Sensory characteristics are properties of milk that the human senses can feel. Although instrumental analyses are needed to precisely determine milk’s properties, sensory analysis also provides important indications for the expert.
The sensory characteristics of milk are color, taste and odor, consistency and structure. In this article, the properties of milk in its natural state and the situations in which these properties deviate from naturalness have been examined.
1. Color
The color of the milk is defined as “porcelain white.” This color emerges due to the effects of water, fat, and protein, which are the main components of milk, and color pigments such as β-carotene (provitamin A) and riboflavin (vitamin B2).
Fat and β-carotene are responsible for the yellowness in milk. Yellowness decreases as the amount of fat or β-carotene decreases. If, some or all of its fat is removed, the milk turns white. An increase in the amount of fat or β-carotene in milk makes the milk more yellow.
β-carotene is found in high amounts in green feeds; therefore, the milk of animals fed on pastures is yellower. On the other hand, buffaloes convert β-carotene to high levels of vitamin A in their body, and therefore, their milk is whiter than cattle fed on the same pasture.
Riboflavin gives milk a greenish-yellow color. Riboflavin is water-soluble, and the effect of riboflavin on color is seen in whey. The reason for the greenish-yellow color of whey is riboflavin.
Water gives the milk a bluish color. Milk with increased water content becomes bluish. This color can be seen where the milk’s surface intersects the glass or container. For example, a milk master takes the milk in his hand and sees whether water is added to the milk by looking at the degree of blueness of the part where the milk comes into contact with his palm. As I said, it takes skill to understand this.
Years ago, in 2012, when one of my masters took the milk in his hand at the dairy factory where I worked, did it get removed fat? How much has been removed? Did it get added water? How much has been added? He would understand immediately. When I asked how he understood this, he had replied “Don’t you see?”. I was looking but was not understanding. He had gave to the milk his 30 years… I’m still not as good at understanding today as he is.
In addition, if the animal is sick, the udder is injured, the milk is not properly stored or handled, and unusual colors such as reddish, brownish, or excessive yellow can be seen in the milk.
On the other hand, the processes applied to the milk can also affect the color of the milk. High heat treatment of milk causes browning, while the homogenization process causes milk to whiten.
2. Taste and Odor
Milk has a unique taste and odor. The main components, especially lactose and fat, protein, mineral content, and some components*, provide milk’s characteristic taste and odor.
*Some components; acetoin, acetaldehyde, diacetyl, acetic acid, lactate, aromatic fatty acids with low carbon numbers such as butyric acid, components formed as a result of the activity of lipase and protease enzymes, and other components such as lactic acid formed as a result of the microbial activity.
Taste and odor are more dominant in milk with high dry matter content.
The fat content of milk is highly variable, and this causes the taste and odor of milk to vary. Milk with a high-fat content is more delicious. Many factors, especially the season, animal breed, and ration used in feeding the animal, cause fluctuations in the fat ratio of milk.
On the other hand, milk fat absorbs aroma substances and, therefore, can attract the odor of the environment it is in. In this context, milk odors like the animal from which it was milked, which is quite natural. Similarly, the odor of the barn may permeates the milk. These odors are quite natural, and if proper storage and cleaning conditions are not provided, the excess of these odors will be disturbing.
You can observe the odor-attracting feature of milk fat best when the butter is stored in the refrigerator. Butter left open in the refrigerator, if there is an odorous food in the refrigerator, quickly absorbs the odor of that food, which is usually not a very pleasant situation.
Milk is a slightly sweet food due to lactose. As it has been repeatedly mentioned in other articles before, lactose is broken down in lactose-free milk. Since the total sweetness of the resulting glucose and galactose is higher than lactose, lactose-free milk is sweeter than normal milk.
The taste and odor of milk are highly affected also by the feed the animal is fed. For example, the taste and odor of the milk of animals grazing on a pasture with thyme in its flora is priceless. On the other hand, feeding the animals with plants such as cabbage, beets, radishes, leeks, onions and garlic is undesirable because the taste and odor of milk are adversely affected.
The lipase enzyme, which is naturally present in the milk, breaks down the fat molecules and the aromatic fatty acids with low carbon numbers, which are released as a result of this breakdown, affect the taste and odor of the milk. The released butyric acid is bitter; caproic, caprylic, and capric acid imparts a goaty taste and odor.
The salty taste of milk is due to the change in the mineral balance in its content. This is usually observed in animals with mastitis, which is exacerbated by the degree of disease. The salty taste is also seen in the milk of animals approaching the end of the lactation period.
The activities of microorganisms in milk are very effective in the taste and odor of milk. As a matter of fact, as the most obvious effect, some microorganisms break down lactose (lactic acid fermentation), and the lactic acid released gives the milk a sour taste. As the microorganism activities continue, more lactic acid accumulates in the environment, and the milk taste becomes more sour. Of course, this situation is undesirable in milk. However, the opposite is desirable in yogurt.
The effect of microorganisms is not limited to just forming lactic acid. Lipolytic and proteolytic activity, and citrate metabolism, exhibited by some microorganism species changes the taste and odor of milk in desired or undesirable ways.
Components such as acetaldehyde (the molecule that gives yogurt its characteristic flavor), diacetyl (the molecule that gives butter its characteristic flavor), and acetoin, which are formed as a result of citrate metabolism, strengthen the taste and odor of milk.*
*An important detail here is that the pH of the milk must be lower than 6.0 for microorganisms to absorb the citrate found in milk. The pH of a natural, fresh milk is around 6.6-6.8. It is considerably higher than the values at which microorganisms can operate the citrate metabolism. Therefore, the formation of these components that affect odor and taste in fresh milk is minimal. However, when the accumulated lactic acid lowers the pH and reaches pH 6.0, these aromatic components begin to form in high amounts. The limit pH value at which milk can be processed into drinking milk or fresh cheese is 6.4. Therefore, these aromatic components are found in minimal amounts in dairy products such as drinking milk and fresh cheese. It is at a very high level in yogurt (pH 4.2) and ripened cheeses (p.H < 4.5), which is desirable.
On the other hand, microorganisms’ proteolytic and lipolytic activity gives milk an undesirable bitter taste.
Exposure of milk to light and oxygen causes oxidation of unsaturated molecules, negatively affecting taste and odor. Similarly, heavy metal contamination of milk causes the taste and odor of milk to be adversely affected due to oxidation.
Processes applied to milk can also affect the taste and odor of milk. The high heat treatment applied to the milk causes the cooked taste, while the homogenization process increases the flavor of the milk.
3. Consistency and Structure
As it is known, oil is insoluble in water and does not form a homogenus solution with water. This physical feature of fat is also seen in milk, and since it is not soluble in water and its density is lower than water, milk fat begins to collect on the surface of the milk after a certain time.
As a result, a yellow and relatively hard oil layer, which we call “cream”, forms on the upper part. Since the lower part of the milk is less fat, it appears whiter than in its original form. This is the natural structure and appearance of milk.
However, the “homogenization” process applied in the industry ensures that the fat globules are reduced and prevents the fat from accumulating above and forming a cream layer. In homogenized milk, fat does not collect on the surface of the milk and is homogeneously distributed throughout the milk.
Since milk is denser than water, it is not as fluid a food as water. Therefore, the fact that milk exhibits a fluidity close to water may indicate that water has been added to milk for fraud.
Some factors denature the milk’s proteins; in this case, milk loses its natural appearance. In general, serum proteins are sensitive to temperature and denature under the influence of temperature. When milk is boiled, the white part that sticks to the pot is mostly serum proteins.
Casein, conversely, is more stable against temperature than serum proteins, but it may denatured by the effect of enzymes, high temperature, or acid. This situation causes the milk to move away from its natural appearance and structure.
The curdling of milk when boiled is due to the denaturing of casein under the effects of acid and temperature. Depending on the degree of denaturation, tiny filaments can be seen in milk, a downright solid structure and serum separation can also be seen. (For more detailed information on the structure and classification of milk proteins, see Milk; Definition, Composition and Nutrition).
Some microorganisms can also form small filament-like structures in milk. Tiny filaments seen on the surface of milk are caused by bacteria such as Alcaligenes viscolactis and Micrococcus freudenreichii.
The strands formed in the whole milk can be caused by many bacterial species, especially coliform bacteria. (Microorganisms can affect the color of milk as well as its structure. For more detailed information on the effects of microorganisms in milk, see Microorganisms in Raw Milk; Species, Effects and Importance).
The fact that milk is more fluid or viscous than average and has a sandy or rough texture indicates cheating or a problem and is not natural.
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