Milk is a white liquid produced by the mammary
glands of mammals.
It is the primary source of nutrition for young mammals before they are able to digest other
types of food. Early-lactation milk contains colostrum,
which carries the mother's antibodies to its young and can reduce the risk of many
diseases. Milk contains many other nutrients[1] and the
carbohydrate lactose.
As an agricultural
product, milk is extracted
from mammals during or soon after pregnancy and is used as food for humans.
Worldwide, dairy
farms produced about 730 million tonnes of milk in 2011,[2] from 260
million dairy cows.[3]
India is the world's largest producer of milk, and is the leading exporter of
skimmed milk powder, yet has little to no other milk product exports.[4][5] The ever
increasing rise in domestic demand for dairy products and a large demand-supply
gap could lead to India being a net importer of dairy products in the future.[6] New
Zealand, the European Union's 28 member states, Australia, and
the United States are the world's largest exporters of milk and milk products.
China and Russia are the world's largest importers of milk and milk products.[7][8]
Throughout the world, there are more
than six billion consumers of milk and milk products. Over 750 million people
live within dairy farming households.[9]
Types
of consumption
There are two distinct types of milk
consumption: a natural source of nutrition for all infant mammals and a food
product for humans of all ages that is derived from other animals.
Nutrition
for infant mammals
A human baby feeding on its mother's
milk
A goat kid feeding on
its mother's milk
In almost all mammals, milk is fed
to infants
through breastfeeding, either directly or by expressing the milk to be stored and consumed later.
The early milk from mammals is called colostrum.
Colostrum contains antibodies that provide protection to the newborn baby as
well as nutrients and growth factors.[10] The
makeup of the colostrum and the period of secretion varies from species to
species.[11]
For humans, the World Health Organization recommends
exclusive breastfeeding for six months and breastfeeding in addition to other
food for at least two years.[12] In some
cultures it is common to breastfeed children for three to five years, and the
period may be longer.[13]
Fresh goats' milk is sometimes
substituted for breast milk. This introduces the risk of the child developing electrolyte
imbalances, metabolic acidosis, megaloblastic anemia, and a host of allergic reactions.[14]
Food
product for humans
Holstein
cattle, the dominant breed in industrialized dairying today
In many cultures of the world,
especially the West, humans continue to consume milk beyond infancy, using the
milk of other animals (especially cattle, goats and sheep) as a food product.
Initially, the ability to digest milk was limited to children as adults did not
produce lactase,
an enzyme necessary for digesting the lactose in milk. Milk was therefore
converted to curd, cheese and other
products to reduce the levels of lactose. Thousands of years ago, a chance
mutation spread in human populations in Europe that enabled the production of lactase in adulthood. This
allowed milk to be used as a new source of nutrition which could sustain
populations when other food sources failed.[15]
Milk is processed into a variety of dairy products such as cream, butter, yogurt, kefir, ice cream,
and cheese. Modern industrial processes use milk to produce casein, whey
protein, lactose, condensed milk, powdered
milk, and many other food-additives and industrial products.
Whole milk, butter and cream have
high levels of saturated fat.[16][17] The
sugar, lactose, is found only in milk, forsythia
flowers, and a few tropical shrubs. The enzyme needed to digest lactose,
lactase, reaches its highest levels in the small intestine after birth and then
begins a slow decline unless milk is consumed regularly.[18]
Those groups who do continue to tolerate milk, however, often have exercised
great creativity in using the milk of domesticated
ungulates,
not only of cattle, but also sheep, goats, yaks, water
buffalo, horses,
reindeer
and camels. The
largest producer and consumer of cattle and buffalo milk in the world is India.[19]
|
Per
capita consumption of milk and milk products in selected countries in 2011[20]
|
|||
|
Country
|
Milk
(liters)
|
Cheese
(kg)
|
Butter
(kg)
|
|
135.6
|
6.7
|
2.4
|
|
|
127.0
|
22.5
|
4.1
|
|
|
105.9
|
10.9
|
3.0
|
|
|
105.3
|
11.7
|
4.0
|
|
|
90.1
|
19.1
|
1.7
|
|
|
78.4
|
12.3
|
2.5
|
|
|
75.8
|
15.1
|
2.8
|
|
|
62.8
|
17.1
|
3.6
|
|
|
55.7
|
3.6
|
0.4
|
|
|
55.5
|
26.3
|
7.5
|
|
|
54.2
|
21.8
|
2.3
|
|
|
51.8
|
22.9
|
5.9
|
|
|
49.1
|
23.4
|
0.7
|
|
|
47.5
|
19.4
|
3.3
|
|
|
39.5
|
-
|
3.5
|
|
|
9.1
|
-
|
0.1
|
|
History
Drinking milk in Germany in 1932
Humans first learned to regularly
consume the milk of other mammals following the domestication of animals during
the Neolithic Revolution[30] or the
development of agriculture. This development occurred independently in several
places around the world from as early as 9000–7000 BC in Southwest
Asia[31]
to 3500–3000 BC in the Americas.[32] The most
important dairy animals—cattle, sheep and goats—were first domesticated in
Southwest Asia, although domestic cattle had been independently derived from
wild aurochs
populations several times since.[33][34] Initially
animals were kept for meat, and archaeologist Andrew
Sherratt has suggested that dairying, along with the exploitation of
domestic animals for hair and labor, began much later in a separate secondary products revolution in the
fourth millennium BC.[35]
Sherratt's model is not supported by recent findings, based on the analysis of lipid residue in
prehistoric pottery, that shows that dairying was practiced in the early phases
of agriculture in Southwest Asia, by at least the seventh millennium BC.[36][37]
From Southwest Asia domestic dairy
animals spread to Europe (beginning around 7000 BC but not reaching Britain and
Scandinavia until after 4000 BC),[38] and South Asia
(7000–5500 BC).[39]
The first farmers in central Europe[40] and
Britain[41]
milked their animals. Pastoral and pastoral nomadic economies, which rely
predominantly or exclusively on domestic animals and their products rather than
crop farming, were developed as European farmers moved into the Pontic-Caspian steppe in the fourth
millennium BC, and subsequently spread across much of the Eurasian
steppe.[42]
Sheep and goats were introduced to Africa from Southwest Asia, but African
cattle may have been independently domesticated around 7000–6000 BC.[43] Camels,
domesticated in central Arabia in the fourth millennium BC, have also been used
as dairy animals in North Africa and the Arabian Peninsula.[44] The
earliest Egyptian records of burn treatments describe burn dressings using milk
from mothers of male babies.[45]
In the rest of the world (i.e., East and Southeast Asia, the Americas and
Australia) milk and dairy products were historically not a large part of the
diet, either because they remained populated by hunter-gatherers
who did not keep animals or the local agricultural economies did not include
domesticated dairy species. Milk consumption became common in these regions
comparatively recently, as a consequence of European colonialism
and political domination over much of the world in the last 500 years.
In the Middle
Ages, milk was called the "virtuous white liquor" because
alcoholic beverages were more safe to consume than water.[46]
Industrialization
Preserved Express
Dairies three-axle milk tank wagon at the Didcot Railway Centre, based on an SR chassis
The growth in urban population
coupled with the expansion of the railway network in the mid-19th century,
brought about a revolution in milk production and supply. Individual railway
firms began transporting milk from rural areas to London from the 1840s and
1850s. Possibly the first such instance was in 1846, when St Thomas's Hospital in Southwark
contracted with milk suppliers outside London to provide milk by rail.[47]
The Great Western Railway was an early and
enthusiastic adopter, and began to transport milk into London from Maidenhead
in 1860, despite much criticism. By 1900, the company was transporting over 25
million gallons annually.[48] The milk
trade grew slowly through the 1860s, but went through a period of extensive,
structural change in the 1870s and 1880s.
Milk transportation in Salem,
Tamil Nadu
Urban demand began to grow, as
consumer purchasing power increased and milk became regarded as a required
daily commodity. Over the last three decades of the 19th century, demand for
milk in most parts of the country doubled, or in some cases, tripled. Legislation in 1875 made the adulteration of
milk illegal - this combined with a marketing campaign to change the image of
milk. The proportion of rural imports by rail as a percentage of total milk
consumption in London grew from under 5% in the 1860s to over 96% by the early
20th century. By that point, the supply system for milk was the most highly
organized and integrated of any food product.[47]
Sources
of milk
Modern dairy farm
in Norway
The females of all mammal species
can by definition produce milk, but cow's milk dominates commercial production.
In 2011, FAO estimates 85% of all milk worldwide was produced from cows.[54]
Human milk is not produced or
distributed industrially or commercially; however, human
milk banks collect donated human breastmilk
and redistribute it to infants who may benefit from human milk for various
reasons (premature neonates, babies with allergies, metabolic diseases, etc.) but who cannot
breastfeed.[55]
In the Western world, cow's milk is
produced on an industrial scale and is by far the most commonly consumed form
of milk. Commercial dairy farming using automated
milking equipment produces the vast majority of milk in developed
countries. Dairy cattle such as the Holstein
have been bred selectively for increased milk production. About 90% of the
dairy cows in the United States and 85% in Great Britain are
Holsteins.[18]
Other dairy cows in the United
Sources
aside from cows
Other
significant sources of milk
Goats (2% of world's milk)
Buffaloes (11%)
Aside from cattle, many kinds of
livestock provide milk used by humans for dairy products. These animals include
buffalo, goat,
sheep,
camel,
donkey, horse,
reindeer and yak. The first four respectively produced about 11%, 2%, 1.4% and
0.2% of all milk worldwide in 2011.[54]
In Russia and Sweden, small moose
dairies also exist.[56]
According to the US National Bison
Association, American bison (also called American buffalo) are
not milked commercially;[57] however,
various sources report cows resulting from cross-breeding bison and domestic
cattle are good milk producers, and have been used both during the European
settlement of North America[58] and
during the development of commercial Beefalo in the
1970s and 1980s.[59]
Production
worldwide
|
Per
capita consumption of milk and milk products in selected countries in 2011[20]
|
|||
|
Country
|
Milk
(liters)
|
Cheese
(kg)
|
Butter
(kg)
|
|
135.6
|
6.7
|
2.4
|
|
|
127.0
|
22.5
|
4.1
|
|
|
105.9
|
10.9
|
3.0
|
|
|
105.3
|
11.7
|
4.0
|
|
|
90.1
|
19.1
|
1.7
|
|
|
78.4
|
12.3
|
2.5
|
|
|
75.8
|
15.1
|
2.8
|
|
|
62.8
|
17.1
|
3.6
|
|
|
55.7
|
3.6
|
0.4
|
|
|
55.5
|
26.3
|
7.5
|
|
|
54.2
|
21.8
|
2.3
|
|
|
51.8
|
22.9
|
5.9
|
|
|
49.1
|
23.4
|
0.7
|
|
|
47.5
|
19.4
|
3.3
|
|
|
39.5
|
-
|
3.5
|
|
|
9.1
|
-
|
0.1
|
|
Health Benefits
Bone health
Milk and dairy products are providers of calcium, phosphorous, magnesium and protein which are all essential for healthy bone growth and development.Adequate consumption of milk and dairy from early childhood and throughout life can help to make the bones strong and protect them against diseases like osteoporosis (a debilitating, brittle bone disorder) in later life.
Teeth
The amounts of calcium and phosphorous in milk and dairy products are also beneficial for the development and maintenance of healthy teeth.The most abundant protein in milk is casein and is protective as it forms a thin film on the enamel surface which prevents loss of calcium and phosphate from the enamel when the teeth are exposed to acids in the mouth.
Studies have suggested that milk also reduces the effects of cariogenic foods on teeth when consumed together with them in the diet.
In fact, dentists recommend that milk is the only safe drink to have between meals (except for water) as it has been shown not to cause tooth decay even in conditions perfect for damaging teeth!
Milk and blood pressure
An increasing number of studies suggest that consuming 3 portions of dairy each day, along with 5 portions of fruit and vegetables as part of a low salt diet can reduce high blood pressure in both adults and children.Although the exact mechanisms involved are not clear, it is thought that the calcium, potassium, magnesium and proteins within milk are all likely to be involved.
Milk and cardiovascular disease
Several studies have linked milk and dairy consumption with a reduced risk for cardiovascular disease.A recent study in Welsh men found that those who drank the most milk had fewer heart attacks than those who had little or no milk in their diets.
This connection could be due to many factors in milk, but epidemiological studies have shown that higher intakes of calcium in particular are linked to a reduced risk of cardiovascular disease.
More specifically, studies have shown that high calcium intakes may reduce high levels of bad cholesterol in the blood, and increase low levels of good cholesterol both of which are known risk factors for cardiovascular disease.
In addition, it is also thought that calcium may bind harmful fats together in the gut and prevent their absorption, which in turn prevents levels in the blood increasing.
Obesity
Contrary to popular belief, research has shown that people who consume milk and dairy foods are likely to be slimmer than those who do not.
Studies have also shown that consumption of milk and dairy foods as part of a calorie controlled diet is associated with increased weight loss, particularly form the abdomen.
This is particularly beneficial since excess fat around the trunk region of the body is associated with greater risks to health.
The precise mechanisms involved are unclear but are likely to involve calcium which is found in milk and dairy foods.
Type 2 diabetes
Studies suggest that regular consumption of low fat dairy products can help to reduce the risk of type 2 diabetes, which has been a longstanding problem in adults, and is becoming increasingly common in children and adolescents.
A recent study of more than 37,000 middle aged women found that those with the highest intakes of dairy had a reduced risk of type 2 diabetes.
The strongest association was found with low fat dairy products.
Similarly a study of men in 2005 found a reduced risk of type 2 diabetes with increased consumption of low fat dairy, interestingly, every extra portion of dairy consumed was associated with increasingly lower risk.
It is thought that this effect may be due to the combined effects of many beneficial nutrients found within dairy foods including calcium and magnesium, or the fact that dairy foods have a low glycaemic index, which helps to control blood sugar levels.
Cancer
There is considerable evidence to suggest that milk has a protective effect on risk of both colorectal and breast cancer with increased intakes.
A recent study of 45,000 Swedish men reported that men who drank 1.5 glasses of milk per day or more, had 35% lower risk of the disease than those who had a low milk intake of less than 2 glasses per week.
Additionally a study of over 40,000 Norwegian women found that those who drank milk as children and continued to do so as adults, had a lower risk of developing breast cancer.
Calcium and a naturally occurring fat in dairy products known as Conjugated Linoleic Acid (CLA) have been suggested as protective components in colon cancer.
Hydration
In order to remain adequately hydrated, it is recommended that we consume 6-8 cups of fluid each day.
If we become dehydrated, it can result in poor concentration and memory function and leave you feeling irritable and unwell.
Milk is an excellent choice of fluid as it not only re-hydrates the body, but provides a host of beneficial nutrients and protects the teeth at the same time!
Re-hydration after exercise is particularly important to replace lost fluids, and a recent study in the USA found that chocolate milk helped the body to recover after exhausting exercise!
Product form milk
ow's milk is the most popular in
many countries, milk can be obtained from many different sources. For example,
milk from goats and sheep makes a substantial contribution to the total milk
production in countries of Eastern and Southern Europe, Malawi, and Barbados,
whereas the water buffalo is a common source of milk in much of Asia. The table
below illustrates some of the differences in composition between these milks.
Milk is a perishable commodity and
spoils very easily. Its low acidity and high nutrient content make it the
perfect breeding ground for bacteria, including those which cause food
poisoning (pathogens).
Bacteria from the animal, utensils,
hands, and insects may contaminate the milk, and their destruction is the main
reason for processing. This preservation of the milk can be achieved by
fermentation, heating, cooling, removal of water, and by concentration or
separation of components, to produce foods such as butter or cheese.
The degree to which milk consumption
and processing occurs will differ from region to region. It is dependent upon a
whole host of factors, including geographic and climatic conditions,
availability and cost of milk, food taboos, and religious restrictions. Where
processing does exist, many traditional techniques can be found for producing
indigenous milk products. These are more stable than raw milk and provide a
means of preservation as well as adding variety to the diet. In addition, the
introduction of western-style dairy products and the subsequent setting up of
small-scale dairies has provided more choice of dairy products to the consumer.
Milk is often regarded as being
nature's most complete food. It earns this reputation by providing many of the
nutrients which are essential for the growth of the human body. Being an
excellent source of protein and having an abundance of vitamins and minerals,
particularly calcium, milk can make a positive contribution to the health of a
nation. The realization of its nutritional attributes is clearly illustrated by
the implementation of numerous 'school milk programmes' worldwide.
Fermented-milk products such as
yoghurt and soured milk contain bacteria from the Lactobacilli group.
These bacteria occur naturally in the digestive tract and have a cleansing and
healing effect. Therefore the introduction of fermented products into the diet
can help prevent certain yeasts and bacteria which may cause illness.
Many people suffer from a condition
known as 'lactose intolerance'. This means that they are unable to digest the
milk fat (lactose). Such people can, however, tolerate milk if it is fermented
to produce foods such as yoghurt. During fermentation, lactic acid producing
bacteria break down lactose, and in doing so eliminate the cause of irritation.
The quality of milk
The type of animal, its quality, and
its diet can lead to differences in the colour, flavour, and composition of
milk. Infections in the animal which cause illness may be passed directly to
the consumer through milk. It is therefore extremely important that
quality-control tests are carried out to ensure that the bacterial activity in
raw milk is of an acceptable level, and that no harmful bacteria remain in the
processed products.
Average composition (%) of milks of
various mammals
|
Species
|
Water
|
Fat
|
Protein
|
Lactose
|
Ash
|
|
Human
|
87.43
|
3.75
|
1.63
|
6.98
|
0.21
|
|
Cow
|
87.2
|
3.7
|
3.5
|
4.9
|
0.7
|
|
Goat
|
87.00
|
4.25
|
3.52
|
4.27
|
0.86
|
|
Sheep
|
80.71
|
7.9
|
5.23
|
4.81
|
0.9
|
|
Indian buffalo
|
82.76
|
7.38
|
3.6
|
5.48
|
0.78
|
|
Camel
|
87.61
|
5.38
|
2.98
|
3.26
|
0.7
|
|
Horse
|
89.04
|
1.59
|
2.69
|
6.14
|
0.51
|
|
Llama
|
86.55
|
3.15
|
3.9
|
5.6
|
0.8
|
FAO photo
Milk fat
The price paid for milk is usually
dependent upon the milk-fat content, and this may be determined either at the
collection stage or at the dairy using a piece of equipment known as a
butyrometer. Additionally the specific gravity can be measured using a
hydrometer. This can also be used as an aid to detect adulteration.
Bacterial activity
Routinely it is necessary to check
the microbiological quality of raw milk using either methylene blue or
resazurin dyes. These tests indicate the activity of bacteria in the milk
sample and the results determine whether the milk is accepted or rejected.
Both tests work on the principle of
the time taken to change the colour of the dye. The length of time taken is
proportional to the number of micro-organisms present (the shorter the time
taken, the higher the bacterial activity). It is preferable to use the resazurin
test as this is less time-consuming. For these tests, basic laboratory
equipment will be needed such as test-tubes, a water bath, accurate measuring
equipment, and a supply of dyes.
After collection the milk should
ideally be stored at a temperature of 4°C or below. This is necessary to slow
the growth of any contaminating bacteria.
Phosphatase test
For pasteurized milk, it is possible
to ensure that pasteurization has been adequately achieved by testing for the
presence of the enzyme phosphatase. The destruction of phosphatase is regarded
as a reliable test to show that the milk has been sufficiently heat-processed,
because this enzyme (present in raw milk) is destroyed by pasteurization
conditions.
It is stressed that pasteurization
is an effective safeguard against spoilage and food poisoning only if the milk
is not re-contaminated after pasteurization.
Liquid milk
Milk can be kept for longer periods
of time if it is heated to destroy the bacteria or cooled to slow their growth.
Pasteurization and sterilization are the two most commonly-used heat
treatments. Technically, it is possible for both to be carried out on a small
scale, but they are most usually performed on a larger industrial scale due to
the need for qualified, experienced staff and accurate and strictly controlled
hygienic processing conditions.
Production stages for pasteurized
and sterilized milk
|
Product
|
Store
raw milk at 4°C
|
Test
for bacterial activity using resazurin/methylene blue
|
Filter
|
Homogenize
|
Pasteurize
|
Fill
into sterilized bottles
|
Sterilize
|
Store
|
|
Pasteurized milk
|
*
|
*
|
*
|
*
|
*
|
*
|
||
|
Sterilized milk
|
*
|
*
|
*
|
*
|
*
|
*
|
*
|
Equipment required
|
Processing
stage
|
Equipment
|
Section
reference
|
|
Store
at 4°C
|
Refrigerated storage
|
15.0
|
|
Thermometer
|
63.0
|
|
|
Test
for fat content
|
Butyrometer
|
64.5
|
|
Test
specific gravity
|
Hydrometer
|
64.4
|
|
Test
bacterial activity
|
Supply of dyes
|
64.6
|
|
Thermometer
Basic laboratory equipment is required for most of the tests |
63.0
|
|
|
Filter
|
Filter cloth
|
0.80
|
|
Filter press
|
29.2
|
|
|
Homogenization
|
Homogenizer
|
37.0
|
|
Fill
into bottles
|
Liquid-filling machine
|
28.1 - refer to Packaging chapter
for notes on the preparation of sterilized bottles
|
|
Capping machine
|
47.2
|
|
|
Pasteurization
|
Boiling pan
|
48.0
|
|
or pasteurizer
|
50.0
|
|
|
Heat source
|
36.0
|
|
|
Thermometer
|
63.0
|
|
|
Sterilization
|
Pressure cooker
|
48.0
|
|
Retort
|
05.1
|
|
|
Heat source
|
36.0
|
|
|
Thermometer
|
63.0
|
|
|
Cool
|
Bottle-cooling system
|
Refer to the Packaging chapter for
details
|
Homogenization
Homogenization breaks up the oil
droplets in milk and prevents the cream from separating out and forming a
layer. This is of particular importance for sterilized milk which has a long
shelf-life and when the formation of a cream layer is not desired. Additional
changes include increased viscosity and a richer taste. Homogenizers are more
usually designed for industrial-scale production, but it is possible to
purchase smaller versions.
Filling
The most common packaging material
for both pasteurized and sterilized milk is glass bottles sealed with either
foil or metal caps, although plastic bottles, plastic bags, and cardboard
cartons are all used when bottles are not available or too expensive.
Pasteurization
Pasteurization is a relatively mild
heat treatment, (usually performed below 100°C) which is used to extend the
shelf-life of milk for several days. It preserves the milk by the inactivation
of enzymes and destruction of heat-sensitive micro-organisms, but causes
minimal changes to the nutritive value or sensory characteristics of a food. Some
heat-resistant bacteria survive to spoil the milk after a few days, but these
bacteria do not cause food poisoning.
The time and temperature combination
needed to destroy 'target' microorganisms will vary according to a number of
complex inter-related factors. For milk, the heating time and temperature is
either 63°C for 30 minutes or alternatively 72°C for 15 seconds. Only the
former combination is possible on a small scale and for this the simplest
equipment required is an open boiling pan. Better control is achieved using a
steam jacketed pan, and this can be fitted with a stirrer to improve the
efficiency of heating. Both of these are batch processes which are suited to
small-scale operation. A higher production rate may be possible using a tubular-coil
pasteurizer. This equipment has been tested and has been successful for some
fruit products but it is presently still at a developmental stage.
Sterilization
Sterilization is a more severe heat
treatment designed to destroy all contaminating bacteria. The milk is
sterilized at a temperature of 121°C maintained for 15-20 minutes. This can be
achieved using a retort or pressure cooker. Unlike pasteurization, this process
causes substantial changes to the nutritional and sensory quality of the milk.
In some countries, flavoured milk has become a very popular product.
However, sterilization is not
recommended for small-scale production for the following reasons:
· The cost of a retort and ancillary equipment is high for
the small-scale processor.
· It is essential that the correct heating conditions are
carefully established and maintained for every batch of milk that is processed.
If the milk is overheated, the quality is reduced, and it may have a rather
burnt taste and aroma.
· If the milk is not heated sufficiently, there is a risk
that micro-organisms will survive and grow inside the bottle. In low-acid foods
such as milk, many types of bacteria including Clostridium botulinum can
grow and cause severe food poisoning.
· Due to the potential dangers from food poisoning, the
skills of a qualified food technologist/microbiologist are required in order to
routinely examine samples of sterilized milk that have been subjected to
accelerated storage conditions. This requires a supply of microbiological media
and equipment.
In summary, the process of
sterilization requires a considerable capital investment, the need for trained
and experienced staff, regular maintenance of sophisticated equipment, and a
comparatively high operating expenditure.
Cooling
Pasteurization does not destroy all
of the micro-organisms, therefore the milk has to be cooled rapidly to prevent
the growth of surviving bacteria. Cooling can be achieved on a small scale by
using a bottle-cooling system. A system is outlined in the Packaging chapter.
Storage
Pasteurized milk has a shelf-life of
2-3 days if kept at 4°C. Maintaining this low temperature causes a substantial
increase to the cost of transportation and distribution and is therefore a
major disadvantage to the development of a small-scale pasteurized milk
business. If packaged in sealed bottles and stored at room temperature,
sterilized milk should have a shelf-life in excess of six months.
Cream
When milk is left to stand for some
time, fat globules rise to the surface forming a layer of fat (or cream). This
can be separated leaving behind skimmed milk as a by-product. There are
different types of cream each with different fat concentrations: single (or
light) cream contains 18 per cent milk fat whereas double (or heavy) cream
normally contains 30 per cent milk fat. Cream is a luxury item and may be used
as an accompaniment to coffee, as a filling in cakes, and an ingredient in ice
cream.
Separation
Separation can very simply be
achieved by removing the cream with a spoon, however this is a slow process
during which the cream may spoil. For this reason it is more usual to use a
manual or powered centrifuge.
Production stages for cream
|
Ingredients
|
Process
|
Equipment
|
Section
reference
|
|
Raw milk tested
|
Store at 4°C
|
Milk churns
|
62.0
|
|
Refrigerated storage
|
15.0
|
||
|
Thermometer
|
63.0
|
||
|
Separation of milk fat
|
Ladle Dairy centrifuge
|
07.1
|
|
|
|
Pasteurization
|
Large boiling pan or steam
jacketed pan
|
48.0
|
|
Pasteurizer
|
50.0
|
||
|
Heat source
|
36.0
|
||
|
Thermometer
|
63.0
|
||
|
|
Fill bottles/pots
|
Funnel or liquid-filling machine
|
28.1
|
|
Capping machine
|
47.2
|
||
|
Pot sealer
|
47.1
|
||
|
Cool bottles
|
Bottle-cooler
|
See Packaging chapter
|
|
|
Store bottles at 4°C
|
Refrigerated storage
|
15.0
|
Pasteurization
Cream may be pasteurized in a
similar way to milk, using a similar time and temperature combination and the
same equipment. Cream can also be sterilized but there is a considerable loss
of quality.
Packaging and storage
Cream can be packaged in glass jars
or plastic pots sealed with foil lids. Pasteurized cream must be stored at a
temperature of 4°C to have a shelf-life of several days. Refrigerated storage
is necessary because cream is prone to rapid spoilage.
Butter
Butter is a semi-solid mass which
contains approximately 80-85 per cent milk-fat and 15-20 per cent water. It is
yellow/white in colour, with a bland flavour and a slightly salty taste. It is
a valuable product that has a high demand for domestic use in some countries
and as an ingredient in other food processing (e.g. for confectionery and
bakery uses).
The principles of preservation are:
· to destroy enzymes and micro-organisms by pasteurizing the
milk
· to prevent microbial growth during storage by reducing the
water content, by storing the product at a low temperature, and optionally by
adding a small amount of salt during processing.
Production stages for butter
|
Ingredients
|
Process
|
Equipment
|
Section
reference
|
|
Cream or soured cream
|
Store at 4°C
|
Milk churns
|
62.0
|
|
Refrigerated storage
|
15.0
|
||
|
Thermometer
|
63.0
|
||
|
Churning
|
Butter churns
|
13.0
|
|
|
Draining (pour off buttermilk)
|
|||
|
Washing
|
|||
|
Draining (pour off washwater)
|
|||
|
Permitted colours and salt
(optional)
|
Kneading/working
|
Butter pats
|
04.0
|
|
Form into blocks
|
Butter pats
|
04.0
|
|
|
Packaging
|
Paper/plastic/ foil wrapping
Wrapping machines |
47.3
|
|
|
Storage at 4°C
|
Refrigerated storage
|
15.0
|
Churning
Churning disrupts the emulsion of
fat and water and as a result the milk-fat separates out into granules. This
process takes place in a butter churn.
Churning is continued until fat
granules are present and at this stage the mixture is drained to remove liquid
that has separated from the granules. This liquid is known as buttermilk and
can be used as either a beverage or as an ingredient in animal feed.
Washing
Clean water equivalent in weight to
the buttermilk is added to the churn in order to wash the butter granules. The
wash water is drained away. Churning is continued for a short time to compact
the butter, and once this has been achieved it is removed from the churn.
Forming and packaging
Butter is kneaded to achieve a
smooth and pliable texture. This can be done using simple hand-tools such as
butter pats. Alternatively for higher production rates a specially-designed
kneader can be used. Once the butter has a uniform and smooth texture it is
formed into blocks with butter pats and packed in either greaseproof paper or
foil wrappers.
Storage
Due to its high fat composition,
butter must be stored at temperatures below 10°C otherwise the fat becomes
rancid and imparts undesirable 'off' flavours. The water droplets in butter (20
per cent) can also allow bacteria to grow if it is not kept under cool
conditions.
Ghee
Ghee is made from butter which has
been heated and clarified. At ambient temperatures it is a semi-solid mass with
a granular texture, but on melting (40°C+) it turns into a clear, thin liquid.
It has a high demand in some countries for domestic use, as an ingredient for
local food production (for example bakeries and confectionery manufacturers),
and as an export commodity.
Alternatively, cream is boiled
gently to evaporate the water. During boiling the product is stirred
continuously until the milk proteins start to coagulate, forming particles, and
the colour of the cream darkens. Heating is stopped and the product is left to
set. The particles settle at the bottom of the vessel and the milk-fat is
separated. The principles of preservation are:
· heating to destroy enzymes and contaminating microorganisms
· to reduce the water-content by evaporation, and in doing so
prevent the growth of micro-organisms.
Production stages for ghee
|
Ingredients
|
Process
|
Equipment
|
Section
reference
|
|
Butter
|
Heating
|
Heat source
|
36.0
|
|
Large boiling pan or steam
jacketed pan
|
48.0
|
||
|
Cool to room temperature
|
Thermometer
|
63.0
|
|
|
Filter
|
Filter cloth
|
08.0
|
|
|
|
Fill into jars/pots
|
Funnel or liquid-filling machine
|
28.1
|
|
Capping machine
|
47.2
|
||
|
Store at ambient temperatures
|
Packaging and storage
Metal containers are normally used.
They should be thoroughly cleaned, especially if they are re-usable, and they
should be made airtight. Alternatives to metal cans include coloured glass jars
with metal lids, or ceramic pots sealed with cork/plastic stoppers.
Ghee is usually stored at room
temperatures as cold storage affects the granular texture. Thus ghee is useful
for those consumers with no access to refrigeration.
The technology of cultured milk
products such as yoghurt, curd, and cheese is based upon the microbial
conversion of the milk-sugar lactose to lactic acid (lactic acid accounts for
the characteristic 'sourness' of such products). In order for the conversion to
take place, lactic acid producing bacteria must be present. This may occur by
allowing the milk to sour naturally, but it is better to introduce the
appropriate bacteria as a starter culture. Starter cultures may be in the form
of a small quantity of previously-cultured product or may be purchased as a
commercially-prepared culture.
Yoghurt/curd
Yoghurt is a fermented milk product
that evolved by allowing naturally-contaminated milk to sour at a warm
temperature. Yoghurt can be either unsweetened or sweetened, set, or stirred.
Curd is the name given to a yoghurt-type product made from buffalo milk.
The principles of preservation for
yoghurt are:
· Pasteurization of the raw milk to destroy contaminating
microorganisms and enzymes.
· An increase in acidity due to the production of lactic acid
from lactose. This inhibits the growth of food-poisoning bacteria.
· Storage at a low temperature to inhibit the growth of
microorganisms.
Production stages for set yoghurt
|
Ingredients
|
Process
|
Equipment
|
Section
reference
|
|
Milk and starter culture (2 per
cent)
|
Preheat to 70°C for 15-20 minutes
|
Heat source
|
36.0
|
|
Thermometer
|
63.0
|
||
|
Boiling pan
|
48.0
|
||
|
Cool to 30-40°C
|
Thermometer
|
63.0
|
|
|
Addition of starter culture
|
Measuring and weighing equipment
|
64.1 and 64.2
|
|
|
|
Pour into bottles/pots
|
Funnel or Liquid filler
|
28.1
|
|
Sealing machine
|
47.1
|
||
|
or Capping machine
|
47.2
|
||
|
|
Incubate at 43-45°C
|
Commercial incubator
|
39.0
|
|
Thermometer
|
63.0
|
||
|
Store at 4°C
|
Refrigerated storage
|
15.0
|
Heating
In the manufacture of yoghurt, milk
is normally heated to 70°C for 15-20 minutes, using an open boiling pan, or
alternatively a steam jacketed pan.
Addition of starter culture
The milk is cooled to between 30
40°C and inoculated with a mixed culture of Lactobacillus bulgaricus and
Streptococcus thermophilus (usually in a ratio of 1:1). If a commercial
starter-culture is used, the directions for use will be given. However, if a
culture from a previous batch is used, then it is usual to add 2-3 tablespoons
per litre of prepared milk.
Yoghurt of the stirred variety can
be fermented in the mixing container. To make set yoghurt the inoculated milk
should be poured into the individual pots before fermentation.
Incubation
The micro-organisms that produce
yoghurt are most active within a temperature range of 32-47°C. Ambient
temperatures are therefore not adequate and a heated incubator is needed. Small
commercially-available yoghurt-makers consist of an electrically-heated base
and a set of plastic or glass containers. Most yoghurt-makers make four or five
individual half litre cups at a time. There are other simple and inexpensive
ways of incubating yoghurt such as an insulated box, keeping the jars/pots
surrounded by warm water, or by using thermos flasks (the latter is only
suitable for stirred yoghurt). Incubation takes approximately five hours.
When fermentation is complete,
stirred yoghurt is cooled and flavoured or sweetened prior to packaging. In set
yoghurt all ingredients are added before fermentation.
Packaging and storage
Yoghurt or curd is commonly packaged
in plastic pots fitted with a plastic lid, or heat-sealed with foil, although
traditionally, curd is packaged in clay pots. Such pots are made from local
materials and can be re-used or later used for cooking. The shelf-life of
yoghurt is usually 3-8 days when stored at temperatures below 10°C.
Cheese
Cheese is made from milk by the
combined action of lactic acid bacteria and the enzyme rennin (known as
rennet). Just as cream is a concentrated form of milk fat, cheese is a
concentrated form of milk-protein. The differences in cheeses that are produced
in different regions result from variations in the composition and type of
milk, variations in the process, and the bacteria used. The different cheese
varieties can be classified as either hard or soft.
Hard cheeses such as Cheddar and
Edam have most of the whey drained out and are pressed. Soft cheeses such as
paneer contain some of the whey and are not pressed. Many indigenous cheeses
are soft types.
The hardness, flavour, and other
qualities of a cheese can be varied by changes to the process conditions, to
suit local tastes. However the principal steps of a cheese-making process are
basically the same.
The principles of preservation are:
· the raw milk is pasteurized to destroy most enzymes and
contaminating bacteria
· fermentation by lactic-acid bacteria increases the acidity
which inhibits the growth of food-poisoning and spoilage bacteria
· the moisture content is lowered and salt is added to
inhibit bacterial and mould growth.
The table, right, outlines the
stages of production and the equipment needed to produce Edam cheese.
Pre-heating
The pasteurized milk is heated to a
temperature at which the starter-culture can work.
Addition of starter culture
Starter-culture is added to the milk
at the rate of approximately 2 per cent of the weight of milk. The vessel used
should be either aluminium or stainless steel.
Addition of rennet
The rennet should be 1 per cent of
the weight of milk. The rennet alters the milk proteins and allows them to form
the characteristic curd.
Incubation
The milk is allowed to stand until
it sets to a firm curd.
Treatment of the curd
The curd is cut into cubes which
facilitate the elimination of whey from the gel. The curd is then cooked at
40°C for a period of twenty minutes which has the action of firming the curd.
After cooling, the whey is drained off. The curd is pressed to ensure that most
of the whey has been removed, and is then cut to fit the cheese-moulds, and
finally pressed with weights.
Ripening
This is the final stage in the
cheese-making process. It is a process which allows the development of gas in
some cheeses and the development of flavour. The longer the ripening process
the stronger the flavour. Ripening usually takes place in ripening rooms, where
the temperature and humidity must be controlled for the optimum development of
the cheese.
Production stages for Edam-type
cheese
|
Ingredients
|
Process
|
Equipment
|
Section
reference
|
|
Pasteurized milk
|
Preheat to 35-40°C
|
Cheese vat
|
10.0
|
|
or boiling pan
|
48 0
|
||
|
Thermometer
|
63.0
|
||
|
Heat source
|
36 0
|
||
|
Starter culture
|
Addition of starter culture
|
Measuring and weighing equipment
|
64.1 and 64.2
|
|
Rennet
|
Add rennet at 30°C
|
Measuring and weighing equipment
|
64.1 and 64.2
|
|
Thermometer
|
63.0
|
||
|
Incubate
|
|||
|
Cut the curd
|
Curd cutters
|
16.1
|
|
|
|
Heat to 40°C for 20 minutes
|
Heat source
|
36.0
|
|
Thermometer
|
63.0
|
||
|
Drain
|
Filter cloth
|
08.0
|
|
|
Cut to fit a cheese-mould
|
Knife
|
17.1
|
|
|
Put into a cheese-mould
|
Cheese-moulds
|
09.1
|
|
|
Press with
|
Cheese-press weights
|
09.2
|
|
|
Cool and dry at 10-12°C
|
Thermometer
|
63.0
|
|
|
Salt
|
Salting in 20 per cent salt
solution at 12°C for 12-16 hours
|
Brine meter
|
64.6
|
|
Thermometer
|
63.0
|
||
|
|
Ripen for 6-8 weeks at 16°C
|
Thermometer (optional)
|
63.0.
|
|
Washing
|
|||
|
Drying for 30 minutes
|
|||
|
Wax with paraffin wax store at 9°C
|
Refrigerated storage
|
15.0
|
Packaging and storage
The packaging requirements differ
according to the type of cheese produced. Hard cheese, for example, has an
outer protective rind which protects the cheese from air, microorganisms,
light, moisture-loss or pick-up, and odour pickup. Cheese should be allowed to
'breathe', otherwise it will sweat. Suitable wrapping materials are therefore
cheesecloth or grease-proof paper. Cheese should be stored at a relatively low
temperature between 4 and 10°C to achieve a shelf-life of several weeks/months.
Soft cheeses are often stored in pots or other containers, often in brine, to help
increase their shelf-life of several days/weeks.
Ice cream
Ice cream is a frozen mixture which
contains milk, sugar, fat, and optional thickeners (e.g. pectin or gelatin),
colouring, and flavouring. It may be sweetened and flavoured in numerous ways
with nuts, fruit pieces, and natural or artificial flavours and colours.
The principles of preservation are:
·
pasteurization to destroy most micro-organisms and enzymes
· freezing to inhibit microbial growth.
· freezing to inhibit microbial growth.
Pasteurization
Pasteurization is carried out by
heating to 65°C for a period of 30 minutes.
Cooling and beating
Ice cream is a complex mix of small
ice crystals and air bubbles in a milk-fat/water emulsion. To achieve this, it
is necessary to cool the mixture quickly to produce small ice crystals and at
the same time incorporate air into the product by beating.
|
Ingredients
|
Processing
stage
|
Equipment
|
Section
reference
|
|
Milk, sugar, fat, thickener,
colours and flavours.
|
Mix ingredients
|
Weighing and measuring equipment
|
64.1 and 64.2
|
|
Liquid mixer
|
43.1
|
||
|
|
Pasteurize
|
Boiling pan or steam jacketed pan
|
48.0
|
|
Thermometer
|
63.0
|
||
|
Heat source
|
36.0
|
||
|
Cool mixture to approximately -5°C
and beat simultaneously
|
Ice cream maker
|
38.0
|
|
|
Fill into containers
|
Filling machine
|
28.0
|
|
|
Freeze at -18°C
|
Freezer
|
32.0
|
Ice cream makers are available
commercially and work on the following principle. The mixture is placed in a
bowl which is kept at a low temperature (either surrounded by ice and salt, or
having been chilled in a freezer). It is then agitated by a manually-operated
rotor or by a powered stirrer. At the end of this process the ice cream should
be at a temperature of approximately -5°C, and be partly frozen.
Packaging and storage
The ice cream is usually packaged in
plastic, waxed paper, or cardboard containers, and is stored at below -18°C.
The storage temperature is important for two reasons:
·
to maintain the texture of the product
· to prevent the growth of micro-organisms.
· to prevent the growth of micro-organisms.
Ice cream may be transported in an
insulated box (e.g. for sale from a bicycle). It is especially important to
guard against thawing and re-freezing as this will cause changes in texture and
mouthfeel, and there is the increased possibility of food poisoning by
contaminating food poisoning microorganisms