Editing Milk (section)

== {{Anchor|Physical and chemical properties}}Composition ==
[[File:TriglycerideDairyButter.png|thumb|Butterfat is a [[triglyceride]] (fat) formed from fatty acids such as [[myristic acid|myristic]], [[palmitic acid|palmitic]], and [[oleic acid]]s.]]
Milk is an [[emulsion]] or [[colloid]] of [[butterfat]] [[globules of fat|globules]] within a water-based fluid that contains dissolved [[carbohydrate]]s and protein aggregates with minerals.<ref>Rolf Jost "Milk and Dairy Products" Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. {{doi|10.1002/14356007.a16_589.pub3}}</ref> Because it is produced as a food source for the young, all of its contents provide benefits for growth. The principal requirements are energy (lipids, lactose, and protein), biosynthesis of non-essential amino acids supplied by proteins (essential amino acids and amino groups), essential fatty acids, vitamins and inorganic elements, and water.<ref name="Fox-1995-v3">Fox, P.F. Advanced Dairy Chemistry, Vol. 3: Lactose, Water, Salts and Vitamins. 2nd ed. Chapman and Hall: New York, 1995.</ref>

===pH===
The [[pH]] of cow's milk, ranging from 6.7 to 6.9, is similar to other [[bovines]] and non-bovine [[mammals]].<ref>{{Cite journal |last1=Elbagerma |first1=Mohamed A. |last2=Alajital |first2=A. I. |last3=Edwards |first3=H. G. M. |date=September 2014 |title=A Comparative Study on the Physicochemical Parameters and Trace Elements in Raw Milk Samples Collected from Misurata- Libya |url=https://www.researchgate.net/publication/266616267 |journal=SOP Transactions on Analytical Chemistry |volume=1 |issue=2 |pages=15–23 |doi=10.15764/ACHE.2014.02002 |doi-broken-date=November 1, 2024 |s2cid=138302219 |via=ResearchGate}}</ref>

===Lipids===
{{Main|Butterfat}}
Full fat milk contains about 33 grams of fat per liter, including about 19 grams of saturated fat, 1.2 grams of omega 6 fatty acids, and 0.75 grams of omega 3 fatty acids per liter. The amount of fat varies for products where (some of) the fat has been removed, such as in skimmed milk.<ref>{{Cite journal |last1=Haug |first1=Anna |last2=Høstmark |first2=Arne T. |last3=Harstad |first3=Odd M. |date=2007-09-25 |title=Bovine milk in human nutrition – a review |journal=Lipids in Health and Disease |language=en |volume=6 |issue=1 |pages=25 |doi=10.1186/1476-511X-6-25 |doi-access=free |issn=1476-511X |pmc=2039733 |pmid=17894873}}</ref>

Initially milk fat is secreted in the form of a fat globule surrounded by a [[Milk fat globule membrane|membrane]].<ref name="Fox-1995-v2">Fox, P.F. Advanced Dairy Chemistry: Vol 2 Lipids. 2nd Ed. Chapman and Hall: New York, 1995.</ref> Each fat globule is composed almost entirely of triacylglycerols and is surrounded by a membrane consisting of complex lipids such as [[phospholipid]]s, along with proteins. These act as [[emulsifier]]s which keep the individual globules from coalescing and protect the contents of these globules from various [[enzyme]]s in the fluid portion of the milk. Although 97–98% of lipids are triacylglycerols, small amounts of di- and monoacylglycerols, free cholesterol and cholesterol esters, free fatty acids, and phospholipids are also present. Unlike protein and carbohydrates, fat composition in milk varies widely due to genetic, lactational, and nutritional factor difference between different species.<ref name="Fox-1995-v2" />

Fat globules vary in size from less than 0.2 to about 15 [[micrometre|micrometers]] in diameter between different species. Diameter may also vary between animals within a species and at different times within a milking of a single animal. In unhomogenized cow's milk, the fat globules have an average diameter of two to four micrometers and with homogenization, average around 0.4 micrometers.<ref name="Fox-1995-v2"/> The [[fat-soluble]] vitamins [[vitamin A|A]], [[vitamin D|D]], [[vitamin E|E]], and [[vitamin K|K]] along with essential fatty acids such as linoleic and linolenic acid are found within the milk fat portion of the milk.<ref name="On Food and Cooking"/>

{| class="sortable wikitable" style="margin:auto"
|+Main milk fatty acids, length, share of total<ref name="LubaryHofland2010">{{cite journal |last1=Lubary |first1=Marta |last2=Hofland |first2=Gerard W. |last3=ter Horst |first3=Joop H. |title=The potential of milk fat for the synthesis of valuable derivatives |journal=European Food Research and Technology |volume=232 |issue=1 |year=2010 |pages=1–8 |issn=1438-2377 |doi=10.1007/s00217-010-1387-3 |s2cid=85373338|doi-access=free}}</ref>
|-
! Fatty acid !! length !! mol% (rounded) 
|-
| Butyryl || C4 ||  12 
|-
| Myristyl || C14 || 11 
|-
| Palmityl || C16 || 24 
|-
| Oleyl || C18:1 || 24 
|}

===Proteins===
Normal bovine milk contains 30–35&nbsp;grams of protein per liter, of which about 80% is arranged in casein [[micelles]]. Total proteins in milk represent 3.2% of its composition (nutrition table).

====Caseins====
{{Main|Casein}}
The largest structures in the fluid portion of the milk are [[casein|"casein micelles"]]: aggregates of several thousand protein molecules with superficial resemblance to a surfactant [[micelle]], bonded with the help of nanometer-scale particles of [[calcium phosphate]]. Each casein micelle is roughly spherical and about a tenth of a micrometer across. There are four different types of casein proteins: αs1-, αs2-, β-, and κ-caseins. Most of the casein proteins are bound into the micelles. There are several competing theories regarding the precise structure of the micelles, but they share one important feature: the outermost layer consists of strands of one type of protein, [[k-casein]], reaching out from the body of the micelle into the surrounding fluid. These kappa-casein molecules all have a negative [[electrical charge]] and therefore repel each other, keeping the micelles separated under normal conditions and in a stable [[colloid]]al [[suspension (chemistry)|suspension]] in the water-based surrounding fluid.<ref name="On Food and Cooking"/><ref name="chem">{{cite web |last=Goff |first=Douglas |url=https://www.uoguelph.ca/foodscience/book-page/raw-milk-quality |title=Raw milk quality |work=Dairy Science and Technology |publisher=University of Guelph Food Science, Guelph, Ontario, Canada |access-date=February 8, 2011 |year=2010 |archive-url=https://web.archive.org/web/20141231022516/http://www.uoguelph.ca/foodscience/book-page/raw-milk-quality |archive-date=December 31, 2014 |url-status=live}}</ref>

Milk contains dozens of other types of proteins beside caseins and including enzymes. These other proteins are more water-soluble than caseins and do not form larger structures. Because the proteins remain suspended in [[whey]], remaining when caseins coagulate into curds, they are collectively known as ''whey proteins''. [[Lactoglobulin]] is the most common whey protein by a large margin.<ref name="On Food and Cooking"/> The ratio of caseins to whey proteins varies greatly between species; for example, it is 82:18 in cows and around 32:68 in humans.<ref name=Applications2017>{{cite book |title=Handbook of Milk of Non-Bovine Mammals |chapter=Potential Applications of Non-Bovine Mammalian Milk in Infant Nutrition |editor1-first=Young W. |editor1-last=Park |editor2-first=George F.W. |editor2-last=Haenlein |editor3-first=William L. |editor3-last=Wendorff |first1=Shane V. |last1=Crowley |first2=Alan L. |last2=Kelly |first3=John A. |last3=Lucey |first4=James A. |last4=O'Mahony |page=630 |date=2017 |publisher=John Wiley & Sons Ltd. |edition=2nd |doi=10.1002/9781119110316.ch13 |isbn=978-1-119-11031-6}}</ref>

{| class="sortable wikitable" style="margin:auto"
|+Ratio of caseins to whey proteins in milk of nine mammals<ref name=Applications2017/>
|-
! Species !! Ratio
|-
| Human || 29.7:70.3 – 33.7:66.3 
|-
| Bovine || 82:18
|-
| Caprine || 78:22
|-
| Ovine || 76:24
|-
| Buffalo || 82:18
|-
| Equine || 52:48 
|-
| Camel || 73:27 – 76:24
|-
| Yak || 82:18
|-
| Reindeer || 80:20 – 83:17
|}

===Salts, minerals, and vitamins===
Bovine milk contains a variety of cations and anions traditionally referred to as "minerals" or "milk salts". Calcium, phosphate, magnesium, sodium, potassium, citrate, and chloride are all included and they typically occur at concentrations of 5–40{{nbsp}}[[Molar concentration#Units|mM]]. The milk salts strongly interact with casein, most notably calcium phosphate. It is present in excess and often, much greater excess of solubility of solid calcium phosphate.<ref name="Fox-1995-v3"/> In addition to calcium, milk is a source of many vitamins: Vitamins A, B1, B2, B5 B6, B7, B12, and D.

====Calcium phosphate structure====
For many years the most widely accepted theory of the structure of a micelle was that it was composed of spherical casein aggregates, called submicelles, that were held together by calcium phosphate linkages. However, there are two recent models of the casein micelle that refute the distinct micellular structures within the micelle.

The first theory, attributed to de Kruif and Holt, proposes that nanoclusters of calcium phosphate and the phosphopeptide fraction of beta-casein are the centerpiece to micellar structure. Specifically in this view unstructured proteins organize around the calcium phosphate, giving rise to their structure, and thus no specific structure is formed.

Under the second theory, proposed by Horne, the growth of calcium phosphate nanoclusters begins the process of micelle formation, but is limited by binding phosphopeptide loop regions of the caseins. Once bound, protein-protein interactions are formed and polymerization occurs, in which K-casein is used as an end cap to form micelles with trapped calcium phosphate nanoclusters.

Some sources indicate that the trapped calcium phosphate is in the form of Ca<sub>9</sub>(PO<sub>4</sub>)<sub>6</sub>;
whereas others say it is similar to the structure of the mineral [[brushite]], CaHPO<sub>4</sub>·2H<sub>2</sub>O.<ref>[http://www.foodsci.uoguelph.ca/dairyedu/chem.html chemistry and physics] {{Webarchive|url=https://web.archive.org/web/20060614214133/http://www.foodsci.uoguelph.ca/dairyedu/chem.html |date=June 14, 2006 }}. Foodsci.uoguelph.ca. Retrieved December 9, 2011.</ref>

===Sugars and carbohydrates===
[[File:Hydrolysis of lactose.svg|thumb|A simplified representation of a [[lactose]] molecule being broken down into [[glucose]] (2) and [[galactose]] (1)]]
Milk contains several different [[carbohydrate]]s, including [[lactose]], [[glucose]], [[galactose]], and other [[oligosaccharide]]s. The lactose gives milk its sweet taste and contributes approximately 40% of the calories in whole cow's milk's. Lactose is a disaccharide composite of two [[Monosaccharide|simple sugars]], [[glucose]] and [[galactose]]. Bovine milk averages 4.8% anhydrous lactose, which amounts to about 50% of the total solids of skimmed milk. Levels of lactose are dependent upon the type of milk as other carbohydrates can be present at higher concentrations than lactose in milks.<ref name="Fox-1995-v3"/>

===Miscellaneous contents===
Other components found in raw cow's milk are living [[white blood cell]]s, mammary gland cells, various bacteria, vitamin C, and a large number of active enzymes.<ref name="On Food and Cooking"/>

===Appearance===
Both the fat globules and the smaller casein micelles, which are just large enough to deflect light, contribute to the opaque white color of milk. The fat globules contain some yellow-orange carotene, enough in some breeds (such as Guernsey and Jersey cattle) to impart a golden or "creamy" hue to a glass of milk. The [[riboflavin]] in the whey portion of milk has a greenish color, which sometimes can be discerned in skimmed milk or whey products.<ref name="On Food and Cooking"/> Fat-free skimmed milk has only the casein micelles to scatter light, and they tend to scatter shorter-wavelength blue light more than they do red, giving skimmed milk a bluish tint.<ref name="chem"/>