MILK

MILK

Milk:  Milk is the natural food for young mammals during the first stages of life when they are unable to obtain food for themselves.  It is a near perfect single food containing proteins, carbohydrates, fats, mineral elements, etc., but is lacking in iron and vitamins C and D.  It is an emulsion, a suspension of minute droplets of fat in a watery solution.  Milk from several species of animals is used for human consumption in different parts of the world (E.g., goats, sheep, mares, water buffalo, etc.)

The Composition of Milk:  The composition of milk obtained from a cow is not the near standard product which is purchased by the caterer or member of the public from his dairy supplier.  Milk is required to contain some 8.5% (minimum) solids not fat and 3% (minimum) fat under the Drinking Milk Regulations, 1976, otherwise it is presumed to have been adulterated.  The largest percentage constituent is water, 80-85%, containing the proteins, carbohydrates, fats, mineral elements, etc.

           

            The variation in the composition of the milk is caused by the following:

The Breed of Cow:  Jersey, Guemsey, and Ayrshire cows are noted for producing milk that is high in butter fat content and non-fatty solids but low in milk yield.

The Phase in the Lactation Period:  The period lasts for approximately 40 weeks during which the maximum yield is obtainable around the third to fourth week followed by a gradual decline in yield.  Therefore to produce a reasonably standard daily/weekly yield of milk the dairy farmer needs to stagger calving by the cows so that the lactation periods of a dairy herd are balanced.

The Type of Feed:  To obtain milk containing at least the minimum statutory amounts of fat and non fatty solids with as high a yield as possible, it is necessary for the diet of the cow to be controlled.  In the winter cows tend to live indoors and to be fed on hay, silage, oil cake and some root vegetables, all of which are expensive but result in milk rich in fat content.  In the spring and summer the cows live outdoors and feed mainly on grass (of differing quality depending mainly on the weather) and produce a high yield of milk richer in vitamin a than in the winter, but frequently low in fat content as a result of a lower roughage content in the diet.

TREATMENTS OF MILK

Pasteurized:  This is milk that has been subjected to the pasteurization; method of heat treatment.  The process heats the milk to a temperature high enough to kill any pathogenic organisms and reduces the number of any others present so as to make the milk safe for human consumption without spoiling its accepted flavor and appearance.  It is done in one of two methods, either heating the milk to 63-65°C (145 - 150°F) for 30 minutes (the Holder method) or more usually by heating it to not less than 72°C (161°F) for 15 seconds (the High Temperature Short time – HTST).  As soon as the heating time ends, the milk is rapidly cooled to below 10° C.  The rapid cooling is to restrict the growth of heat-resistant organisms and to ensure that the cream line will be distinct in bottled milk.

            These two groups of milk are sold as pasteurized milk.

Homogenized:  This is milk which has been treated to make it of uniform composition, i.e., the cream is distributed evenly throughout the milk.  The process consists of heating the milk to about 60°C (139°F) and then forcing it through a very small tube at a high pressure breaking up the fat globules into very small droplets.  The droplets remain suspended in the milk and do not float to the top forming a cream layer as in ordinary pasteurized milk.  The treatment produces milk which has a smoother creamier taste.  The milk is then subjected to heat treatment being either pasteurized or sterilized.

Sterilized:  This is milk that has been heated to a temperature higher than boiling temperature to ensure the destruction of all micro-organisms (i.e., more completely than pasteurization).  The milk is first pre-heated, homogenized and then filled into bottles which are closed with a hermetic seal.  The `sterilization’ is then done in two main ways.

(A)      In bottle sterilization in which the milk is filled into bottles and then heated for 30 – 40 minutes at a temperature of 104 - 110°C (220 – 230 F) in large autoclaves or pressure steam chamber and then allowed to cool.

(B)      Continuous flow sterilization by an ultra-high temperature method (see UHT below)

Because of the high temperature to which the milk has been subjected it will have a `cooked’ flavor due to the slight caramerlization of the sugar (lactose) and a creamy appearance.

Ultra heat treatment (UHT):  This is milk that is first homogenized and then subjected to a high temperature of between 135 - 150°C (276 – 304 F) for at least one second.  The UHT process kills off all micro-organisms and the very short holding temperature reduces the changes in color and flavor that occur with the in-bottle sterilization process.

            The shelf life of UHT milk can be extended to several months by filling the milk into specially designed Tetra-pak cartons which are plastic coated and lined with an aluminum foil.  The packs are usually date stamped.  This reduces the need for daily deliveries of milk and for refrigeration.

Untreated Milk or Farm-bottled Milk:  This is raw milk which has not been subjected to any form of heat treatment at all before being offered for sale.  A special license is required by the farmer or supplier to produce a sell this milk and it is therefore unlikely to be readily available in all parts of the country.

Semi-skimmed Milk:  This is milk which has had some of the cream content removed reducing its calorie content per pint from 380 to between 263 – 280.  The Regulations require that its milk fat content be a minimum of 1.5% and a maximum of 1.8%

Skimmed Milk:  This is milk that has had virtually all of the cream removed and with it the vitamins A and D.  It has calorie content per pint of 195 and a maximum fat constant of 0.3%

Concentrated Milks:  The concentration of milk serves four main purposes:

(a)   it reduces bulk by the reduction of the water content;

(b)  the heat treatment (and high sugar content in condensed milk) gives the product a very good shelf life;

(c)  it is a convenient outlet for skimmed milk from the production of butter and cream;

(d)  it requires no special form of storage

There are two main types of concentrated milks (evaporated and condensed) either being produced from whole milk (full cream) or skimmed milk.

Evaporated milk:  This is unsweetened concentrated milk.  It is produced by first pasteurizing the milk, evaporating it under reduced pressure in steam-heated vacuum pans until the volume has been reduced to 60% when it is then homogenized, cooled, and canned.

Condensed Milk:  This is sweetened concentrated milk.  It is produced by a similar process to that of evaporated milk except that prior to the evaporating stage sugar is added.  The sugar content in condensed milk is about a 62% concentration in the water of the condensed milk.  The milk is then cooled very carefully so that the crystallization of the lactose is in very fine crystals before being canned.

Dried Milk:  The drying of milk serves similar purposes to that of the liquid concentrated milk.  In this case the water content is removed entirely and thereby the bulk is reduced.  It requires no special storage facilities and is an outlet for the skimmed milk from butter and cream production.  It includes any milks which have been sweetened, modified, or compounded (Dried Milk Regulations, 1977)

            There are two main processes for drying milk.

ROLLER DRYING:  The milk is usually concentrated in an evaporator until it has about 17% total solids (as against 11.5% minimum in fresh liquid milk) before it is then homogenized.  The milk is then fed onto the very smooth surface of single or twin heated drums operating at a temperature of about 150°C (303 F).  The water content in the milk evaporates very quickly leaving a very thin film of dried milk which is scrapped off by a blade fixed to the roller.  The thin film is then quickly cooled before being ground into a powder and sieved before being packed in air-tight containers.  (Milk powders are hygroscope, i.e., they will quickly absorb moisture from the atmosphere and the powder will deteriorate becoming stale in flavor and less soluble on reconstitution).

            Roller drying is the cheaper process of the two and produces a product that is practically free from bacteria owing to the severe heat treatment it receives.  However it has the disadvantages of having a slight `cooked’ flavor, it does not reconstitute easily having a solubility of about 85%, and the structure of the fat globules in the milk is destroyed causing an oily layer to form on the surface when reconstituted in warm water.

SPRAY DRYING:

            The milk is pre-heated to a temperature of 80 - 90°C (175 – 196 F) for some 10 seconds, homogenized and then concentrated at about 43°C (110 F) to about 40% total solids.  The milk is then atomized to a fine mist in a drying chamber containing hot air at 165°C (330 F).  The very minute particles of milk give off their moisture almost instantaneously and drop to the bottom of the drying chamber as tiny grains of dried milk.  The powder is removed to cool as quickly as possible before being packed into air right containers.

            Spray drying is the more expensive process but it has a very good solubility of over 98% with less pronounced flavor changes than roller dried milk.

INSTANT MILK POWDER:

            A more recent development has been he production and marketing of instant nonfat (skimmed) milk powders which dissolve easily and completely when added to water, tea, coffee, etc.  The process used is based on a modification of the spray drying method in which the dried milk powder is moistened with steam to give a lumpy porous structure, the lumps of powder then being dried, cooled, and then reduced to a more standard size.  When added to a liquid they quickly absorb it and dissolve completely.  The cost of this type of milk powder is higher than that of the conventional milk powders and its main use appears to be in hot beverages.