The sensory characteristics of milk are the properties we perceive through our senses. Although instrumental analyses are needed to determine its properties precisely, sensory assessment also provides valuable clues to a trained evaluator.
This article examines milk’s natural characteristics and the circumstances in which they change.
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 a high proportion of β-carotene into 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.
In 2012, at the dairy factory where I worked, an experienced colleague could tell simply by holding milk in his palm whether cream had been removed or water added—and roughly how much. When I asked how he could tell, he would say, “Can’t you see?” I could see it, but I could not interpret it. He had spent 30 years working with milk. Even today, I am still not as skilled as he was.
Unusual colors, including reddish or brownish hues or excessive yellowing, may occur if the animal is ill, the udder is injured, or the milk is stored or processed improperly.
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, short-chain fatty acids 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.
Milk may therefore take on the smell of the animal from which it was obtained, which is natural. Barn odors can also pass into milk. Without proper storage and hygiene, these odors can become excessive and unpleasant.
Butter stored in a refrigerator illustrates how milk fat absorbs odors. If butter is left uncovered near a strong-smelling food, it can quickly absorb that food’s odor, usually with unpleasant results.
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 also strongly affected by the animal’s diet. 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 short-chain fatty acids, 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 acids impart a goaty taste and odor.
A salty taste results from changes in the balance of minerals in milk. It is commonly observed in milk from animals with mastitis, becoming more pronounced as the disease becomes more severe. It can also occur toward the end of lactation.
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 tastes increasingly 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 change 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 (pH < 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
Fat does not dissolve in water to form a true solution. This property is also apparent in milk: because milk fat is insoluble in water and less dense than it, the fat gradually rises to the surface. A yellow, firm layer of cream forms on top. The milk underneath contains less fat and appears whiter than it did initially. This is milk’s natural structure and appearance.
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 as a form of adulteration.
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 be 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 fully coagulated mass 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.
Ropiness occurring throughout the 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).
Milk that is unusually thin or thick, or has a sandy or rough texture, shows signs of adulteration or another problem rather than its normal characteristics.
References
Akgül, H. İ. (2020). Tereyağlarından ekzopolisakkarit (EPS) üreten laktik asit bakterilerinin izolasyonu ve tereyağı üretiminde starter kültür olarak kullanılabilirliğinin araştırılması. Atatürk Üniversitesi, Fen Bilimleri Enstitüsü.
Akgül, H. İ. (2015). Trabzon tereyağlarının bazı kalite parametrelerinin belirlenmesi. Atatürk Üniversitesi, Fen Bilimleri Enstitüsü.
Metin, M. (2014). Süt Teknolojisi: Sütün Bileşimi ve İşlenmesi (13. baskı). Ege Üniversitesi Basımevi.
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Halil İbrahim Akgül
Halil Ibrahim Akgul, Ph.D., is an Assistant Professor of Food Engineering at Bayburt University, where he works in the Division of Food Sciences within the Faculty of Engineering. He received his B.Sc. in Food Engineering from Akdeniz University in 2010, followed by an M.Sc. and a Ph.D. in Food Engineering from Atatürk University in 2015 and 2020, respectively. His formally recognised academic fields are Food Sciences and Engineering, Food Technologies, Dairy and Dairy Products Technology, and Food Microbiology.
Dr. Akgul has more than fifteen years of professional, research, and academic experience in food science. Between 2011 and 2013, he worked as the food engineer responsible for operations at the Pilot Dairy and Bread Factory of Atatürk University’s Directorate of Agricultural Enterprises. This industrial experience, combined with his subsequent academic research, gave him extensive practical and scientific knowledge of milk processing, dairy production, food quality, hygiene, and product development.
His research is primarily focused on milk and dairy products, dairy microbiology, starter cultures, lactic acid bacteria, exopolysaccharides, butter technology, and the physicochemical, microbiological, sensory, and nutritional properties of foods. His studies have examined the quality and fatty-acid composition of traditional butter, aroma formation in butter produced with different starter cultures, raw-milk quality, the characterisation and antioxidant activity of bacterial exopolysaccharides, traditional dairy products such as çökelek and karın kaymağı, colostrum powder, and the use of fruit-derived ingredients in ice cream production.
His academic record includes peer-reviewed research articles published in national and international journals, including Mljekarstvo, Acta Scientiarum Technology, the Journal of Food Safety and Food Quality, the International Journal of Food Engineering, the Atatürk University Journal of Agricultural Faculty, and ATA Food Journal. Four of the research articles currently listed in his university profile were published in SCI-Expanded-indexed journals. He has also contributed to twelve national and international conference papers and presentations covering dairy products, food microbiology, butter quality, fatty-acid composition, starter cultures, exopolysaccharide-producing bacteria, colostrum, traditional foods, and functional ice-cream formulations.
Dr. Akgul has participated as a researcher or adviser in eleven completed scientific and applied research projects. These projects have addressed the modernisation of pilot dairy-processing facilities; the production and quality characteristics of colostrum powder; the physicochemical and microbiological quality of butter and raw milk; the identification of exopolysaccharide-producing lactic acid bacteria; the development and production optimisation of starter cultures for mould-ripened cheese; the standardisation of traditional dairy products; and the use of sloe berry and persimmon fibre in ice-cream production.
His teaching experience spans undergraduate, associate-degree, and graduate education. The courses he has taught include Food Chemistry, Analytical Chemistry, General Chemistry, Organic Chemistry, Fermented Dairy Products Technology, Butter Production and Processing Technology, Healthy Living and Nutrition, Organic Agriculture and Food, Popular Science, Digital Literacy, and Information Technologies. Through these courses, he combines fundamental food chemistry and engineering principles with dairy technology, nutrition, scientific literacy, and the practical communication of scientific knowledge.
He is the founder and Editor-in-Chief of Gıda ve Sağlık, a multilingual digital publication devoted to food, nutrition, health, food chemistry, food microbiology, and culinary science. Through this platform, he draws upon his academic background, teaching experience, research activity, and practical knowledge of food production to present scientifically grounded information in a detailed and accessible form for an international readership.