Milk Grading

An understanding of the principles of sensory evaluation are neccessary for grading

Sense of Taste

Taste buds are mostly on the top surface of the tongue

The taste buds, must make contact with the taste agent before a taste sensation occurs. There are four different types of nerve endings on the tongue which detect the four basic "mouth" flavours - sweet, salt, sour, and bitter. Samples must, therefore, be moved around in the mouth in order to make positive flavour identification. In addition to these basic tastes, the mouth also allows us to get such reactions as coolness, warmth, sweetness, astringency, etc.

 

The Sense of Smell

We are much more perceptive to the sense of smell than we are to taste. For instance, it is possible for a pungent material to be detected in 20 billion parts of air.

Note: A good judge does not try to examine more than one sample per minute and rinsing the mouth with water between samples may help to restore sensitivity and clean the palate

Milk Grading Techniques
Temperature should be between 60-70° F (15.5-21° C) so that any odour present may be detected readily by sniffing the container. Also, we want a temperature rise when taking the sample into the mouth; this serves to volatize any notable constituents.

Noting the odour by placing the nose directly over the container immediately after shaking and taking a full "whiff" of air. Any off odour present may be noted.

Need to make sure we have a representative sample; mixing and agitation are important.

Agitation leaves a thin film of milk on the inner surface which tends to evaporate giving off odour if present.

During sampling, take a generous sip, (milk must be heat treated if it is to be swallowed) roll about the mouth, note flavour sensation, and expectorate. Swallowing the untreated milk is not recommended.

You can enhance the after-taste by drawing a breath of fresh air slowly through the mouth and then exhale slowly through the nose. With this practice, even faint odours can be noted.

Milk has a flavour defect if it has an odour, a foretaste or an aftertaste, or does not leave the mouth in a clean, sweet, pleasant condition after tasting.

Characterization of Flavour Defects
Lipolytic or Hydrolytic rancidity
Rancidity arises from the hydrolysis of milkfat by an enzyme called the lipoprotein lipase (LPL). The flavour is due to the short chain fatty acids produced, particularly butyric acid. LPL can be indigenous or bacterial. It is active at the fat/water interface but is ineffective unless the fat globule membrane is damaged or weakened. This may occur through agitation, and/or foaming, and pumping. For this reason, homogenized milk is subject to rapid lipolysis unless lipase is destroyed by heating first; the enzyme (protein) is denatured at 55-60° C. Therefore, always homogenize milk immediately before or after pasteurization and avoid mixing new and homogenized milk because it leads to rapid rancidity.

Some cows can produce spontaneous lipolysis from reacting to something indigenous to the milk. Late lactation, mastitis, hay and grain ratio diets (more so than fresh forage or silage), and low yielding cows are more suseptible.

Lipolysis can be detected by measuring the acid degree value which determines the presence of free fatty acids. Lipolytic or hydrolytic rancidity is distinct from oxidative rancidity, but frequently in other fat industries, rancid is used to mean oxidative rancidity; in dairy, rancidity means lipolysis.

Characterized: soapy, blue-cheese like aroma, slightly bitter, foul, pronounced aftertaste, does not clear up readily

Oxidation
Milk fat oxidation is catalysed by copper and certain other metals with oxygen and air. This leads to an autooxidation reaction with c haracteristic: metallic, wet cardboard, oily, tallowy, chalky; mouth usually perceives a puckery or astringent feel

Cooked
This defect is a function of the time-temperature of heating and especially the presence of any "burn-on" action of heat on certain proteins, particulary whey proteins. Whey proteins are a source of sulfide bonds which form sulfhydryl groups that contribute to the flavour. The defect is most obvious immediately after heating but dissipates within 1 or 2 days.
Characterized: slightly cooked or nutty-like to scorched or caramelized

 

Transmitted flavours
Cows are particulary bad for transmitting flavours through milk and milk is equally as susceptible to pick-up of off flavours in storage. Feed flavours and green grass can be problems so it is necessary to remove cows from feed 2-4 hrs before milking. Weeds, garlic/onion, and dandelions can tranfer flavours to the milk and even subsequent products such as butter. Barny flavours can be picked up in the milk if there is poor ventilation and the barn is not properly cleared and cows breathe the air. These flavours are volatile so can be driven off through vacuum de-aeration.
Characterization: hay/silage, cowy/barny

Microbial
There are many flavour defects of dairy products that may be caused by bacteria, yeasts, or moulds. In raw milk the high acid/sour flavour is caused by the growth of lactic acid bacteria which ferment lactose. It is less common today due to change in raw milk microflora. In both raw or processed milk, fruity flavours may arise due to psychrotrophs such as Pseudomonas fragi. Bitter or putrid flavours are caused by psychrotrophic bacteria which produce protease. It is the proteolytic action of protease that usually causes spoilage in milk. Malty flavours are caused by S.lactis var. maltigenes and is characterized by a corn flakes type flavour. Ropy milk is caused by bacteria, specifically those which produce exopolysaccharides.

Miscellaneous Defects
astringent
chalky
chemical/medicinal - disease - associated or adulteration
flat - adulteration (water)
foreign
salty - disease associated
bitter - adulteration

Milk flavour is graded on a score of one to 10. Some flavour defects, even if only slightly present, can decrease the score drastically. The following are suggested flavour scores for milk with designated intensities of flavour defects.