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seeq

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About seeq

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    Bicester, Oxfordshire
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    Cooking, Photography, Coffee, Physics
  1. I thought I would share with you some of my true geek side. I am a keen cook at home and very interested in the science behind it (you could call me Heston 2!) and I think that's partly what has sparked my interest in coffee, since some of the science can be useful. Last night, while working a 12 hour night shift I was thinking to myself what is the science behind milk foam? After a few coffee's (dissapointinly I was stuck for 12 hours with work issued Kenco Instant due to forgetting my french press) I thought it may make an interesting read. Before you get into this and think "that's not telling me anything about how to froth milk", I would like to state that is not the purpose of this post, I am merely going to talk about what happens to milk at various stages. You may or may not choose to use some of this science in perfecting your technique.... anyway enough chit chat.... Milk in the form we know it as is created in the mammary glands of cows. Milk is food and as such is enriched in fats, sugars and proteins and bacteria. The amount of fat and protein is large enough to deflect light, which gives milk its white, opaque appearance. in general it's very slightly acidic. Lactose is the sugar that is found in milk and is unique to it, it is a combination of glucose and galactose. As Lactose is so unique, some human's lack the enzyme's to digest it and thus are 'lactose intolerant'. Bacteria also grow on lactose immediately after it comes from the cow and produces lactic acid. Lactic acid is what turns the milk sour, but it also prevents any other bacteria prevailing and stops milk spoiling (necessary when making cheese). Milk Fat is a very good source of nutrition and counts for most of the vitamins the body needs. Fat in milk is packages into globules, surrounded by fatty acids and proteins this becomes important in the heating of milk and prevents the fat from breaking down and becoming rancid. Milk fat is able to tolerate heat and can be boiled for hours without breaching the surrounding membrane, in fact the proteins begin to unfold and the membrane becomes thicker. The proteins are made of the very well known 'curds' and 'whey'. Curd proteins have most effect on other methods of yoghurt and cheese making, but not so much impact on foam. Whey has overall less impact, but is responsible for stabilizing foam. Milk is pasteurized in the UK and most other places using the 'High temperature, short time' method. Milk is heated in large quantities to a minimum of 162F (72C) for 15 seconds. The HTST method denatures 10% of whey proteins and generates hydrogen sulfide giving a 'cooked' flavour. This has an important impact on milk foaming as pasteurized milk has already reached at least 162F (72C) and this therefore is your maximum temperature. Contrary to popular belief even heating your milk to 71C is not going to 'burn' the lactose any more than it already has done. Milk in the UK has to be pasteurized using the HTST method to be able to label it as pasteurized. The other form of pasteurization (excluding UHT) is batch pasteurization where the milk is heated to 145F (62C) for 30 to 35 minutes. This prevents denaturing of the whey proteins and is not hot enough to produce hydrogen sulfide. This will be sold in shops as 'raw milk' or 'unpasteurized'. Generally this is safe to drink as it has still killed the pathogens and microbes that are unsafe. Most doctors will advise against it if pregnant, have illnesses, etc. But due to the lower temperature raw milk would make for better tasting and as you will understand shortly, due to the lack of denaturing in the whey, more stable foam. Milk fat can be separated using centrifugal force. Whole milk is around 4% fat, semi skimmed around 2% and skimmed about 0.1%. Fat by itself makes absolutely no difference to the foaming of milk. However higher milk fat gives a creamier and 'fuller' texture and flavour to the milk itself, generally resulting in a more pleasing foam. A foam is simply a portion of liquid filled with bubbles of air. Milk foams are quite fragile and have to be made immediately before serving. They are useful also in preventing a skin (caused when water evaporates from milk leaving dry proteins on the surface) and insulating the milk, keeping it hot. When foaming milk the proteins surround the air causing bubbles. Protein only constitutes around 3% and does not unfold and combine well. If milk is heated to around 160F (70C) the whey does unfold and bind together causing a more stable foam, all be it briefly. Ironically most skimmed and semi skimmed milks are often fortified with whey and therefore are easier to foam than full fat milk. Milk starts to sour very quickly and the older the milk the more it will curdle very slightly. Therefore milk should be as fresh as possible. Using steam to foam milk is actually a bit of a myth. Steam is purely water vapour and all this does is simply condense into the colder water of the milk. The steam provides two elements of frothing. It introduces air and mixes it with milk(hence the need to have the wand close to the surface) and simultaneously heats the bubbles enough to unfold the whey proteins into a stable web. The tricky part is that hot milk doesnt hold foam well. The foam collapses when gravity pulls the liquid from the bubble wall and ideally you need a large volume (at least 150ml) of milk to make sure it doesnt heat too fast and collapse before the foam has started. This can also be reduced by starting with milk and jug as cold as possible. Ideally the milk should be as cold as you can get it without freezing. When milk is frozen the water forms sharp ice crystals which damage the membrane surrounding the fat and when heated causes the fat to clump together in a substance similar to butter and the proteins develop an oil like consistency.
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