Transcript Food Biotechnology Dr. Kamal E. M. Elkahlout Applications of

Food Biotechnology
Dr. Kamal E. M. Elkahlout
Applications of Biotechnology to
Food Products 3
Production of Fermented Foods
(Fermented Foods Made from Milk)
• Composition of Milk
• Milk is the fluid from the mammary glands of animals
which is meant for feeding the young of mammals.
• It is a complex liquid consisting of several hundred
components of which the most important are proteins,
lactose, fat, minerals, enzymes, and vitamins in which
emulsified fat globules and casein micelles are present.
• Its composition varies from breed to breed, as shown in
Table 19.1.
• Proteins: Milk proteins are divided into two: caseins and
whey proteins.
• Caseins consist of carbohydrate, phosphorus, and
protein ( glyco-phspho-protein) and make up 85% of
the total milk proteins.
• Casein exists in milk as the calcium salt, ie, as calcium
caseinate in globules (micelles) ranging from 40-300
mμ in diameter.
• Casein exists in four types designated, , and and
depending on their electric charges.
• The proportion of the various types in milk depends on
the breed of the cow producing the milk.
• The letter ‘s’ after s - caseins indicates its sensitivity to
precipitation by calcium.
• Whey proteins consist of different components which
are normally stable to acid, but very sensitive to heat.
• Lactoglobulin forms about 66% of the total whey
proteins, followed by -Lactabumin (22%).
• The immune globulins from about 10% of the total, and
contribute towards the immunity derived by the young
from the consumption of colostrum.
• Lactose: The main carbohydrate in milk is lactose, which
is found only in milk.
• It is a dissacharide of glucose and galactose and has a
low sweetening ability, as well as low solubility in water.
• Fat: Fat consists of one molecule of glycerol and three
of fatty acids.
• Over 60 different acids are known in butter, many of
them, being of low molecular weight of about 10
carbon atoms or less, and include saturated and
unsaturated fatty acids.
• Enzymes: Enzymes found in milk include proteases,
carbohydrases, esterases, oxidases/reductases.
• Minerals: Milk is a major source of calcium; other
minerals in milk are phosphorous, magnesium, sodium,
potassium, as well as sulphate and chloride ions.
• When fat is removed from milk such as during butter
making, the remnant is skim milk.
• On the other hand, when casein is removed such as
during cheese manufacture, the remnant is known as
whey.
• Whey is high in lactose and its disposal sometimes
poses some problem as not all microorganisms can
break down whey. It is however used in the production
of yeasts to be used as food or fodder.
• Cheese
• Cheese is a highly proteinaceous food made from the
milk of some herbivores.
• Cheese is believed to have originated in the warm
climates of the Middle East some thousands of years
ago, and is said to have evolved when milk placed in
goat stomach was found to have curdled.
• The scientific study and manipulation of milk for cheese
manufacture is however just over a hundred years old.
• Most cheese in the temperate countries of the world
such as Western Europe and the USA is made from
cow’s milk, the composition of which varies according
to the breed of the cattle, the stage of lactation, the
adequacy of its nutrition, the age of the cow, and the
presence or absence of disease in the breasts (udders),
known as mastitis.
• In some subtropical countries milk from sheep, goats,
the lama, yak, or ass is also used.
• Sheep milk is used specifically for the production of
certain special cheese types in some parts of Europe
(e.g. Roquefort in France, and Brinsen in Hungary).
• Milk from the water buffalo may be used in India and
other countries, while milk from the reindeer and the
mare may be used in northern parts of Scandinavia and
in Russia, respectively.
• Cheese made from the milk of goat and sheep has a
much stronger flavor than that made from cow’s milk.
• This is because the fat in goat and sheep milk contain
much lower amounts of the lower fatty acids, caproic,
capryllic, and capric acids.
• These acids confer a sharp taste (similar to that of
Roquefort cheese) to cheese made from these
mammals.
• Future discussion of cheese in this chapter will however
refer to that made from cow’s milk.
• About a thousand types of cheese have been
described depending on the properties and
treatment of the milk, the method of production,
conditions such as temperature, and the properties
of the coagulum, and the local preferences.
• Stages in the manufacture of cheese
• The manufacture of all the types of cheeses include
all or some of the following processes:
• (a) Standardization of milk
• The quality of the milk has a decided effect on the
nature of cheese.
• Cheese made from skim milk is hard and leathery;
the more fat a cheese contains the smoother its feel
to the palate.
• The fat/protein ratio is often adjusted through fat
addition in order to yield a cheese of consistent quality.
• In the US, pasteurization (High Temperature Short
Time) or (Long Temperature Short Time) must be given
to milk to be in certain types of cheeses, such as
cottage or cream cheese.
• For others the milk need not be pasteurized but must
be stored at for at least 60 days at 2°C.
• If however the ‘starter’ is slow acting or souring is
delayed, food-poisoning staphylococci could develop
and produce toxins in the cheese.
• Sub-pasteurization temperatures are often the legal
compromise.
• Pasteurization gives a better control over the processes
of cheese production.
• However, the organisms present in raw milk are
important during the ripening processes.
• The milk may also be homogenized by forcing it at high
speed through small orifices to reduce the milk fat
globules for use in producing soft cheeses.
• (b) Inoculation of pure cultures of lactic acid bacteria as
starter cultures.
• In the past, lactic acid was produced by naturally
occurring bacteria.
• Nowadays they are inoculated artificially, by specially
selected bacteria termed starters.
• Indeed lactic acid formation is necessary in all kinds of
cheese.
• The propagation and distribution of lactic acid bacteria
for use in cheese manufacture is an industry in its own
right in the US.
• For cheese prepared at temperatures less than 40°C
strains of Lactococcus lactis are used.
• For those prepared at higher temperatures the
more thermophilic Streptococcus thermophilus,
Lactobacillus bulgaricus, and Lact. helveticus are
used.
• Lactic acid has the following effects:
• (i) It causes the coagulation of casein at pH 4.6, the
isoelectric point of that protein, which is used in the
manufacture of some cheeses, e.g. cottage cheese.
• (ii) It provides a favorably low pH for the action of
rennin the enzyme which forms the curd from
casein in other types of cheeses.
• (iii) The low pH eliminates proteolytic and other
undesirable bacteria.
• (iv) It causes the curd to shrink and thus promotes the
drainage of whey.
• (v) Metabolic products from the lactic acid bacteria
such as ketones, esters and aldehydes contribute to the
flavor of the cheese.
• Problems of lactic acid bacteria in cheese-making
• (i) Attack by bacteriophages: Bacteriophages
sometimes attack the lactic acid starters and besides
choosing strains that are resistant to phages, rotations
(i.e., using different lactic mixtures every three or four
days) helps eliminate them.
• (ii) Inhibition by penicillin and other antibiotics: Lactic
acid bacteria, being Gram-positive are particularly
susceptible to penicillin used to treat diseased udder in
mastitis if the antibiotic finds its way into the milk;
other antiobiotics also have an inhibitory effect on
them.
• (iii) Undesirable strains: Some strains of lactic acid
bacteria are undesirable in cheese making because they
produce too much gas, undesirable flavors, or produce
antibiotics against other lactic acid bacteria.
• They arise by mutation.
• (iv) Sterilant and detergent residues: Sterilant and
detergent residues may inhibit the growth of starter
bacteria.
• The minimum concentration required for inhibition
varies with the different anti-microbial agents and
between different strains of starter bacteria.
• Residues gain entry to milk at the (a) farm, (b) during
transportation to the factory, and (c) the factory due to
careless use of sterilants or detergents, incomplete
draining or inadequate rinsing of equipment.
• The inhibitory effects of sterilant and detergent
residues are prevented by the correct and ethical use of
these materials.
• Proper use includes the use of the chemical at the
correct concentration and adequate rinsing and
draining.
• Their presence is mitigated by dilution with
uncontaminated milk.
• (c) Adding of rennet for coagulum formation
• The classical material used in the formation of the
coagulum is ‘rennet’ which is derived from the fourth
stomach, abomasum or vell of freshly slaughtered milkfed calves.
• Besides those of calves, the abomasum of kids (young
goats), lamb or other young mammals have been used.
• Rennet is produced by soaking and/or shredding airdried vells under acid conditions with 12-20% salt.
• Extracts from young calves contain 94% rennin and
6% pepsin and from older cows, 40% rennin and
60% pepsin.
• Rennin (chymosin) is the enzyme responsible for
the coagulation of the milk.
• Pepsin is proteolytic and too high an amount of
pepsin can result in the hydrolysis of the coagulum
and a resulting low yield of cheese, and a bitter
taste may result from the amino acids.
• Due to the high cost of animal rennet, other
sources, mostly of microbial origins, have been
found (Table 19.2).
• The major effect of the milk-clotting enzymes is the
conversion of casein from a colloidal to a fibrous
form.
• First the pH of the milk is brought down from pH 6.8
-7 to pH 5.5 by the action of lactic acid bacteria
which produce lactic acid from lactose in the milk.
• On addition of rennet, the active component,
rennin, catalyses the hydrolyses of k-casein to
release para-k-casein and k-casein macropeptide.
• The latter goes into whey, while the para-k-casein
remains part of casein micelles, which now bind
together to form the curd following the removal of
carbohydrates with the k-casein macropeptide and
the exposure of binding surfaces.
• The events up this coagulation are aided by lowered pH
and by increasing temperatures up to 45°C.
• Most of the bacteria, fat, and other particulate matter
are entrapped in the curd.
• When casein is removed the remaining liquid
containing proteins, lactalbumin, globulin, and yellowgreen riboflavin (vitamin B2) is whey.
• The whey proteins may be precipitated by heat, but not
acid or rennin and they are used in making whey
cheese.
• The enzymes used in cheese making are now obtained
from microorganisms, mainly fungi.
• (d) Shrinkage of the curd
• The removal of whey and further shrinkage of the curd
is greatly facilitated by heating it, cutting it into smaller
pieces, applying some pressure on it and lowering the
pH.
• In many types of cheeses, such as Parmesan,
Emmenthal and Gruyere, there is a stage known as
‘scalding’ in which the temperature can be as high as
56°C in the preparation.
• Acid produced by the lactic starters introduce elasticity
in the curd, a property desirable in the final qualities of
cheese.
• (e) Salting of the curd and pressing into shape
• Salt is added to most cheese varieties at some stage in
their manufacture.
• Salt is important not only for the taste, but it also
contributes to moisture and acidity control.
• Most importantly however it helps limit the growth of
proteolytic bacteria which are undesirable.
• The curd is pressed into shape before being allowed to
mature.
• (f) Cheese ripening
• The ripening or maturing of cheese is a slow joint
microbiological and biochemical process which
converts the brittle white curd or raw cheese to the
final full-flavored cheese.
• The agents responsible for the final change are
enzymes in the milk, in the rennet and those from the
added starter microorganisms as well as other microorganisms which confer the special character of the
cheese to it.
• Among the cheese whose peculiar characteristics are
dependent on particular microorganisms are the blueveined cheese Roqueforti, Gorgonzola, Stilton,
conferred by Penicillium roquefort.
• Swiss cheese, with its characteristic flavor and holes
produced by the fermentation products and gases
from Propionibacterium spp. Yeasts, micrococci, and
Brevibacterium linens impart the characteristic
flavor of Limburger cheese.
• In soft cheese, such as Camembert, the protein is
completely broken down to almost amino acids,
whereas in the hard cheese, the protein remains
intact.
• Yoghurt and Fermented Milk Foods
• Many types of fermented milks are produced and drunk
around the world (Table 19.3).
• Yoghurt is a fermented milk traditionally believed to be
an invention of the Turks of Central Asia, in whose
language the word yoghurt means to blend, a reference
to how the milk product is made.
• Although accidentally invented thousands of years ago,
yoghurt has only recently gained popularity in the
United States.
• While yoghurt has been present for many years, it is
only recently (within the last 30-40 years) that it has
become popular.
• This is due to many factors including the introduction of
fruit and other flavorings into yoghurt, the convenience
of it as a ready-made breakfast food and the image of
yoghurt as a low fat healthy food.
• In the manufacture of yoghurt, two kinds of lactic
acid bacteria, Lactococcus spp. & Lactobacillus spp.,
are generally used with usually unpasteurized milk.
• Most commonly used are Lactococcus salivarius and
thermophilus, and Lactobacillus spp., such as Lacto.
acidophilus, bulgaricus and bifidus.
• The bacteria produce lactic acid from lactose in the
milk causing the pH to drop to about 4-5 from
about 7.0.
• This drop in pH causes the milk to coagulate.
• The lactic acid gives yoghurt its sour taste and limits
the growth of spoilage bacteria.
• Yoghurt is flavored usually with fruits.