Flavours and Off- Flavours Related to Milk

Flavours of foods can be divided into two categories. Natural, good or normal flavour which are desirable, or essential flavours. The other category is off-flavour which are undesirable and are not natural or normal flavours.

For consumer acceptance of any food including milk, flavour is of primary importance.Even if the food is wholesome, nutritious, attractively packaged, reasonably priced, but if its flavour is poor it will be rejected by the consumers. Milk and milk products are in the top list of foods where flavour plays most important role. Right from production of milk at the farm to the point of processing of milk into milk and milk products needs proper handling of milk to avoid flavour contamination and deterioration. Thus knowledge of flavour is indispensable in the production of milk and milk product that are consistently acceptable to consumers.

Studying flavour involves study of three fields together. These include dairy technology, chemistry and physiology. Dairy technology involves control of flavour,chemistry deals with micro-chemical changes involved in flavour development including the chemistry of compounds involved. Physiology, deals with sensory evaluation which includes odour and taste of milk and milk products.

The nature of flavours: Three basic sensory aspects are involved in flavour.They include olfactory, gustatory and tactual components. On tasting, food vapours go into the olfactory area which is related with odour of milk. Taste is concerned with sensation in the mouth and tongue. These include sweetness, sourness, saltiness and bitterness. Sensation is detected by taste buds. Actual sensation gives an index of the milk feels in the mouth. This aspect deals with the feeling aspects such as tender, grainy, etc.

Measurement of flavour: Flavour is measured psychometrically and by chemical analysis. Chemical analysis only shows the nature of component responsible for flavour. However, chemical methods cannot evaluate like or dislike of flavour. They can be differentiated only by psychometrically as stated above.

Flavour identification: Compounds responsible for a given flavour needs identification. This is essential so that off flavours may be prevented and desirable flavours can be preserved.

Flavour components are present in minute amount in (parts per million or parts per billion). These components can be isolated and identified chromatographically involving TLC (Thin Layer Chromatography), GLC (Gas Liquid Chromatography),HPLC (High Pressure Liquid Chromatography) and often combined with mass spectrophotometery. Infrared (IR) is also used in flavour analysis.

i. Flavour of milk

Flavour of milk can be divided into two categories, normal and natural flavour that are desirable. Milk should be free from oxidized, rancid, feed/weed, unclear, malty and other off flavours.

On tasting milk, one finds milk sweet and slightly salty. Normally it is neither bitter nor sour. However, these variations differs with individuals. Some detect milk as sweet, others flat and still others a slight salty. Sweet-salt taste varies with individuals.

Milk has sweet taste due to the presence of sugar lactose (4.8-5.0%). Salty taste is observed in mastitis milk due to its high chloride content. This is also true for late lactation milk.

Normal milk has a sweet characteristic flavour, which is faint in nature. At present it cannot be said conclusively what compounds cause natural flavour of milk.However, some low weight compounds definitely contribute to the flavour of milk though they are present in trace amount. They include acetone, acetaldehyde,butyric and certain other free falty acids contributing towards the flavour of milk.

A high predominance of these compounds gives abnormal flavour to milk. Normal milk has smooth feeling in mouth.A notable compound methyl sulphide present in p.p.b (parts per billion) level up to 12 p.p.b in water gives flavour similar to that of milk. This compound is present in milk and cow’s breath and significantly contributes towards the flavours of milk.

Off-flavours are referred to the flavours, which are not typical of the food such as milk and are considered undesirable. These undesirable flavours are often off shoot from undesirable compounds generated generally as post contamination.Milk is particularly susceptible to off-flavours. These generally start from the milking animal such as cow or buffalo and include feed and fodder, weeds and barn contamination. These factors may create problem for flavour of milk. Contaminations such as bacteria, metals etc., also contribute towards off-flavours. Problems of off-flavours can be eliminated or avoided by carefully considering the above factors.

ii. Chemical flavour deteriorations

a) Oxidized flavour: Oxidized flavour is one of the most important aspect of flavour deterioration of milk and milk products. It is perhaps one of the single most important factor of off-flavour of milk. Oxidized flavour is a general term and include off-flavours such as cardboard, metallic, oily and tallow.

Cause of off-flavour of milk: The component responsible for off-flavour is the fat or lipid part of milk which undergo oxidation and generates off-flavours. Phospholipids of milk serve as the origin of oxidized flavour of milk. During milk fat separation one third of the phospholipids are found in skim milk. Sweet cream buttermilk is highly susceptible to off-flavour due to high content of phospholipids in butter milk.

Mechanism of oxidized flavour development in milk: Oxidized flavour is produced from hightly unsaturated fatty acids such as linoleic acid present in phospholipids. Oxygen from the air attacks the methylene group adjacent to the double bond present in these acids, resulting in the formation of peroxides,hydroperoxide and breakdown products such as various aldehydes and ketones.One compound 2-octenal and 2-nonenal has the most characteristic oxidized property.b) Rancid Flavour (hydrolytic rancidity): Hydrolytic rancidity is caused by natural milk enzyme lipase present is milk. Lipase releases free fatty acids from glycerides of milk fat. One of the most notable fatty acid butyric acid when released causes rancid flavour

Glyceride D Glycerol +Butyric acid

Lipase is inactive in the freshly drawn milk because it is present in the serum phase of milk. However, during handling of milk lipase is absorbed on the fat which can cause lipolysis and produce rancid flavour. Amongst the factors which cause and enhance rancidity are homogenization, vigorous agitation, warming and cooling of milk.

Prevention of Lipolysis: Problems of lipolysis is the release of lower chain fatty acids of milk fat such as butyric acid which renders it unacceptable for human consumption.

Lipolysis can be reduced or avoided by inactivating lipase. For this the following parameters should be considered:

1. Pasteurization of milk destroys lipase in milk

2. Avoid excessive agitation which should not be prolonged for a long period especially accompanied by foaming

3. Homogenization of milk

4. Separation or clarification.

5. Warming milk to 80-90 0 C and cooling again to low temperature

6. Secretion of milk during advanced stage of lactation

7. Freezing and thawing of milk

8. Mixing of raw milk with cream or homogenized milk should be avoided.

c) Sunlight Flavour: When milk is exposed to direct sunlight it leads to the development of sunlight flavour which is undesirable. Sunlight off-flavour is caused due to two-reasons which are -
  •  Development of oxidized flavour as a result of light exposure.
  •  Sunlight flavour as such which is sometime refered as burnt flavour.
Methional is the sunlight off flavour compound produced due to direct exposure of sunlight. It gives off-flavour to as low as 1 part in 20 million. It is formed due to the reaction of methionine released from hydrolysis of protein in the presence of riboflavin.

Methionine + Riboflavin D Methional


Avoid exposure of milk to direct sunlight or artificial light.

iii. Heated Flavours

Milk is always subjected to heating to preserve the same. Heat is required in the preparation of milk products. Heating process involve pasteurization, forewarming,boiling, superheating and sterilization of milk.Pasteurization: Pasteurization whether holder or short-time process has hardly any effect on the development of flavour in milk. However, pasteurization is now an acceptable process for milk preservation without any change in flavour

Pre-heating/Forewarning: At or above 74 0 C distinct flavour changes occur in milk. Notable amongst is the development of cooked flavour caused by the formation of H 2 S. Hydrogen Sulfide (H 2 S) is formed by the amino-acid methionine with a lowering of oxidation-reduction potential of milk.

Superheating: When heating is carried out for 75 0 C for long period cooked flavour is changed to caramelized flavour. Chemical nature of caramelized flavour is not known. Since caramelized flavour is absent in whey it appears that casein plays an important part in caramelization.

Browning: However, caramelizing is accompanied by browning reaction. The reaction is known as Stecker degradation. This reaction occurs between the amino group of a basic amino acid such as lysine with free aldehyde group of sugar resulting in browning.

iv. Other off - flavours

Coconut Flavour: Coconut flavour originates from milk fat. Recent research shows that a compound delta lactone is formed from milk fat. It is a storage related defect and is favoured at high storage temperature rather than cooling.

Microbial flavour: Milk is a favourable medium for the growth of microbes particularly bacteria. These microbial changes are also accompanied by chemical changes. Such types of flavour changes are encountered with defects including bitter, fruity, rancid, stale and putrid-type of off-flavours.

Absorbed flavour: Absorbed flavours are those, which are other than those off-flavour caused by microbial or chemical action. These off-flavours are accidentaly absorbed in milk from several sources. These flavours can absorb either before or after milking. These flavours can enter through milking animals by the nose or mouth, to the lungs, to the blood stream, to the udder cells, and into the milk. Feeds and fodders fed to the animal are the main cause of absorbed flavours. Thus if a cow eats onion or garlic the flavour is transfered to the milk within 20 to 30 minutes. Milk can also absorb flavour from the atmosphere, improperly clean utensils and equipments. Residual disinfectant or any other odorous substance sticking to the metal surface has the potential to cause off-flavour in milk.

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