with similar flavors. Information from bacterial counts of the milk, inspection of equipment, and checking on sanitizing procedures and cooling practices indicates whether bacterial flavors may be involved. If the defect appears to be rancid flavor, a chemical test for free fatty acids could be used for confirmation.
Rancidity that occurs in raw milk supplies usually is induced by an activating treatment: either excessive agitation of warm milk or temperature fluctuations between 50 and 86 F. Eliminating the activating condition usually prevents development of the defect. Determining the susceptibility of milk from individual cows is also helpful. Cows that produce the most susceptible milk are usually in advanced lactation, and there is little loss in production resulting from drying them off.
In correcting problems with oxidized flavor. If samples taken from individual cows indicate that a high proportion of the cows in the herd are producing milk in which the defect develops without metal contamination, the most practical control would be through the herd ration.
In 'trouble-shooting' causes of defects in processed milk, samples should be collected from the raw milk storage tanks and at every step in processing where feasible. A good practice is to compare 'first-off' samples with samples collected near the end of a processing run. If product from one processing line feeds several fillers, samples should be taken from each filler. As for raw milk, the samples should be tasted fresh and after storage, except that a storage period of at least a week is preferable for most pasteurized products.
Identification of the defect, using prepared reference samples for comparison if necessary, is the first step toward effective control. Corrective measures for the most common defects were indicated in the first two papers in this series, and are summarized briefly here.
Limited shelf-life resulting from bacterial growth is usually caused by post-pasteurization contamination. Phosphatase tests may be run on freshly pasteurized products to check on adequacy of pasteurization. The unclean, fruity, bitter and putrid flavors that develop in pasteurized milk are usually attributable to growth of psychrophilic bacteria that do not survive pasteurization. Hence, critical evaluation of cleaning and sanitizing procedures is necessary in order to detect sources of post-pasteurization contamination. Checks on temperature and time of storage, and rotation of stock should also be included.
Rancid flavor should not develop in a properly pasteurized product, but activating treatments at some stage of processing may induce its development before pasteurization. Possible causes include mixing pasteurized homogenized products with raw milk, warming cooled milk to about 85 F and recooling, and excessive agitation and foaming of raw milk. If the intensity of the flavor increases during storage, the possibility that the milk was improperly pasteurized or was contaminated by raw milk should be checked by use of the phosphatase test.
Problems with oxidized flavor may be caused primarily by low oxidative stability of the milk as produced, but are aggravated by abuse at any stage of processing and distribution. Particular attention should be given to avoiding copper contamination from white metal and other copper-containing alloys, and to minimizing exposure to light. Use of higher pasteurizing temperatures may be helpful if the more severe heat treatment does not produce objectionable heated flavors. Milk that is susceptible to oxidized flavor may be directed into homogenized products, as homogenization inhibits oxidized flavor. Locating causes of some atypical flavors in processed milk is sometimes a baffling assignment, even for people with extensive experience in flavor-control work. The approaches outlined above, however, provide guides to step-by-step elimination of possible causes of a defect.
The objective of quality control personnel should be more than to prevent customer complaints. Routine daily examination of all processed products, fresh and after storage, usually results in recognition of off- flavors before they reach such intensity that they become objectionable to consumers. Much of the pasteurized milk sold today could be made more palatable by selecting milk of high quality initially, and giving increased attention to protecting its flavor during processing and distribution. Maintaining consumer confidence in the dependable high quality of milk is essential for maximum consumption.
Professor Emeritus W. L. Dunkley and John C. Bruhn,
University of California,
Department Food Science and Technology,
Davis, CA 95616-8598