Milk Grading and Defects

Milk Grading and Defects and soft palates of young people. These buds, about 900 in number, The importance of milk grading lies must make contact with the in the fact that dairy products are flavouring agent before a taste only as good as the raw materials sensation occurs. Saliva, of course, from which they were made. It is is essential in aiding this contact. important that dairy personnel have There are four different types of knowledge of sensory perception nerve endings on the tongue which and evaluation techniques. The detect the four basic "mouth" identification of off-flavours and flavours - sweet, salt, sour, and desirable flavours, as well as bitter. Samples must, therefore, be knowledge of their likely cause, spread around in the mouth in order should enable the production of high to make positive flavour quality milk, and subsequently, high identification. In addition to these quality dairy products. basic tastes, the mouth also allows us to get such reactions as coolness, Milk Grading warmth, sweetness, astringency, etc.

 Sense of Taste The Sense of Smell  Sense of Smell  Techniques We are much more perceptive to the sense of smell than we are to taste. Milk Defects For instance, it is possible for an odouriferous material such as  Lipolyzed mercaptain to be detected in 20  Oxidation billion parts of air. The centres of  Sunlight olfaction are located chiefly in the  Cooked uppermost part of the nasal cavity.  Transmitted To be detectable by smell, a  Microbial substance must dissolve at body temperature and be soluble in fat Milk Grading solvents. An understanding of the principles of sensory evaluation is necessary for Note: The sense of both taste and grading. All five primary senses are smell may become fatigued during used in the sensory evaluation of steady use. A good judge does not dairy products: sight, taste, smell, try to examine more than one touch and sound. The greatest sample per minute. Rinsing the emphasis, however, is placed on mouth with water between samples taste and smell. may help to restore sensitivity.

The Sense of Taste Milk Grading Techniques

Taste buds, or receptors, are chiefly Temperature should be between 60- on the upper surface of the tongue, 70 °F (15.5 – 21 °C) so that any but may also be present in the cheek odour present may be detected

1 readily by sniffing the container. produced, particularly butyric acid. Also, we want a temperature rise LPL can be indigenous or bacterial. when taking the sample into the It is active at the fat/water interface mouth; this serves to volatize any but is ineffective unless the fat notable constituents. globule membrane is damaged or weakened. This may occur through Noting the odour by placing the nose agitation, and/or foaming, and directly over the container pumping. For this reason, immediately after shaking and taking homogenized milk is subject to rapid a full "whiff" of air. Any off odour lipolysis unless lipase is destroyed present may be noted. by heating first; the enzyme (protein) Need to make sure we have a is denatured at 55-60° C. Therefore, representative sample; mixing and always homogenize milk immediately agitation are important. before or after pasteurization and avoid mixing new and homogenized Agitation leaves a thin film of milk on milk because it leads to rapid the inner surface which tends to rancidity. evaporate giving off odour if present. Some cows can produce spontaneous lipolysis from reacting During sampling, take a generous to something indigenous to the milk. sip, roll about the mouth, note flavour Late lactation, mastitis, hay and sensation, and expectorate. grain ratio diets (more so than fresh Swallowing milk is a poor practice. forage or silage), and low yielding Can enhance the after-taste by cows are more susceptible. drawing a breath of fresh air slowly Lipolysis can be detected by through the mouth and then exhale measuring the acid degree value slowly through the nose. With this which determines the presence of practice, even faint odours can be free fatty acids. Lipolytic or hydrolytic noted. rancidity is distinct from oxidative rancidity, but frequently in other fat Milk has a flavour defect if it has an industries, rancid is used to mean odour, a foretaste or an aftertaste, or oxidative rancidity; in dairy, rancidity does not leave the mouth in a clean, means lipolysis. sweet, pleasant condition after Characterized: soapy, blue-cheese tasting. like aroma, slightly bitter, foul, pronounced aftertaste, does not clear up readily Characterization of Flavour Defects - ADSA Oxidation

Lipolytic or Hydrolytic rancidity Milk fat oxidation is catalysed by copper and certain other metals with Rancidity arises from the hydrolysis oxygen and air. This leads to an of milk fat by an enzyme called the autooxidation reaction consisting of lipoprotein lipase (LPL). The flavour initiation, propagation, termination. is due to the short chain fatty acids

2 RH --- R + H initiation - free radical but tend to diminish after storage of several days. R + O2 ---- RO2 propagation Characterized: burnt-protein or burnt-feathers-like, "medicinal"-like RO2 + RH --- ROOH + R flavour

R + R --- R2 termination Cooked

R + RO2 --- RO2R This defect is a function of the time- temperature of heating and It is usually initiated in the especially the presence of any "burn- phospholipids of the fat globule on" action of heat on certain membrane. Propagation then occurs proteins, particularly whey proteins. in triglycerides, primarily double Whey proteins are a source of bonds of unsaturated fatty acids. sulfide bonds which form sulfhydryl During propagation, peroxide groups that contribute to the flavour. derivatives of fatty acids accumulate. The defect is most obvious These undergo further reactions to immediately after heating but form carbonyls, of which some, like dissipates within 1 or 2 days. aldehydes and ketones, have strong flavours. Dry feed, late lactation, Characterized: slightly cooked or added copper or other metals, lack nutty-like to scorched or caramelized of vit E (tocopherol) or selenium (natural antioxidants) in the diet all Transmitted flavours lead to spontaneous oxidation. It can be a real problem especially in Cows are particularly bad for winter. Exposure to metals during transmitting flavours through milk processing can also contribute. and milk is equally as susceptible to Characterized: metallic, wet pick-up of off flavours in storage. cardboard, oily, tallowy, chalky; Feed flavours and green grass can mouth usually perceives a puckery be problems so it is necessary to or astringent feel remove cows from feed 2-4 hrs before milking. Weeds, garlic/onion, Sunlight and dandelions can transfer flavours to the milk and even subsequent Often confused with oxidized, this products such as butter. Barny defect is caused by UV-rays from flavours can be picked up in the milk sunlight or florescent lighting if there is poor ventilation and the catalyzing oxidation in unprotected barn is not properly cleared and milk. Photo-oxidation activates cows breathe the air. These flavours riboflavin which is responsible for are volatile so can be driven off catalyzing the conversion of through vacuum de-aeration. methionine to methanol. It is, therefore, a protein reaction rather Characterization: hay/silage, than a lipid reaction. However, the cowy/barny end product flavour notes are similar

3 Microbial also caused by bacteria, specifically those which produce There are many flavour defects of exopolysaccharides. dairy products that may be caused by bacteria, yeasts, or moulds. In Miscellaneous Defects raw milk the high acid/sour flavour is caused by the growth of lactic acid  astringent bacteria which ferment lactose. It is  chalky less common today due to change in  chemical/medicinal - disease - raw milk microflora. In both raw and associated or adulteration processed milk, fruity flavours may  flat - adulteration (water) arise due to psychrotrophs such as  foreign Pseudomonas fragi. Bitter or putrid  salty - disease associated flavours are caused by  bitter - adulteration psychrotrophic bacteria which produce protease. It is the proteolytic Milk flavour is graded on a score of action of protease that usually one to 10. Some flavour defects, causes spoilage in milk. Malty even if only slightly present, can flavours are caused by S. lactis var. decrease the score drastically. The maltigenes and is characterized by a following are suggested flavour corn flakes type flavour. Although scores for milk with designated more of a tactile defect, ropy milk is intensities of flavour defects.

Flavour Intensity of Defect Criticisms Slight Definite Pronounced ------Astringent 8 7 5 Barny 7 5 3 Bitter 7 5 3 Cooked 9 8 6 Cowy 6 4 1 Feed 9 7 5 Flat 9 8 7 Foreign 5 3 0 Garlic/onion 5 3 1 High acid 3 1 0 Bacterial 5 3 0 Lacks Freshness 7 5 3 Malty 7 5 3 Oxidized 7 5 3 Rancid 7 5 3 Salty 8 6 4 Unclean 7 5 3