Milk can be defined as the liquid produced by a mammal’s mammary glands; it is the first source of nutrition for an infant mammal before it is able to digest other foods. While this remains largely true, of all the animals on planet Earth, only humans purposefully consume the milk of other animals. Not only do they consume the milk of other species, members of the population have adapted the ability to digest the milk beyond infancy.
Milk and Human Evolution:
In order to be able to consume milk without ill-effect, humans need to be able to digest the sugar lactose – a disaccharide consisting of a glucose molecule bound to a galactose molecule. Lactase is the enzyme responsible for the cleavage of this sugar.
Lactose intolerance (hypolactasia) is actually the natural state of human beings after weaning off of their mother’s milk. However, with agriculture and animal husbandry, animal milk became a viable source of nutrition and energy to be taken advantage of when other foods could fail.
Evolutionary theory suggests that those humans with mutations allowing them to digest lactose as adults would have had an advantage in acquiring additional nutrition. Those with the mutation to digest lactose also initially encountered less competition for food, as they would be the only adult humans able to digest milk. The rate of milk availability was also greater than that of animal meat; one animal could produce milk, requiring less energy and time than it would take to raise and slaughter an animal for food. This gave lactase-persistent humans a competitive advantage as the ability to digest milk would confer a strong selective pressure for the gene, particularly in times of starvation and famine.
With 80% of modern Europeans being lactase-persistent, rising to as much as 100% in Northern Europe, we can surmise that humans in Northern Europe were the first to become lactase-persistent, occurring approximately 5000 to 10,000 years ago. Compared to most Europeans, lactase-persistence is significantly lower in sub-Sahara Africa and southeast Asia.
The information provided below is in regards to cow’s milk, due to its consumption worldwide.
|Cow’s Milk, 1% fat, pasteurised (per 100g)|
|Energy (kcal)||41 kcal|
Cow’s milk is an excellent source of vitamin B12, containing 60% of an adult’s Daily Recommended Value (DRV), at 0.9µg per 100g. Cow’s milk is also a good source of iodine, riboflavin, calcium and phosphorus, providing 21.4%, 17.7%, 12.3% and 12.2% of the nutrients’ DRVs, respectively.
For reference, an average glass of milk will contain approximately 200g of milk.
The nutritional content of milk will, naturally, vary depend on the animal the milk is sourced from and any alterations made to the milk before consumption. For instance, goat’s milk contains significantly more vitamin D than cow’s milk.
Human Modification of Milk:
Milk has been modified by humans in an effort to meet food hygiene requirements and to satisfy customers, compared to the raw milk that our human ancestors drank and perhaps people of older generations will recall. Raw milk would now only be available directly from dairy farms. Standard milk is pasteurised, skimmed and homogenised.
Pasteurisation is the process by which a foodstuff is heat treated in order to reduce the number of pathogenic microbes present. In the UK, milk is rapidly heated to 71.7°C for 15 seconds, then quickly cooled to 3°C; the entire process is done by a heat exchanger. You may also see ultra high temperature/heat treated milk, commonly abbreviated to UHT, which is milk that has been heated to 135-150°C for 4-15 seconds, then rapidly cooled to room temperature. Pasteurisation improves the safety of milk, gives it a longer life shelf, but also reduces the amount of vitamins present in milk compared to raw milk, as vitamins are broken down by heat.
Skimming is the reduction or removal of the cream, or butter/milkfat, of whole milk. This allows the milk to have a reduced fat and energy content. In the UK, milk is found in three main varieties of skim: whole milk (blue cap), semi-skimmed (green cap) and skimmed milk (red cap). The butterfat content of each is 3.5%, 1.5-1.8% and less than 0.3%, respectively.
Finally, homogenisation is the process by which two immiscible (non-mixing) liquids are converted into an emulsion (mixed together, but not dissolved). As milk fat normally separates from the water to form a cream, homogenisation breaks the fat globules into smaller sizes so that the water and fat emulsifies. This is accomplished by mixing large amounts of milk to a constant and then forcing the milk through small holes at a high pressure. Homogenisation ensures that the flavour and fat concentration remains constant through the entirety of the milk. In the UK, milk is not legally required to be homogenised but is done so by customer demand. Jersey and Guernsey milk, colloquially known as gold cap milk, is a milk that is not homogenised for traditional reasons.
What about plant milks?
The term plant milk is something of a misnomer, as these are not true milks by definition. They are made by blending a plant nut, seed or grain with water until smooth. Plant milks offer a means to have a milk substitute suitable for consumption by the lactose-intolerant, those with animal welfare or environmental reasons, and vegetarians and vegans.Plant milks include soy, almond, coconut, rice and many others. Of note, commercial plant milks may be fortified with vitamin D and calcium, and may also be sweetened to make the milk more palatable.
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