A
dairy is a facility for the extraction and
processing of animal
milk—mostly from
cows or
goats, but also from
buffalo,
sheep,
horses or
camels —for human
consumption. Typically it is a farm (
dairy farm) or
section of a farm that is concerned with the production of milk,
butter and cheese.
Terminology differs slightly between countries. In particular, in
the U.S. a
dairy can also be a facility that processes,
distributes and sells
dairy products,
or a room, building or establishment where milk is kept and butter
or cheese is made. In
New Zealand
English a
dairy means a corner
convenience store, or
Superette—and
dairy factory is the term
for what is elsewhere called a
dairy.
As an attributive, the word
dairy refers to milk-based
products, derivatives and processes, and the animals and workers
involved in their production: for example
dairy cattle,
dairy
goat. A
dairy farm produces milk and
a dairy
factory processes it into a variety
of
dairy products. These
establishments constitute the dairy industry, a component of the
food industry.
History
Milk producing animals have been domesticated for thousands of
years. Initially, they were part of the subsistence farming that
nomads engaged in. As the community moved about the country, their
animals accompanied them. Protecting and feeding the animals were a
big part of the symbiotic relationship between the animals and the
herders.
In the more recent past, people in
agricultural societies owned dairy animals that
they milked for domestic and local (village) consumption, a typical
example of a
cottage industry. The
animals might serve multiple purposes (for example, as a draught
animal for pulling a
plough as a youngster,
and at the end of its useful life as
meat). In
this case the animals were normally milked by hand and the herd
size was quite small, so that all of the animals could be milked in
less than an hour—about 10 per milker. These tasks were performed
by a
dairymaid (
dairywoman) or
dairyman.
The word
dairy harkens back to Middle English
dayerie,
deyerie, from
deye (female
servant or dairymaid) and further back to Old English
dæge
(kneader of bread).
With
industrialisation and
urbanisation, the supply of milk became
a
commercial industry, with
specialised
breeds of
cattle being developed for dairy, as distinct from
beef or
draught
animals. Initially, more people were employed as milkers, but it
soon turned to
mechanisation with
machines designed to do the milking.
Farmer milking a cow by hand.
Historically, the
milking and the
processing took place close together in
space and time: on a
dairy farm.
People milked the animals by hand; on farms where only small
numbers are kept, hand-milking may still be practiced. Hand-milking
is accomplished by grasping the
teats (often
pronounced
tit or
tits) in the hand and
expressing milk either by squeezing the fingers progressively, from
the
udder end to the tip, or by squeezing the
teat between thumb and index finger, then moving the hand downward
from udder towards the end of the teat. The action of the hand or
fingers is designed to close off the milk duct at the udder (upper)
end and, by the movement of the fingers, close the duct
progressively to the tip to express the trapped milk. Each half or
quarter of the udder is emptied one milk-duct capacity at a
time.
The
stripping action is repeated, using both hands for
speed. Both methods result in the milk that was trapped in the
milk duct being squirted out the end into
a bucket that is supported between the knees (or rests on the
ground) of the milker, who usually sits on a low stool.
Traditionally the cow, or cows, would stand in the
field or
paddock
while being milked. Young stock,
heifer,
would have to be trained to remain still to be milked. In many
countries, the cows were tethered to a post and milked. The problem
with this method is that it relies on quiet, tractable beasts,
because the hind end of the cow is not restrained.
In 1937, it was found that
bovine
somatotropin (bST or bovine growth hormone) would increase the
yield of milk.
Monsanto Company
developed a synthetic (recombinant) version of this hormone (rBST).
In February 1994, rBST was approved by the Food and Drug
Administration (FDA) for use in the U.S. It has become common in
the U.S., but not elsewhere, to inject it into milch kine (dairy
cows) to increase their production by up to 15%.
However, there are claims that this practice can have negative
consequences for the animals themselves.A
European Union scientific commission was
asked to report on the incidence of mastitis and other disorders in
dairy cows, and on other aspects of the welfare of dairy cows. The
commission's statement, subsequently adopted by the European Union,
stated that the use of rBST substantially increased health problems
with cows, including foot problems, mastitis and injection site
reactions, impinged on the welfare of the animals and caused
reproductive disorders. The report concluded that on the basis of
the health and welfare of the animals, rBST should not be used.
Health Canada prohibited the sale of rBST in
1999; the recommendations of external committees were that, despite
not finding a significant health risk to humans, the drug presented
a threat to animal health and, for this reason, could not be sold
in Canada
.
Structure of the industry
While most countries produce their own milk products, the structure
of the dairy industry varies in different parts of the world. In
less developed countries the producer generally sells directly to
the public, whereas in major milk-producing countries most milk is
distributed through wholesale markets. In Ireland and Australia,
for example, farmers' co-operatives own many of the large-scale
processors, while in the United States many farmers and processors
do business through individual contracts. In the United States, the
country's 196
farmers'
cooperatives sold 86% of milk in the U.S. in 2002, with five
cooperatives accounting for half that. This was down from 2,300
cooperatives in the 1940s.
As in many other branches of the food industry, dairy processing in
the major dairy producing countries has become increasingly
concentrated, with fewer but larger and more efficient plants
operated by fewer workers. This is notably the case in the United
States, Europe, Australia and New Zealand. In 2009, charges of
anti-trust violations have been made
against major dairy industry players in the United States.
Government intervention in milk markets was common in the 20th
century. A limited anti-trust exemption was created for U.S. dairy
cooperatives by the Capper-Volstead Act of 1922. In the 1930s, some
U.S. states adopted price controls, and Federal Milk Marketing
Orders started under the Agricultural Marketing Agreement Act of
1937 and continue in the 2000s. The Federal Milk Price Support
Program began in 1949. The
Northeast Dairy Compact regulated
wholesale milk prices in New England from 1997 to 2001.
Plants producing liquid milk and products with short shelf life,
such as yogurts, creams and soft cheeses, tend to be located on the
outskirts of urban centres close to consumer markets. Plants
manufacturing items with longer shelf life, such as butter, milk
powders, cheese and whey powders, tend to be situated in rural
areas closer to the milk supply. Most large processing plants tend
to specialise in a limited range of products. Exceptionally,
however, large plants producing a wide range of products are still
common in Eastern Europe, a holdover from the former centralized,
supply-driven concept of the market.
As processing plants grow fewer and larger, they tend to acquire
bigger, more automated and more efficient equipment. While this
technological tendency keeps manufacturing costs lower, the need
for long-distance transportation often increases the environmental
impact.
Milk production is irregular, depending on cow biology. Producers
must adjust the mix of milk which is sold in liquid form vs.
processed foods (such as butter and cheese) depending on changing
supply and demand.
Operation of the dairy farm
See
dairy farming and
dairy cattle for more information.
When it became necessary to milk larger numbers of cows, the cows
would be brought to a
shed or
barn that was set up with
bails (
stall) where
the cows could be confined while they were milked. One person could
milk more cows this way, as many as 20 for a skilled worker. But
having cows standing about in the yard and shed waiting to be
milked is not good for the cow, as she needs as much time in the
paddock grazing as is possible. It is usual to restrict the
twice-daily milking to a maximum of an hour and a half each time.
It makes no difference whether one milks 10 or 1000 cows, the
milking time should not exceed a total of about three hours each
day for any cow.
As
herd sizes increased there was more need to
have efficient milking machines, sheds, milk-storage facilities
(
vat), bulk-milk transport and shed
cleaning capabilities and the means of getting cows from paddock to
shed and back.
Farmers found that cows would abandon their grazing area and walk
towards the milking area when the time came for milking. This is
not surprising as, in the flush of the milking season, cows
presumably get very uncomfortable with udders engorged with milk,
and the place of relief for them is the milking shed.
As herd numbers increased so did the problems of
animal health.
In New Zealand
two approaches to this problem have been
used. The first was improved
veterinary medicines (and the government
regulation of the medicines) that the farmer could use. The other
was the creation of
veterinary clubs where groups of
farmers would employ a
veterinarian (vet) full-time and share
those services throughout the year. It was in the vet's interest to
keep the animals healthy and reduce the number of calls from
farmers, rather than to ensure that the farmer needed to call for
service and pay regularly.
Most dairy farmers milk their cows with absolute regularity at a
minimum of twice a day, with some high-producing herds milking up
to four times a day to lessen the weight of large volumes of milk
in the udder of the cow. This daily milking routine goes on for
about 300 to 320 days per year that the cow stays in milk. Some
small herds are milked once a day for about the last 20 days of the
production cycle but this is not usual for large herds. If a cow is
left unmilked just once she is likely to reduce milk-production
almost immediately and the rest of the season may see her
dried
off (giving no milk) and still consuming feed for no
production. However, once-a-day milking is now being practised more
widely in New Zealand for profit and
lifestyle reasons. This is effective because the
fall in milk yield is at least partially offset by labour and cost
savings from milking once per day.
This compares to some intensive farm
systems in the United
States
that milk three or more times per day due to higher
milk yields per cow and lower marginal labor
costs.
Farmers who are contracted to supply liquid milk for human
consumption (as opposed to milk for processing into
butter,
cheese, and so on—see
milk) often have to manage their
herd so that the contracted number of cows are in milk
the year round, or the required minimum milk output is maintained.
This is done by mating cows outside their natural mating time so
that the period when each cow in the herd is giving maximum
production is in rotation throughout the year.
Northern hemisphere farmers who keep cows in barns almost all the
year usually manage their herds to give continuous production of
milk so that they get paid all year round. In the southern
hemisphere the
cooperative dairying
systems allow for two months on no productivity because their
systems are designed to take advantage of maximum grass and milk
production in the spring and because the milk processing plants pay
bonuses in the dry (winter) season to carry the farmers through the
mid-winter break from milking. It also means that cows have a rest
from milk production when they are most heavily pregnant. Some
year-round milk farms are penalised financially for over-production
at any time in the year by being unable to sell their
overproduction at current prices.
Artificial insemination (AI)
is common in all high-production herds.
Industrial processing
Dairy plants process the raw milk they receive from farmers so as
to extend its marketable life. Two main types of processes are
employed: heat treatment to ensure the safety of milk for human
consumption and to lengthen its shelf-life, and dehydrating dairy
products such as butter, hard cheese and milk powders so that they
can be stored.
Cream and butter
Today, milk is separated by large machines in bulk into cream and
skim milk. The cream is processed to produce various consumer
products, depending on its thickness, its suitability for culinary
uses and consumer demand, which differs from place to place and
country to country.
Some cream is dried and powdered, some is condensed (by
evaporation) mixed with varying amounts of
sugar and canned. Most cream from New Zealand
and Australian factories is made into
butter.
This is done by
churning the cream
until the fat globules coagulate and form a monolithic mass. This
butter mass is washed and, sometimes, salted to improve keeping
qualities. The residual
buttermilk goes
on to further processing. The butter is packaged (25 to 50 kg
boxes) and chilled for storage and sale. At a later stage these
packages are broken down into home-consumption sized packs. Butter
sells for about US$3200 a
tonne on the
international market in 2007 (an unusual high).
Skimmed milk
The product left after the cream is removed is called skim, or
skimmed, milk. Reacting skim milk with
rennet
or with an acid makes
casein curds from the milk solids in skim milk, with
whey as a residual. To make a consumable liquid a
portion of cream is returned to the skim milk to make
low fat milk (semi-skimmed) for human
consumption. By varying the amount of cream returned, producers can
make a variety of low-fat milks to suit their local market. Other
products, such as
calcium,
vitamin D, and flavouring, are also added to
appeal to consumers.
Casein
Casein is the predominant
phosphoprotein found in fresh milk. It has a
very wide range of uses from being a filler for human foods, such
as in
ice cream, to the manufacture of
products such as
fabric,
adhesives, and
plastics.
Cheese
Cheese is another product made from milk.
Whole milk is reacted to form curds that can be compressed,
processed and stored to form cheese. In countries where milk is
legally allowed to be processed without
pasteurisation a wide range of cheeses can be
made using the bacteria naturally in the milk. In most other
countries, the range of cheeses is smaller and the use of
artificial cheese curing is greater. Whey is also the byproduct of
this process.
Cheese has historically been an important way of "storing" milk
over the year, and carrying over its nutritional value between
prosperous years and fallow ones. It is a food product that, with
bread and
beer, dates back
to
prehistory in Middle Eastern and
European cultures, and like them is subject to innumerable variety
and local specificity. Although nowhere near as big as the market
for cow's milk cheese, a considerable amount of cheese is made
commercially from other milks, especially goat and sheep (see
Roquefort cheese for a notable
example).
Whey
In earlier times whey was considered to be a waste product and it
was, mostly, fed to pigs as a convenient means of disposal.
Beginning about 1950, and mostly since about 1980,
lactose and many other products, mainly food
additives, are made from both casein and cheese whey.
Yogurt
Yoghurt (or yogurt) making is a process
similar to cheese making, only the process is arrested before the
curd becomes very hard.
Milk powders
Milk is also processed by various drying processes into powders.
Whole milk, skim milk, buttermilk, and whey products are dried into
a powder form and used for human and animal consumption. The main
difference between production of powders for human or for animal
consumption is in the protection of the process and the product
from contamination. Some people drink milk reconstituted from
powdered milk, because milk is about 88% water and it is much
cheaper to transport the dried product. Dried skim milk powder is
worth about US$5300 a
tonne (mid-2007 prices)
on the international market.
Other milk products
Kumis is produced commercially in
Central Asia. Although it is traditionally made
from
mare's milk, modern industrial variants
may use cow's milk instead.
Transport of milk
Historically, the milking and the processing took place in the same
place: on a dairy farm. Later, cream was separated from the milk by
machine, on the farm, and the cream was transported to a
factory for buttermaking. The skim milk was fed to
pigs. This allowed for the high cost of
transport (taking the smallest volume high-value product),
primitive trucks and the poor quality of roads. Only farms close to
factories could afford to take whole milk, which was essential for
cheesemaking in industrial quantities, to them. The development of
refrigeration and better road
transport, in the late 1950s, has meant that most farmers milk
their cows and only temporarily store the milk in large
refrigerated
bulk tanks, whence it is
later transported by truck to central processing facilities.
Milking machines
The milking machine extracts milk
from all teats.
Milking machines are used to harvest milk from cows when manual
milking becomes inefficient or labour intensive. The milking unit
is the portion of a milking machine for removing milk from an
udder. It is made up of a claw, four teatcups, (Shells and rubber
liners) long milk tube, long pulsation tube, and a pulsator. The
claw is an assembly that connects the short pulse tubes and short
milk tubes from the teatcups to the long pulse tube and long milk
tube. (Cluster assembly) Claws are commonly made of stainless steel
or plastic or both. Teatcups are composed of a rigid outer shell
(stainless steel or plastic) that holds a soft inner liner or
inflation. Transparent sections in the shell may allow
viewing of liner collapse and milk flow. The annular space between
the shell and liner is called the pulse chamber.
Milking machines work in a way that is different from hand milking
or calf suckling. Continuous vacuum is applied inside the soft
liner to massage milk from the teat by creating a pressure
difference across the teat canal (or opening at the end of the
teat). Vacuum also helps keep the machine attached to the cow. The
vacuum applied to the teat causes congestion of teat tissues
(accumulation of blood and other fluids). Atmospheric air is
admitted into the pulsation chamber about once per second (the
pulsation rate) to allow the liner to collapse around the end of
teat and relieve congestion in the teat tissue. The ratio of the
time that the liner is open (milking phase) and closed (rest phase)
is called the pulsation ratio.
The four streams of milk from the teatcups are usually combined in
the claw and transported to the milkline, or the collection bucket
(usually sized to the output of one cow) in a single milk hose.
Milk is then transported (manually in buckets) or with a
combination of airflow and mechanical
pump to a
central storage
vat or
bulk
tank. Milk is refrigerated on the farm in most countries either
by passing through a
heat-exchanger
or in the bulk tank, or both.
In the photo above is a bucket milking system with the stainless
steel bucket visible on the far side of the cow. The two rigid
stainless steel teatcup shells applied to the front two quarters of
the udder are visible. The top of the flexible liner is visible at
the top of the shells as are the short milk tubes and short
pulsation tubes extending from the bottom of the shells to the
claw. The bottom of the claw is transparent to allow observation of
milk flow. When milking is completed the vacuum to the milking unit
is shut off and the teatcups are removed.
Milking machines keep the milk enclosed and safe from external
contamination. The interior 'milk contact' surfaces of the machine
are kept clean by a manual or automated washing procedures
implemented after milking is completed. Milk contact surfaces must
comply with regulations requiring food-grade materials (typically
stainless steel and special plastics
and rubber compounds) and are easily cleaned.
Most milking machines are powered by
electricity but, in case of
electrical failure, there can be an alternative means of motive
power, often an
internal
combustion engine, for the vacuum and milk pumps. Milk cows
cannot tolerate delays in scheduled milking without serious milk
production reductions.
Milking shed layouts
Bail-style sheds—This type of milking facility was
the first development, after open-paddock milking, for many
farmers. The building was a long, narrow,
lean-to shed
that was open along one long side. The cows were held in a yard at
the open side and when they were about to be milked they were
positioned in one of the bails (stalls). Usually the cows were
restrained in the bail with a breech chain and a rope to restrain
the outer back leg. The cow could not move about excessively and
the milker could expect not to be kicked or trampled while sitting
on a (three-legged) stool and milking into a bucket. When each cow
was finished she backed out into the yard again. The UK bail,
developed largely by Rex Patterson, was a six standing mobile shed
with steps that the cow mounted, so the herdsman didn't have to
bend so low. The milking equipment was much as today, a vacuum from
a pump, pulsators, a claw-piece with pipes leading to the four
shells and liners that stimulate and suck the milk from the teat.
The milk went into churns, via a cooler.
As herd sizes increased a door was set into the front of each bail
so that when the milking was done for any cow the milker could,
after undoing the leg-rope and with a remote link, open the door
and allow her to exit to the pasture. The door was closed, the next
cow walked into the bail and was secured. When milking machines
were introduced bails were set in pairs so that a cow was being
milked in one paired bail while the other could be prepared for
milking. When one was finished the machine's cups are swapped to
the other cow. This is the same as for
Swingover Milking
Parlours as described below except that the cups are loaded on
the udder from the side. As herd numbers increased it was easier to
double-up the cup-sets and milk both cows simultaneously than to
increase the number of bails. About 50 cows an hour can be milked
in a shed with 8 bales by one person. using the same teat cups for
successive cows has the danger of transmitting infection, mastitis,
from one cow to another. Some farmers have devised their own ways
to disinfect the clusters between cows.
Herringbone Milking Parlours—In herringbone
milking sheds, or parlours, cows enter, in single file, and line up
almost perpendicular to the central aisle of the milking parlour on
both sides of a central pit in which the milker works (you can
visualise a fishbone with the ribs representing the cows and the
spine being the milker's working area; the cows face outward).
After washing the udder and teats the cups of the milking machine
are applied to the cows, from the rear of their hind legs, on both
sides of the working area. Large herringbone sheds can milk up to
600 cows efficiently with two people.
Swingover Milking Parlours—Swingover parlours are
the same as herringbone parlours except they have only one set of
milking cups to be shared between the two rows of cows, as one side
is being milked the cows on the other side are moved out and
replaced with unmilked ones. The advantage of this system is that
it is less costly to equip, however it operates at slightly better
than half-speed and one would not normally try to milk more than
about 100 cows with one person.
Rotary Milking sheds—Rotary milking sheds consist
of a turntable with about 12 to 100 individual stalls for cows
around the outer edge. A "good" rotary will be operated with 24–32
(~48–50+) stalls by one (two) milkers. The turntable is turned by
an electric-motor drive at a rate that one turn is the time for a
cow to be milked completely. As an empty stall passes the entrance
a cow steps on, facing the centre, and rotates with the turntable.
The next cow moves into the next vacant stall and so on. The
operator, or milker, cleans the teats, attaches the cups and does
any other feeding or whatever husbanding operations that are
necessary. Cows are milked as the platform rotates. The milker, or
an automatic device, removes the milking machine cups and the cow
backs out and leaves at an exit just before the entrance. The
rotary system is capable of milking very large herds—over a
thousand cows.
80-stand rotary dairy that is fully
computerised and records milk production
Automatic Milking sheds—
Automatic milking or 'robotic milking'
sheds can be seen in
Australia, New
Zealand and many European countries. Current automatic milking
sheds use the
voluntary
milking (VM) method. These allow the cows to voluntarily
present themselves for milking at any time of the day or night,
although repeat visits may be limited by the farmer through
computer software. A
robot arm is used to
clean teats and apply milking equipment, while automated gates
direct cow traffic, eliminating the need for the farmer to be
present during the process. The entire process is computer
controlled. There is a description of an automatic system
here—
Supplementary accessories in sheds—Farmers soon
realised that a milking shed was a good place to feed cows
supplementary foods that overcame local dietary deficiencies or
added to the cows' wellbeing and production. Each bail might have a
box into which such feed is delivered as the cow arrives so that
she is eating while being milked. A computer can read the eartag of
each beast to ration the correct individual supplement. A close
alternative is to use 'out-of-parlour-feeders', stalls that respond
to a transponder around the cow's neck that is programmed to
provide each cow with a supplementary feed, the quantity dependent
on her production, stage in lactation, and the benefits of the main
ration
The holding yard at the entrance of the shed is important as a
means of keeping cows moving into the shed. Most yards have a
powered gate that ensures that the cows are kept close to the
shed.
Water is a vital commodity on a dairy farm:
cows drink about 20 gallons (80 litres) a day, sheds need water to
cool and clean them. Pumps and reservoirs are common at milking
facilities. Water can be warmed by heat transfer with milk.
Temporary milk storage
Milk coming from the cow is transported to a nearby storage vessel
by the airflow leaking around the cups on the cow or by a special
"air inlet" (5-10 l/min free air) in the claw. From there it is
pumped by a mechanical pump and cooled by a
heat exchanger. The milk is then stored in a
large vat, or
bulk tank, which is usually
refrigerated until collection for processing.
Processing facilities
Topics:
Waste disposal
In countries where cows are grazed outside year-round, there is
little waste disposal to deal with. The most concentrated waste is
at the milking shed, where the animal waste is liquefied (during
the water-washing process) and allowed to flow by gravity, or
pumped, into
composting ponds with
anaerobic bacteria to consume the
solids. The processed water and nutrients are then pumped back onto
the pasture as
irrigation and
fertilizer.Surplus animals are slaughtered for
processed
meat and other
rendered products.
In the associated milk processing factories, most of the waste is
washing water that is treated, usually by composting, and returned
to waterways. This is much different from half a century ago, when
the main products were butter, cheese and casein, and the rest of
the milk had to be disposed of as waste (sometimes as animal
feed).
In areas where cows are housed all year round, the waste problem is
difficult because of the amount of feed that is bought in and the
amount of bedding material that also has to be removed and
composted. The size of the problem can be understood by standing
downwind of the barns where such dairying goes on.
In many cases, modern farms have very large quantities of milk to
be transported to a factory for processing. If anything goes wrong
with the milking, transport or processing facilities it can be a
major disaster trying to dispose of enormous quantities of milk. If
a road
tanker overturns on a road, the
rescue crew is looking at accommodating the spill of 5 to 10
thousand gallons of milk (20 to 45 thousand litres) without
allowing any into the waterways. A derailed rail
tanker-train may involve 10 times that amount.
Without refrigeration, milk is a fragile commodity, and it is very
damaging to the environment in its raw state. A widespread
electrical power
blackout is another
disaster for the dairy industry, because both milking and
processing facilities are affected.
In dairy-intensive areas, the simplest way of disposing of large
quantities of milk has been to dig a large hole in the ground and
allow the clay to filter the milk solids as it soaks away. This is
not very satisfactory.
Associated diseases
- Leptospirosis is one of the most
common debilitating diseases of milkers, made somewhat worse since
the introduction of herringbone sheds, because of unavoidable
direct contact with bovine urine
- Cowpox is one of the helpful diseases; it
is barely harmful to humans and tends to inoculate them against
other poxes such as small
pox.
- Tuberculosis (TB) is able to be
transmitted from cattle mainly via milk products that are unpasteurised. TB has been eradicated from
many countries by testing for the disease and culling suspected
animals.
- Brucellosis is a bacterial disease
transmitted to humans by dairy products and direct animal contact.
Brucellosis has been eradicated from certain countries by testing
for the disease and culling suspected animals
- Listeria is a bacterial
disease associated with unpasteurised
milk, and can affect some cheeses made in traditional ways. Careful
observance of the traditional cheesemaking methods achieves
reasonable protection for the consumer.
- Johne's Disease (pronounced
"yo-knees") is a contagious, chronic and sometimes fatal infection
in ruminants caused by a bacterium named
Mycobacterium
avium subspecies paratuberculosis (M. paratuberculosis). The bacteria are
present in retail milk, and are believed by some researchers to be
the primary cause of Crohn's disease
in humans. This disease is not known to infect animals in Australia
and New Zealand.
Notes
References
- Jay, J.M. (1992). Modern Food Microbiology. Fourth
Edition. New York: Chapman & Hall. pp. 237-9.
- Potter, N.N. & J. H. Hotchkiss. (1995). Food
Science. Fifth Edition. New York: Chapman & Hall.
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