​Proteins and lipids are organized in spherical structures called micelles, such that the outside of the structure is hydrophilic and the inside is hydrophobic. Milk is an emulsion of milkfat globules and casein (protein) micelles in an aqueous environment. These properties are responsible for milk’s opaque white color as the emulsion scatters/deflects the full spectrum of light. Similarly, skim milk may have a blueish color as fat removal results in scattering the blue range of the spectrum. Food scientist Patrick Polowsky, notes that milk is primarily composed of water (87.4%) with the remaining (12.6%) of milk being composed of solids [2]. These solids, as noted and described in Hill and Ferrer (2021), in the chapter Raw milk composition and Quality are composed of and influenced by the following [3]

Ingredients: Milk

Composition of Milk Solids

• Lactose: 38.1%
  
• Fat: 29.4%
  
• Protein (casein 22.2% and whey 4.8%)
  
• Vitamins and Minerals [5]
  

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Composition of Milk Solids

Lactose: 38.1%
The only milk sugar, this disaccharide made up of glucose and galactose.

Influence on Cheese
Lactic acid bacteria (LAB), the primary cheese fermentation organism, utilize lactose as energy by secreting the lactase enzyme which hydrolyzes the molecule. As the metabolism of lactose by lactic acid bacteria is slow, increasing aging lowers the lactose in the resulting cheese. The LAB in cheesemaking can be inoculated, as is the case with starter cultures, or spontaneously occurring.

Once metabolized, lactic acid can then be converted into [4]:

• DL-Lactate (crystals) by Non-starter lactic acid bacteria.
• Formate, acetate, and CO2 by Non-starter Lactic Acid bacteria
• Propionate, acetate, CO2, Water by Propionic bacteria
• Butyrate, and hydrogen gas by Clostridium.

     A human’s digestive tract also produces lactase, with low levels of production resulting in lactose intolerance or sensitivity. When this occurs, lactic acid bacteria in the colon ferment the lactose and create gas.
     For more on lactose intolerance: Foster, P. (2019, April 11). Can changing the microbiome reverse lactose intolerance?: 2019 News: News: News & Events: Department of Biology: Indiana University Bloomington. Indiana University Bloomington Biology.  Retrieved April 24, 2023, from biology.indiana. edu/news-events/news/2019/foster-lactose-intolerance.html

Fat: 29.4%
Fat exists in milk as complex, globular structures with multiple layers and membranes. The homogenization process breaks up the larger fat globules, thereby distributing the fat in the aqueous solution and reducing the likelihood that it forms a “cream line” by rising to the top.

Influence on Cheese
Important milk fat properties include:

• Fatty acid diversity as milk fat contains over 50 fatty acids that are 4–22 carbons in length. This makes it the most diverse of all natural fats. Of the short-chain fatty acids, butyric acid (4 carbons in length) in particular is the cause of the dairy’s primary flavor. However, it is often lost during cheesemaking and added back in by the metabolism of LAB. For more see the section on “Cheese Flavor Compounds”.
• Major spoilage reactions in milk fat include the decomposition of the fat’s triglycerides, which produces a rancid flavor because of excessive amounts of fatty acids, and the oxidation of unsaturated fats, which creates an oxidized/flat/cardboard flavor.
• Though white when in solution, cow milk fat is typically yellow due to the beta-carotene pigments from eating grass. Goat and sheep milk is typically white due to its ability to metabolize beta-carotene into Vitamin A.

Protein (casein 22.2% and whey 4.8%)
Casein and whey are the two most important proteins in milk.

Influence on Cheese

• Casein forms the backbone of cheese structure - the micelle. There are various types of casein including: alpha s-1 (αs1), alpha s-2 (αs2), beta (β), and kappa (κ). Of these, κ-casein is the most important as it forms a “hairy” surface around the casein micelle, thereby preventing these micelles from sticking together. Converting milk into cheese requires the manipulation of these κ-casein hairs in a way that allows the proteins to coagulate into curds (see “Coagulation” section) by “clotting”. This can be done by the usage of enzymes, acidification, or the usage of heat, and will be discussed in later sections.
• Whey is typically removed from cheese as it denatures when heated and ends up in the aqueous portion of the milk (curds). Beta-lactoglobulin, which constitutes ~ 40% of whey protein, readily reacts with κ-casein at temperatures greater than 65°C thereby interfering with rennet coagulation (Hill and Ferrer, Raw Milk Quality). This results in high-cook-temperature cheeses like ricotta having a higher percentage of whey than other styles of cheese.

Vitamins and Minerals [5]

• Minerals: Calcium, in the form of calcium phosphate, is significant because it cross-links casein proteins, providing the structure of the cheese. However, beyond the phosphate bound directly to casein, some is freely floating in the milk (soluble), and some form insoluble clusters with calcium called colloidal calcium phosphate.
• Vitamins: As ~90 % of the milk fat is retained in the cheese curd, fat- soluble vitamins like Vitamins A, D, E, and K and their precursors like beta-carotene are also retained. However, water-soluble vitamins are expelled from the whey during curd manufacture. Microbial synthesis of water-soluble B-Vitamins, like Vitamin B12 synthesized by propionic acid bacteria in Swiss- style cheeses, may occur during ripening.

Cheese Production
Cheese production's goal is to concentrate casein (milk protein) and milk fat by the removal of whey proteins, other milk proteins, and lactose.

Factors Influencing Milk Composition
A common practice in cheesemaking is the standardization of milk to adjust the afore- mentioned key components so the resulting liquid meets the requirements of the intended cheese style. However, variations in milk can still influence the final flavor. Species The approach to cheesemaking generally remains the same for cows and goats, while sheep and buffalo have higher concentrations of solids that require some technique modifications. Additionally, its influence on flavor will depend on the cheesemaking technique as the flavor of fresh cheeses is more correlated to the flavor of the milk compared to aged cheese, which are more related to fat and protein content. Goat milk (versus cow milk) [6], [7]

• Fat: Goat milk’s higher concentration of fat and smaller fat globules allow higher fat recovery and potentially a smoother texture than larger fat molecules that have a heavier mouthfeel and are harder to digest. This higher fat content also means goat milk cheese is more influenced by lipolysis (fat breakdown).
• Protein: Goat milk is typically low in αS1-casein; therefore, the milk does not gel as easily as cow milk. This makes it more suitable for varieties with easily crumbling curds like Feta, but less suitable for varieties strongly dependent on the breakdown of αS1-casein by rennet for typical flavor development like Cheddar.
• Lactose: Typically contains less lactose making it potentially more suitable for those with lactose sensitivities.
• Vitamins and minerals: Goat milk has typically high levels of minerals and vitamins like A, E, K, B6, and B3 (niacin). However, goat milk typically contains less vitamin D and riboflavin.

• Fatty Acid Composition by Species [8]
  
• Stage of Lactation
• Feed [12] [13]

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Fatty Acid Composition by Species [8]
In fresh cheeses, due to their higher water content, short (C:4–10) and medium-chain (C:11–15) fatty acids fatty acid composition is influential to the final flavor, particularly of fresh cheeses. In lower water content cheeses many of the animal produced fatty acids are water soluble and have less impact. In particular, milk from sheep and goats contains higher levels than that of cows.

• Cow milk: 11% short-chain fatty acids.
• Water Buffalo (originally from Asia): SFAs (65–75 g/100 g of total FAs) are comparable to cow milk. • Goat milk: At least 20% of the FAs in goat milk are short-chain FAs, and the content of medium-chain FAs is relatively high. Of these, Butyric (C4:0), Caproic (C6:0), Caprylic (C8:0), Capric (C10:0), Lauric (C12:0), Myristic (C14:0), Palmitic (C16:0), and Linoleic (C18:2) acids are particularly influential to goat milk’s flavor [8].
• Sheep milk: They have significantly higher levels than cow’s milk of Caproic (C6:0), Caprylic (C8:0), Capric (C10:0), and Lauric (C12:0) acids, and higher levels that than other ruminant milk of butyric acid (C4:0) and ω-3 Fatty acid [8].

Animal Genetics/Breed
Historically breeding has focused on higher milk fat content. Development for higher protein content has been more recent, however, protein typically increases with increases in fat. Additionally, the breed can influence variations in caseins and whey proteins. Of the common breeds of cow in the United States:

• Breeds like Jersey are popular as their milk has high concentrations of fats and proteins.
• The k-CN β allele is associated with faster coagulation, firmer curd, and improved cheese yield [9] [10].

Stage of Lactation
Cows produce milk for ~305 days after calving. During that period, the milk’s contents change.

• First 60 days: Production increases and fat content decreases.
• Thereafter: Production gradually decreases and fat and protein content increases, though fat content varies more than the protein content. Protein distribution also changes with αs1-caseins decreasing during lactation while the proportion of β-casein increases.

Season
The protein to fat ratio changes by the season. However, the “ideal” PF ratio varies by the type of cheese being produced. For this reason, adjustments may need to be made to the milk.

• Summer: Higher protein/fat ratios
• Winter: Lower protein/fat ratios
  

​For more on the protein to fat ratios of various kinds of cheese: Hill, A., & Ferrer, M. A. (2021). Standardization of Milk for Cheese Making – Cheese Making Technology e-Book. University of Guelph Open Books. Retrieved April 24, 2023, from https://books.lib. uoguelph.ca/cheesemakingtechnologyebook/chapter/3-3-standard- ization-of-milk-for-cheese-making/

Feed [12] [13]
When comparing the fatty acid profile of milk from cows managed under three feeding systems in the United States Ritz et al (2021) found:

"Conventional" cows are fed rations in which forage-based feeds ac- count for an estimated 53 percent of daily dry matter intake, with the other 47 percent coming from grains and concentrates. Conven- tional management accounts for over 90 percent of the milk cows on U.S. farms. This feed type, primarily composed of high-energy food sources like grain, encourages lower milk fat content with little decrease in protein content [3]

"Organic" cows receive, on average, about 80 percent of their dai- ly dry matter intake from forage-based feeds and 20 percent from grain and concentrates. "Grassmilk" cows which essentially receive a 100% organic grass and legume forage-based diet, via pasture and stored feeds like hay and silage resulted by far in the highest level of omega-3s (0.05 grams per 100 grams of milk), compared to 0.02 g/100 g in conventional milk.

Grassmilk also had 52 percent less omega-6 than conventional milk, and 36 percent less omega-6 than organic milk. Regional and seasonal variations were observed:

• The highest levels of omega-3 in grassmilk were from the Midwest (1.60%) and Northeast (1.58%). California had the lowest (1.40%).
• The Midwest and Northeast had the highest concentrations of omega-6 in grassmilk.
• The omega-6/omega-3 ratio was the highest in July while cows were on pasture and were lowest in December.
• Seasonal variations may be due to climate conditions that are most extreme during drought or flooding. The duration of the grazing period also impacts forage quality, as does management attention to sustaining a good mix of grasses and legumes in pastures.

For more on Milk Composition and Quality: Hill, A., & Ferrer, M. A. (2021, January 1). Raw milk composition and Quality. Cheese Making Technology eBook. Retrieved April 16, 2023, from https://books.lib.uoguelph.ca/cheesemakingtechnologyebook/ chapter/raw-milk-quality/​

​Ingredients – Microbial inoculants [14]

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