rural dairy technology
Post on 05-Jul-2018
223 Views
Preview:
TRANSCRIPT
-
8/15/2019 Rural Dairy Technology
1/123
Rural
Dairy
Technology
C.B.O'Connor
International Livestock
Research
Institute
Addis Ababa,Ethiopia
January 1995
-
8/15/2019 Rural Dairy Technology
2/123
-
8/15/2019 Rural Dairy Technology
3/123
5.3
Y
easts
.......................................................
23
5.4
V
iruses ......................................................
23
5.5 Milk m icrobiology
.............................................
24
5.5.1
Pasteurisation
......................................... 25
5.5.2
Sterilisation ..........................................
25
5.6
M
icrobiology of
butter .........................................
26
5.6.1
Sources
ofcontamination
...............................
26
5.6.2
Control
of
micro-organisms
it.butter
......................
26
6.
Clean
m
ilk
production
................................................
29
6.1 Sourcesofcontamination
.......................................
29
6.1.1
The interior of
the
udder
................................
29
6.1.2
The cow
as
a
source
of
pathogens
.........................
29
6.1.3
Milking utensils
.......................................
30
6.1.4 Miscellaneous sources
ofbacteria in
milk ..................
30
6.2 Cooling
m
ilk
.................................................
30
7. Milkreception, dairy
accounting and record
keeping ........................
33
7.1
Reception
....................................................
33
7.1.1
Q
uality
..............................................
33
7.1.2 Quantity .............................................
33
7.1.3
Composition
of
m ilk
...................................
33
7.1.4 Adulteration of m
ilk
...................................
33
7.2. Dairyaccounting
and recordkeeping
.............................
34
7.2.1
M
ilk
quantityand
quality ................................
34
7.2.2
Processingrecords .....................................
34
7.2.3
Records ofproduct
quality and
sales
.......................
34
7.2.4 Suggested
formats for recordsof
milkintake,
payment
and
utilisation
.........................................
35
8.
M
ilk
processing
..................................................... 37
8.1
M
ilk
separation ...............................................
37
8.1.1
G
ravity
separation
.....................................
37
8.1.2
Centrifugal separation ..................................
38
8.1.3
Hand
separator
........................................
40
8.1.4
Separator
maintenance ..................................
42
8.1.5 Calculations
..........................................
42
8.1.6
Standardisation
ofmilkandcream
........................
44
8.2
Buttermaking
with
fresh milk
or
cream
............................
46
iv
-
8/15/2019 Rural Dairy Technology
4/123
8.2.1 Buttermaking
theory
....................................
47
8.2.2 Theory
ofthe
mechanism of
churning
......................
47
8.2.3 Chum
preparation ......................................
47
8.2.4
Churning
temperature ...................................
47
8.2.5
W
ashing the
butter
.....................................
48
8.2.6 Salting, working andpacking
the
butter
....................
48
8.2.7
Washing thechurn
and
buttermaking
equipment after
use
48
8.2.8
Overrun
and
produce in buttermaking
......................
49
8.2.9 Butterquality
.........................................
50
8.3 Buttermaking
with
sour
whole
milk ...............................
51
8.3.1
Buttermaking .............................................
51
8.4 Ghee,
butteroiland dry butterfat
..................................
53
8.5
Cheesemaking
using fresh milk ...................................
53
8.5.1 Rennet
coagulation theory
... ...........................
54
8.5.2
Cheese
varieties
.......................................
54
8.5.3
Cheese yield ..........................................
67
8.6 Cheesemaking
with sour
skim
milk
................................
67
8.7
M
ilk
ferm entations .............................................
68
8.7.1 Ferm
ented
m ilks .......................................
69
8.7.2 Concentrated
fermented milk .............................
71
9. Cleaning,sanitising
and sterilisingdairy
equipment
......................... 73
9.1
Dairy
water
supplies ............................................
73
9.1.1
Bacteriological
quality ..................................
73
9.1.2
Chem ical quality .......................................
73
9.1.3
W
ater softeners ........................................
74
9.2
Chemicals used for
cleaning .....................................
74
9.2.1
Sterilisers
............................................ 75
9.3
Cleaning
procedure ............................................
75
10. Sampling and
analysis of
milk,
milkproducts
and
water
...................... 77
10.1 Sampling
.....................................................
77
10.2
Milk
pH
.....................................................
77
10.2.1
Measuring pH
using indicator ............................
77
10.2.2
Electrometric
measurement
of
pH .........................
77
10.3 Titratable
aciditytest
...........................................
78
10.3.1
Using
N10
sodium hydroxide
...........................
79
10.3.2
Using
N/9
sodiumhydroxide
(milk) .......................
79
v
-
8/15/2019 Rural Dairy Technology
5/123
10.3.3 Using N/9sodium
hydroxide
(cream)
...................... 80
10.4 A lcohol
test
.................................................. 80
10.5 Clot-on-boiling
test
............................................ 80
10.6
Fatdetermination
.................................
81
10.6.1
M
ilk ................................................ 81
10.6.2
Skimmilk,
buttermilkand
whey
..........................
84
10.6.3
Cream
............................................... 84
10.6.4 Cheese ..............................................
85
10.7 Specific gravityof
milk
......................................... F5
10.8 Total solids(TS) inmilk ........................................ 87
10.8.1 Lactometermethod ....................................
87
10.8.2
Oven-drying method
...................................
87
10.9
Formaldehyde in milk ..........................................
88
10.10 Methylene
blue
reduction
test
.................................... 88
10.11 Resazurin
10-minute test
........................................ 90
10.12 Sediment orvisible dirt test ...................................... 91
10.13 M
oisture content of
butter ....................................... 91
10.14
Saltcontent
ofbutter ...........................................
92
10.15
Protein content
of
milkby
formaldehyde
titration
....................
92
10.16
Estimationofhardness
in
water .................................. 93
10.16.1 Temporaryhardness ...................................
93
10.16.2 Permanent hardness .................................... 93
10.16.3 W
ater-testing tablets ...................................
94
11.
Dairybuilding design and
construction
...................................
95
11.1 Site selection 95
................................................
11.1.1
W
atersupply .........................................
95
11.1.2 Land
................................................
95
11.1.3 Other
buildings
....................................... 95
I1.LIA Proximity to the road ...................................
95
11.1.5 Effluent
disposal
...................................... 95
11.2 Type ofbuilding ..............................................
96
11.2.I Construction
materials
..................................
96
11.3 Arrangement
and
installation of
equipment
.........................
96
11.3.1 Arrangem
ent
.........................................
96
11.3.2 Installation
........................................... 96
vi
-
8/15/2019 Rural Dairy Technology
6/123
List
ofAppendices
Appendix
1.
Dairycalculations
........................................ 99
Appendix 11. Glossary
ofterms
.......................................
103
Appendix III.
Composition of
somefoods
...............................
10'
Appendix
IV. Temperature
conversion
..................................
106
Appendix
V.
Titratable
acidity expressed
in
differentways .................
107
Appendix
VI.
Volumeunitconversions
.................................
108
Appendix
VII. Tableof
atomic weights
..................................
109
Appendix
VIII.
Somestandard solutions
for volumetric
analysis
...............
110
Appendix
IX.
Approximate
strengths of
somecommercial
laboratory
reagents .
.III
Appendix
X. Indicators forvolumetric
analysis
...........................
112
Appendix
XI.
Length
and area
units
....................................
113
Appendix
XII.
The improved
traditional chum for
buttermaking
..............
114
Appendix
XIII. Useful
references, names and
addresses ......................
118
vii
-
8/15/2019 Rural Dairy Technology
7/123
Listof
Tables
Table
I.
Composition
(%)
of
milk
of
somespecies
of
mammal
..................
3
Table
2.
Average
composition (%)of
cow
milk................................
7
Table 3.Composition
of
cow
milk..........................................
II
Table 4.
Composition of lipids inwhole
bovine
milk
...........................
II
Table
5.
Principal
fatly
acids
found in milk
triglycerides
........................
13
Table 6.
Distribution ofmilk saltsbetween the
soluble
and
colloidal phases ......... 18
Table
7.Stability
olf
itam ins..............................................
18
Table 8.Bacterial
types
commonlyassociated
with
milk
........................
24
-
oftemperattire
different
conditions..................................
............
3
I
Table 9. flect on the
growth
of
bacteriainmilk produced
under
Table 10. Manufacluring procedures
for yoghurt,acidophilus milkandkefir.
.......
70
viii
-
8/15/2019 Rural Dairy Technology
8/123
List
of
Figures
Figure 1.Flow chart illustrating
the incorporation
of
the
majormilk
solid fractions
in m ilk products . ................................................. 5
Figure 2.
Changes in
the concentrations offat, protein and lactose
over
a
lactation of
Figure
4.
Structural formulaeof
four
18-carbon fattyacids varying in degree of
acow. .......................................................... 8
Figure 3.Fat globules
in m
ilk
.
............................................. II
saturation
....................................................... 12
Figure 5.
Milk-protein
fractions............................................. 15
Figure
6.
Struciure
of
a
lactose molecule
.............................. 17
Figure 7. Rod-shaped
(bacilli)
andspherical (cocci) bacteria...................... 19
Figure 8.
Schematic
illustrationofbacterial stucture............................ 20
Figure
9.
The fourphases of
bacterial growth
..................................
21
Figure
10. Structure
ofmoulds
.
............................................. 23
Figure
11. Structureofa
yeast
cell. ..........................................
23
Figure 12.
Batch
separation ofmilkby
gravity
.................................. 38
Figure
13.
Cutaway diagrams
of
(a)
hand-operated milkseparatorand
(b)
thebowl
showing
thepaths
ofmilk and cream fractions......................... 39
Figure 14. Products andby-productsofbuttermaking
from
sourwhole
milk
......... 52
Figure 15.
Adding
lemonjuiceand stirring the
milk.............................. 55
Figure 16.Stirring
the
curds andwhey........................................ 56
Figure
!7.
Separating
thecurds fromthewhey using
a
muslin cloth...............
56
Figure 18. Adding curd
to
a muslin-lined mould................................. 57
Figure 19. Pressing cheese.................................................. 58
Figure20. Cuttingthecurd mass
.............................................
59
Figure 21. Floatingcheese
pieces
. ........................................... 59
Figure 22.
Cheesz
pieces folded
over
aftersalting............................... 60
Figure 23. Ladlingcurds andwhey into a cheese mould....................... 62
Figure 24. Whey draining
and,
in the foreground,
cheesepieces incool water ........
62
Figure 25.
Checking
the coagulum before cutting................................ 63
Figure
26.
Cutting thecoagulum with avertical knife............................
64
Figure
27.
Cutting the cheese
curd
............................................
64
Figure 28. Covering thecurd with
cheese
clo'h tokeep it
warm
duringcheddaring
..... 65
ix
-
8/15/2019 Rural Dairy Technology
9/123
Figure
29. Putting
thecheese
intoamuslin-lined
mould
..........................
65
Figure
30.Cheese
ready for
pressing .........................................
66
Figure 31.
Outline offour
important
lactose fermentations
compared
to a
76%fat
recovery whenusing
the
clay
pot fittedwith
the
internalwooden
agitator...
68
Figure 32.
Flow diagram of
fermented milk manufacture
.........................
70
Figure
33. pH
meter .......................................................
78
Figure
34. Apparatus
for the
Gerber test .......................................
82
Figure
35. Transferring
milk
to the butyrometer
and reading
the fatresult
83
Figure
36. Lactom eter.....................................................
86
Figure 37. Improved
traditional
churn
. ......................................
114
Figure38.
Componentsof
the
improved traditional chum
........................
115
Figure
39.
Components
of
the internal agitator
................................
116
x
-
8/15/2019 Rural Dairy Technology
10/123
Acknowledgements
The production
of this
training manual
has
been
made possible
through the efforts
and
collaboration ofseveral
people.
Sincerethanks are
due
toProfessor
PA
Morrissey,
University
College,
Cork,Ireland and
to
Mrs
PABorland,
DR
and
SS,
Harare,
Zimbabwe
for
their
verymany
helpful
comments
and
suggestions.
The
interestandencouragement
ofDrM
E
Smalley,
Director
ofTraining
and
Information,
in
thepreparation
of
the manual
isvery
much
appreciated.
Many
thanks
are
due
to
PaulNeate,
Head
of Publications,
and
to
Anne Nyamu, Scienc.
Writer/Editor,
for their
constructive
comments,
patient,
critical
andcarefulediting
and
overall
layout
andpresentation
of
themanual.
To
thestaff
of
the
Publications
Department
fortheir
skill
and
care
inprinting the
manual
and
toFantu
Yimer
for typingthe
script I
offersincere thanks.
Charles
B.O'Connor
xi
-
8/15/2019 Rural Dairy Technology
11/123
Foreword
This training manual
is
based largely on ILCA Manual No.
4
written by the
late
Frank
O'Mahony.
As a result of
experiences
and
suggestions
obtained
particularly
from
participants
in our
Rural Dairy
Processing
training
courses
it
was
considered necessary
to
provide
a more
comprehensive
manual
withmore
up
to
date technical
information.
Emphasis
has
been
given
to clean
milk
production
as milk
is our raw
material
for
processing and
preservation.
In
recognition
ofthe need
tocomply
with
hygienic
and
compositional
requirements
ofmilk and
milk
products
the
chapter
on
analytical
methods
has
been
expanded.
To
recognise
the
importance
of
water
in milk
processing,
water-quality
standards are
discussed
and methods
for the
determination
of
itschemical
quality
are detailed.
Milk production
and
processing
is
an
important activity
of
smallholders throughout
the
worldand
withthe
helpof
this
manual itis
hoped
thatdairytechnologists
andextensionworkers
willfurtherassist
andpromote
thedevelopment
ofmilk
processing
particularly
incountries
with
a
developing
dairy
industry.
Michael
E. Smalley
Director ofTraining
and Information
Previous
Page
Blank
xiii
-
8/15/2019 Rural Dairy Technology
12/123
1.Introduction
Milk
and milkproducts
have been
used
by
mansince
prehistoric
times.
There
is evidence
thatbutter was
made
as far
hack
as
2000
BC.
It
is
thought that cheesemaking was
discovered
accidentally
and
initially
developed
in
Iraqcirca7000-6000
BCand
spread
withthe
migrationof populations
dueto famines,
conflicts
and invasions.
Examples
of
these migrations
are
the development
ofSwiss cheeses
by
the
Hclveti
tribe
in
Switzerland
and the introduction
ofcheesemaking
into
England
by the Romans.
Cheese
varieties peculiar
to
each region
developed
because
ofthe different
agricultural
conditions
prevailing
in eachcountry.
There
are,at present,
almost
2000 recognised
varieties of
cheese.
Fermented
milks have
been
prepared for
more
than
2000years.
Allowing
milk to
ferment
naturally
gives
an
acidic
product
that does not
putrefy.
Fermented
milks are wholesome
and readily digestible;
examples
of
suchproducts
are yoghurt,
kefir,
koumiss and
acidophilus
milk.
Thedevelopment
of
the
milk
separator
in the
19
'hcentury
made
centralised milkprocessing
possible.
Initially,
cream
was
separated
and
retained for
buttermaking
and the
fresh skim
milkwas
returned to
the
milk producers.
As the nutritional
importance
of the
non-fat component
(skim milk)
became
recognised,
processes
weredeveloped
toconserve
milk
solids-itot-fat
(SNF).Cascin
andcasein
products as
wellaslactose
and
dried
milk were prepared. Today,
up to
60%
of
the
milk produced
in the
world is converted
into
dehydrated
milk
products and
foods containing
a large
proponion
of
milk solids. In
countries
with
commercial
dairying
these processes
are carried
out in large-capacity
processing
plants.
In Africa,
milk is produced
in most
agricultural
production
systems. Itiseither
sold fresh, consumed
as
fermented milk or
manufactured
into products
such
a.
butter, ghce
and cheese. Sour
milk
is !he
most
common
product,
and
vnilk
is
usually
soured
before further
processing.
While there
are several
milk
processing
plants
in Africa,
much
of
the
milkproduced
by
rural
smallholders
is processed
on-farm
using
traditional technologic:.
It
is important,
therefore,
to
consider these processes
and
look
to
possible
technological interventions
at
this level
when considering
dairy
development
in
the
rural
sector.
Farmers in many
Africancountries
produce
sourmilk,
butterand
cottage
cheese for
home
consumption
and
sale.The
Maasaii
in Kenya
makeghee from sourmilk.
Fernented
milks
aremade
throughout
sub-Saharan
Africa,
andconce:itratcd
fermented
milks
aremade
in sonic
parts
of
the continent.
While theprocesses
used
have
not
been
subject to extensive
scientific
investigation,
they
appear to
be effective
methods
of
converting
milk into
stablemarketable products
and
have long
been used forprocessing
,urplus
milk.
Milk is
processedprimarily
to convert it into
a
more stable
product,e.g fermented
milk
canbe
stored
forabout
20days
compared
with
less
than oneday
for fresh
milk. Milk
products
are morestable
than
fresh
milk
because they
are more acidic
and/orcontain
less moisture. Preservatives,
e.g.
salt may
also beadded
tom ilk
products.Thus,
by increasing
the acidity and
reducing the
moisture
content, the
storagestability
of
milk
can
be
increased.
This manual deals with
milkprocessing
in a
rural and
small-scale environment.
It
concentrates
on
traditional
productsor
on
products
that
are
easily
made,
need little specialised
equipment
and
can
beeasily
adapted
to the rural
processing
plant. Some
background
information
in
the areas
of
milk
chemistry, dairy
microbiology
andmilk and milk-product
analysis
is also
gi,,en.
-
8/15/2019 Rural Dairy Technology
13/123
2.
Milk
as afood
Milk is
secretedbythemammary
glandsofmammals
tofeedtheiryoung.Cow
milk
-
awhite fluid
oflow
viscosity
and slightly sweet
taste
-
is most
commonly used
as human food. There are, however,
wide
variations
in
the
chemical and
physical propertiesof
themilk ofvarious
mammalian
species
as
shown
in
Table
1.The
tablegives the
average
gross composition
values ofmilk
from
somecommon
species.
Table 1. Composition
( ) of milk
of
some species
of
mammal.
Species
Total solids Fat
Protein
Lactose Ash
Human
12.4
3.8 1.0
7.0
0.2
Cow
12.7 3.7
3.4
4.8
0.7
Goat
12.3
4.5
2.9 4.1 0.8
Sheep
19.3
7.4
5.5
4.8
1.0
Horse
11.2
1.9
2.5
6.2 0.5
Donkey
11.7
1.4 2.0
7.4
0.5
Domestic ranbit
32.8
18.3 13.9
2.1 1.8
Camel
12.9
4.2 3.7
4.1
0.9
Milk is
the sole
sourceofnutrients
for mostyoung
mammals for lengths oftime
which vary with
the
species.
Overall, milk serves the following broad functions for both young
and
old:
(a)
growth,
(b)
reproduction,
(c) supply of
energy, (d) maintenance
and repairs and
(e) appetite satisfaction.
The
requirements of
these
categories vary with
the individual, andin
some instances
not
all
the
statedfunctions
ofthe
food
need to
be
served,
e.g.
adults
do
not require
food
for growth whereas infants
do.To
fulfil
its
functions as a food
milk contains various
nutritionally important
components, namely
proteins,
carbohydrates,
lipids, minerals,
vitamins and
water.
The gross energy
supplied
by
milk
can
be calculated
from
its
lactose,
protein and
fat contents.
The
metabolically available
energy is approximately
4.0,4.1 and8.9
kcal/g(16.8, 17.0 and37.0
kJ/g) forlactose,
protein
andfat,
respectively.
On thebasis
of
the
data
inTable
I
human
andcowmilk
contain
670-720
kcallkg
(2.8-3.0
MJ/kg).
The
chief
function
of
lactose in
milk
is to
supplyenergy formuscular
activity and maintenance
ofbody
temperature.
Like
other
disaccharides,
lactose must
be
hydrolysed
to
its monosaccharide
components,
glucose and
galactose, before it is
absorbed across
the
intestinal membrane
into
the
bloodstream.
Some
peoplecannot
toleratelactose because
they lack
the
enzyme
(lactase)
whichis required
to
hydrolyse
it.
Lack
of
lactase may
result inabdominal
cramps,bloating anddiarrhoea
ondrinkingmilk.Whenlactose is removed
from
milk,
or
converted into lactic
acid
during
cheese
manufacture,
milk products
can be consumed
by
lactose-intolerant people.
Lactose has certain
therapeutic properties
and is known to enhance
the
intestinal
absorption
of
calciumand
phosphorus.
Itspresence inthe
intestine favoursan
acid-type
fermentation
which
may
prevent
intestinal
disorders.
Fermented
milks
may
be
preferable
to
fresh milk
because
theyprevent the
propagation of
infectiousdiseases.
Proteins
are
essential
for
thegrowth and maintenanceofall
cells inthebody.
Thevalue
of
milkproteins
depends
primarily
on their
content
of
some nine essential
aminoacids
whichcannot be
synthesised
by the
body. Cow
and
human milk
contain
all
the essential aminoacids required
for
human infants.
Fortunately,
both
cow
and human milkareeasily
digestedand theamino
acids arereadily absorbed.
3
kPreVioUS.
kage.b~ara
-
8/15/2019 Rural Dairy Technology
14/123
Cow milk
(3.4%
protein) forms
a rather
frm curd
in
the stomach
and
digestion
is slower
than
with
human
milk,
whichcontains
about
1%
protein.
Diluting
cow
milk with
wateror
high heat
treatment
softens
the curd.
Lipids
supply
the
body
withaconcentrated
source
ofenergy
and
are
also
important
contributors
to
both
desirable
and
undesirable
flavours
in milk
and
milk
products. Certain
fatty
acids
are
not
synthesised
by the
animal.
They
include
the
polyunsaturated
acids,
linoleic
(Ct8:
2
)acid
and probably
linolenic
(C18:3) aced.
It
is
considered
that
2-4%
of
the total energy
of
the diet should
he
supplied
by
polyunsaturated
acids.
The
linoleic
acid contentin
human
milk
fat accounts
for
approximately
5%
of
the
energy
in
milk. This
is
much
higher
than for
cow milk fat
which
accounts
for only
about 1%of
the
total
energy.
Human
and
cow
milkare excellent
sources of
vitamins.
Vitamins
A, D,
E
and K
occur in
the fat phase
and
theothers
in the
aqueous
phaseof
milk.Milk isa
major
source of
someof
thevitamins
needed
by
infants
and
adults. It
is reltively
rich in
vitamins
Aand
E,
thiamin,
riboflavin,
folic
acidand
vitamin
B
12. However,
large
variationsoccur
between
humanand
cow milk.
Humanmilk
contains
only
about
35%as much
thiamin,
25%
as
much riboflavin
and5%
as much
Bt2 as
cow
milk.On
the other
hand,
human
rmilkcontains
about
I0
times
as much
vitamin
Eand
2.5
timesas
much
ascorbic
acidas
cowmilk.In
many
countries
milk
is
fortified
with
vitamins
A and
D. Vitamin
A
is central
to
the visual
process
as aconstituent
of
the
visual pigment
rhodopsin.
Vitamin
D
is
essential
for
the
calcification
processes
in
the body,
including
bone
and teeth
formation.
Milk is
alsoanexcellent
sourceof
many
minerals
and
supplies virtually
allof
the
minerals
required
by
humans.
Cow milk
furnishes
a major
portion
of
the
total calcium
consumed
in many countries.
The
high
levels of
calcium
and
phosphorus
in
milk are
important
in
bone and
tooth
formation
in young
children;
both
these elements
play
a
significant
role in preventing
osteoporosis
in elderly
people.
Milk also
contains
high
levels of
magnesium,
zinc
and
iodine. Ilowever,
milk is apoor
source of
ironand
neither
human
nor
cow
milk
supply
enough
forhuman
infants.
Fortunately,
infants
havea
store of
iron
in the
liverwhich
is sufficient
to
meetthe
needs of
the
body during
thefirst
six
months.
The nutritive
value
ofmilk
may
be
considerably
altered by processes
such as
separation,
concentration
of
the components,
addition
of non-milk
constituents
and heat
treatment.
For
example,
duringbuttermaking
thefat
and fat-soluble
vitamins
are
retained in
thebutter
while
the
protein, lactose,
minerals
and B
vitamins
remain
in thebuttermilk.
'art
of
the fatin
butter
can
be replaced
by vegetable
oiltogive
better
spreadability.
Diluting
cow
milk
with water
or
severe
heat treatment
greatly
softens
the casein
curd
and allows
for
easy
digestion.
When
mother's
milk
is not available
milk
formulations
for
babies are
prepared
by mixing
cow
milk,
cream,
whey
proteins,
lactose
and water.
The
ratio of casin
to
whey, protein,
the
lactose content
and
salts in milk
formulations
are similar
to
those ofhuman
milk.
Mild
heat
treatment
uch aspasteurisation
or ultra
high temperature
(UtlT)processing
cause
very
little
change
in nutritive
value.
Severe
heat
treatment
results
in
some loss
of
available
lysine,
but this
has little
effect
on
the nutritional
quality
because
milkproteins
are rich
in lysine.
The interaction
between
lysine
and
lactose
during
heating
results information
of
a
brown
pigment
(Maillard
browning)
that causes
off-flavours
to
develop
during
storage
of
milk products.
Figure
I
shows
the major
milk
constituents
and a
range of
products
that
can be manufactured
from
these
constituents.
4
-
8/15/2019 Rural Dairy Technology
15/123
FRlprel.
Flowchartillustrating
the incorporation
ofthe major milk-solidfractions
in milk products.
S
Whole milk
solids
en
a.t [Pr"i I
L
actos.
milk
Precipitate
pH46
Supernatant
IFCasein
Wheyprotein
B
t
r
l
fermen-f
team
35%
tat
tto
SChum
cream
Whey ,
Lactic
ac
I
utter
82%
fat
Acetic ai
1Propionic
ai
Aldehyde
Remove
Rmoture
Ketone
mosue.
Hard
C
I
ottage
cheese
cheese
uttr
B
-
8/15/2019 Rural Dairy Technology
16/123
3.
The
composition
of
milk
Milk
composition
isaffected
byanunber
offactors
includinggenetic
andenvironmental
factors.
3.1
Genetic
factors
3.1.1
Breedand
Individuality
of
thecow
Both
milkyieldand
composition
varyconsiderably
among
breeds
of dairy
cattle.
Jersey
andGuernsey breeds
give
milk
withabout5%fatwhile
the
milk
of
ShorthornsandFriesians
containsabout
3.5%
fat.Zebu
cows
cangive
milk
containing
up
to7%
fat.
Table2gives
the
averagecomposition
ofmilk
from different
breedsofcow.
Table2.
Average composition
( )
of
cow milk
Breed
Fat
Protein Lactose
Ash
Zebu
5.6
3.1
4.6
0.71
Ayrshire
3.8
3.4
4.8
0.70
Fresian
3.4
3.2
4.6
0.74
Guernsey 4.9
3.8 4.8 0.75
Jers,y
5.1 3.8 4.9
0.75
Shorthorn
3.6
3.4
4.8
0.70
Milk
of
individualcows within
a
breed
varies
over
a
widerange
bothinyield
and
in
thecontent
ofthe
various
constituens.
The
potential
fatcontentof
milk
from anindividual
cow is
determined
genetically,
as areprotein
and
lactose
levels.
Thus
selection
forbreeding
on
thebasis ofindividualperformance
iseffective
in
improving
milkcompositional
quality.
Herdrecording
oftotalmilk
yieldsand
fatandsolids-not-fat
(SNF) percentages
willindicate
themost
productivecows,
and
replacement
stock
should
be
bred
from
these.
3.2
Environmental factors
3.2.1
Interval
between milkings
Thefatcontent
of milkvaries
considerablybetweenthemorning
and
evening
milking
because
there isusually
a
much
shorter
interval
between
morning and
evening
milking
thanbetween
eveningandmorning
milking.
If
cows
were
milkedat t2-hour
intervalsthevariation in
fat
content
between
milkings
would
be
negligible,
butthis
isnotpracticable
onmostfarms.Normally,
SNFcontent
doesnotvarywith
thelength
of
timebetween
milkings.
3.2.2
Stage
oflactation
Thefat, lactose
and protin
contents
ofmilkvary according
to
stage
oflactation. Solids-not-fat
content
is
usually highest
during the
first two to three
weeks,after
which it
decreases
slightly. Fat
content
is high
immediately
aftercalving butsoon begins
to
fall,
and
continues todo
so for
10to
12
weeks,
afterwhich
it
tends
torise
again
untilthe
end
of
the
lactation.
The
highprotein
content
of
earlylactationmilk
is
due
mainly
to thehighglobulin
content.
The
variation
in milk
constituents
throughout
lactation
is
shownin
Figure
2.
7
')J*V1OUS
Pa~ge
Blaznk
-
8/15/2019 Rural Dairy Technology
17/123
Figure2.
Changes in the
concentrations
offa, protein
and lactose
over
a lactation
of
a cow.
Solids
concentration
(g/litre)
55.
I
Fat
5
.........--
Lactose
45
I
Protein*..*
40
\
35.
30.,
, 40
80
120 160 200
240 280
320
360
Parturition
Stage
of lactation
(days)
Dry
period
3.2.3 Age
and health
As
cows growolder
the fatcontentoftheirmilk
decreases byabout
0.02
percentage unitsperlactationwhile
the
fallin
SNFcontent
isabout0.04percentage
units.Bothfat
and
SNF
contentscan
be reducedby disease,
particularly
mastitis.
3.2.4
Feedingregime
Underfeeding
reduces both the fat
and theSNF
content
of
milk,although
SNF
content
isthemore
sensitive
to feeding
level.Fatcontent
and fatcomposition
are influenced
more by
roughage(fibre) intake.
The SNFcontent
may
fall
if thecow
is fedalow-energy
diet,but isnotgreatly
influenced
by
protein
deficiency,
unless
the deficiency
isacute.
3.2.5 Completeness
ofmilking
The
first
milk
drawn from
theuddercontains
about1.4% fatwhilethe
last
milk
(or
strippings)
contains
about
8.7%
at.
Thus, it
isessential
to
milk
the
cow
completelyand
thoroughlymix all
the milkremovedbefore
taking asample
foranalysis.
The fat left in theudder
at theend
of
a
milking is
usually
picked
up
during
subsequent
milkings,
sothere isnonet loss
of
fat.
-
8/15/2019 Rural Dairy Technology
18/123
4.
Milkchemistry
4.1
Physical
status
of
milk
About
87%
ofmilk
is
water, in
which
the
otherconstituents are distributed invariousforms.
Several
kinds
ofdistribution
aredistinguished
according
tothetypeand
sizeofparticle
presentin
theliquid.
Kind
of
solution Particlediameter(nm)
Ionicsolution 0.01-1
Molecular
solution 0.1-1
Colloid
(fine
dispersion)
1-100
Coarse
dispersion
(suspension
or
emulsion) 50-100
Inmilk,
examples
of
emulsions,
colloids,
molecularand
ionic
solutions
are found.
4.1.1 Ionicsolutions
Anionic solution
isobtained whenthe forcesthathold the ions together
in
a
solidsaltare overcome.The
dissolved
salt
breaks
up
into
ions which float freely
in
the solvent.
Thus
when
common salt
-
sodium
chloride -
isdissolved
in
wateritbecomes
an
ionicsolution
of
freesodium
andchlorideions.Ionicsolutions
are composed
largely
of
inorganic compounds.
4.1.2
Molecular
solutions
In
a molecularsolution
the
moleculesareonlypartly,
if
at
all,
dissociated
into
ions.
The
degree
of
dissociation
represents anequilibriumwhich isinfluenced
by othersubstances
in
thesolutionandby thepH
(or
hydrogen
ion
concentration) ofthe
solution.
Molecular solutionsare
usuallycomposed
oforganic
compounds.
4.1.3
Colloids
In
a colloid, one substance isdispersed in
another ina
finer
state thanan emulsion but the
particle
size
is
larger
than
that
ina truesolution.Colloidal
systemsare
classifiedaccording to
thephysicalstate
ofthe two
phases. Inacolloid,solidparticles consisting
of
groupsofmolecules
float
freely.
Theparticles
in
a colloid
aremuch smaller
than
those in
a
suspensionand
a
colloid is
much morestable.
4.1.4Emulsions
An emulsionconsists ofone
immiscible
liquid
dispersedinanother
in
theform ofdroplets
- the
disperse
phase.
The
otherphase
is
referred to
as
the
continuous
phase.The
systems
haveminimal
stabilityand
require
a surface-activeoremulsifying agent,
e.g. lecithin inmilk, forstability.In
foods,
emulsionsusually contain
oilandwater. If
water
is
thr-continuous
phase
andoil
the
disperse
phase, it
isanoil-in-water (o/w)emulsion,
e.g.milkorcream. In the reverseoasethe emulsion is
awater-in-oil (w/o)type,e.g. butter.
4.1.5 Dispersions
A dispersioa is
obtained
when particles of
a
substance
are
dispersed
in aliquid. A
suspension
consists
of
solid particlesdispersed in
a
liquid,and the force
of
gravity
cancause themtosink
to
the
bottom
orfloat
to
the top.
For
example,
fine
sand,
dispersedin
water,
soonsettlesout.
9
-
8/15/2019 Rural Dairy Technology
19/123
4.2pH
and
acidity
An acid
isasubstance
which
dissociates
toproducehydrogen
ions
in solution.
A
base
(alkaline)
isasubstance
whichproduces
hydroxyl
ions
in
solution.
It
can
equallybe
stated
that
an
acid
isasubstance
whichdonates
a
proton
and
abase
is
a
substance which
acceptsaproton.
The symbol
pH
isused to
denoteacidity; it
is
inversely
related
to hydrogen ion
concentration.
On a
scaleof
0-14:
Neutrality
=
pH7
Acidity
is
pH7
Fresh milkhas
apHof
6.7
andis therefore
slightly
acidic.
When
an acid
is
r.ixed
with
a
base,
neutralisation
takes
place;
similarly
a
base will
be
neutralised
by
an acid.
4.2.1
Buffer
solutions
Buffersare
defined
as materials
that
resistchange
in
pH
on
addition
of
acid
or
alkali.
Characteristically
they
consist of
aweak
acidor
a
weak
base
and its
salt. Milk
contains
alarge
number
of these
substances and
consequently
behaves
as
a
buffersolution.
Fresh cow
milkhas
apH
of
between
6.5and
6.7.
Values
higher
than
6.7 indicate mastitic
milk
and values below
pH
6.5 indicate
tie presence of
colostrum
or bacterial
deterioration.
Because
milk is
abuffer solution,
considerable
acid
development
may occur
before
thepH
changes.
A
pH
lower
than6.5 therefore
indicates
that
considerable
aciddevelopment
has
taken place.
This
is
normally
dueto
bacterial
activity.
Litmus
test
papers,
which
indicate
pH,
are
used
to
test
milk
acidity;
pH
measurements
are
often
used
as acceptancetests
for
milk.
Milk
acidity
is
an important
indicator
of
milk
quality.Acidity
measurementsare
also
used
to
monitor
processes suchas
making
cheese
and
yoghu t.
Thetitratable
acidity
oftmilk
isexpressed
in
termsofpercentage
lactic
acid
- theprincipal
acid
produced
by
fermentation
after
milk
is
drawn
from the
udder. Fresh
milk
contains
onlytracesof
lactic
acid.
However,
C_e tothe
buffering
capacity
oftheproteins
andmilk
salts
fresh
milk,
in
which
no
lactic
acid has been
produced,
normally
exhibitsan initial
acidity
of0.14
to 0.16%
when
titrated
using
sodiumhydroxide
toaphenolphthalein
end-point.
4.3
Milk
constituents
The
quantities
ofthe
main
milk
constituents
can
vary
considerably
depending
on the individual
animal,
its
breed,
stage of
lactation,
ageand
health status. Herd
management
practices
and
environmental
conditions
also
influence
milk
composition.
The
averagecomposition
ofcow
milkis
shownin
Table3.
Water
is themain
constituent
of
milkandmilk
processing
is usually
designed
to
remove
water
from
milkor
reduce the moisture
content
oftheproduct.
4.3.1
Fat
If
milk
is left
tostand,
alayerof
cream
formson
thesurface.
Thecream
differs
considerably
in appearance
from the
lowerlayer
of
skim
milk.
10
-
8/15/2019 Rural Dairy Technology
20/123
Table
3. Composition of cow
milk
Main
constituent
Water
Totalsolids
Fat
Proteins
Lactose
Minerals
Cream consists
of
a
Range(%)
Mean(%)
85.5-89.5
87.0
10.5-14.5
13.0
2.5-6.0
4.0
2.9-5.0
3.4
3.6-5.5
4.8
0.6-0.9
0.8
large
number
of
spherical
microscopic
globules
ofvarying sizes
floating
in
the
milk.Each
globule is surroundedby
athin
skin
-
the fat
globulemembrane-
which
actsas the
emulsifying
agent for
the
fat suspended
in
milk
(Figure
3).
The
membrane
protects
the
fat fromenzymes
and prevents
the
globules
coalescing into
buttergrains.
The
fat is present
as
anoil-in-water
emulsion
that
can
be
broken
by
mechanical
action
such
as shaking.
Figure
3. Fat
globules in milk.
Fat
globule
0
E0
Serum
Fats are partly
solid at
room
temperature. The
term
oil is reserved
for
fats
that arecompletely liquid
at
room
temperature. Fats and
oilsare
soluble
in non-polar
solvents, e.g. ether.
The lipidcontent
of
milk
is
usually
defined
as
the fraction
which
is
extracted
by
organic
solvents. Table4
gives themain
lipid
classes
of
milk fat.
Table
4.
Composition
of lipids
in whole
bovine
milk.
Lipid
Carotenoids
+vitamin A
Cholesterol esters
Triglycerides
Diglycerides
Monoglycerides
Free fattyacids
Cholesterol
Phospholipids
il
Weight (%)
trace
0.02
98.3
0.3
0.03
0.1
0.20-0.40
0.20-1.0
http:///reader/full/0.20-0.40http:///reader/full/0.20-0.40
-
8/15/2019 Rural Dairy Technology
21/123
About
98%
of
milk
fat is a mixture
oftriacyl
glycerides.
The
partial glycerides
(diglyceridesand
monoglycerides)
andfree
fattyacids
are probably
partly
leftoverfrom
the
biosynthesis
process. Also
present
are fat soluble
vitamins
A,
D,E and K
and
pigments,
e.g.
carotene
which
gives
butter its natural
yellow
colour.
Examples
ofeach
type offatty
acid
are shown
in Figure
4. The main variables
arcas follows:
I. Chain
length.
Fatty
acids
vary
in
chain
length from
4
carbon
atoms,
as
in butyricacid,
to20 carbon
atoms,
as
in arachidic
acid. Nearly
all
thefatty
acids
in
milk
contain
an
even
number
of
carbonatoms.
Milk
fat
contains
significant
levels ofshort
and
medium chain
fattyacids.
Butyric
acid
(C4) is specific for
milk
fatof
ruminantspecies.
2. Number
ofdouble
bonds.
A fatty-acid
molecule comprises
a hydrocarbon
chain
and
a carboxyl
group
(-COOH).
In
saturated
fatty
acids the
carbon
atomsare
linked
in achain
by single
bonds
(e.g.
stearic acid,
C18:0, in
Figure 4).
Unsaturated
fatty
acids
have one double
bond, e.g.
oleic acid, CI8:t,
while
polyunsaturated
fatty
acids
havemore
thanonedouble
bond, e.g.
linoleic
acid,
C18:2
(two
double
bonds),
and
linolenic
acid, Ct8:3 (three
double
bonds).
It is the
double
bonds
in
thecarbon
chain
that
make
the
fatty acid unsaturated.
Twohydrogenscan
be
added
perdoublebond
at high
temperature
witha
suitable
catalyst.
This
process
is called hydrogenation
and has the effect
of
converting
a
soft fat to a hard
fat at
room
temperature.
Figure
4.
Structuralformulae
offourl8.carbonfittt.acidthmra.ingintdegreeofsatiuraItion.
Stearic
acidC18:0
H-
H
I
C-
H
HHHHHHH
I I I
I
I I
I I
C-C-C-C--C-C-C-C-C-C-C-C-C-
H
I
H
I
H H
I
H
I
H
H H
I
I I
C-C-C
,0
SI
I I
I
I I
I I I I
I I
I I
I
H H
H H H
H H H
H H H H
H H H
H H OH
18
15
13
11
9
7
5
3 1
Oleic
acid
C18:1
H
I
H
I
H
I
H
I
H
I
H
I
H
I
H
I
H
I
H
I
H
I
H H
I
H
I
H
I
H
I
H
I
H-C-
C-
I
I
CC-
I
C-
I
-C-
C-C
I 1
i
C-C-
C-
C-C-
C-C-C-
I
I I I
I I
I I
H H
H H H H
H H H
H H H H
H H OH
18
16
14
12 10
9
7 5
3 1
Unoleicacid C18:2
H
H H
H H
H
H H
H H
H H
H
H H
H H
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
1
A
H-C-CIC-C-C-
C C-C-C-C-
C-
C- C-
C- C-
C-
-
I I
I I I
I I
I I
I I
I I I
I
II
H
H H
H
H HHH
H H
H
OH
18
13
12 11 10 9
7 5
3 1
Linoleic
acidC18:3
H
H H
H H
H H
H H
H H
H
H H
HHH
I
I I
I I
I
I I
I I I
t I
t
t I
A
H-C-C-C=C-C-CC-CCC=C-C-
C-
C-C-C--C-
H H H
H H
H H H H H H
OH
18
16
14
11
9
7 5
3
1
12
-
8/15/2019 Rural Dairy Technology
22/123
3.
Positionofdoublebond. The double
bond
can occur
in
many
positions (called isomers).
Olcicacid has
thedoublebond
atthe
ninthpositionwhich may
be indicatedas
follows:
CI8:t
9. Linoleicacidhas
two
double
bonds
at the
ninthandtwelfth
positionswhich
may
be
indicated
as
follows:
C18:2
9, 12.
4.
The
proportion
of
saturated
fatty
acidspresent
in
milk
fatisabout
63%.
5.
Oleicacid is
the
mostabundant
of
the
unsaturated
fatty
acids.
Table5. Principalfatty
acids found
in milk triglycerides.
Average
Melting
Fattyacid
Molecularformula
amount
in milk Chainlength point
fat
(%)
(No.ofcarbons)
'C)
Butyric
CH3(CH
2
)
2
COOH
3.7 4
-8
Caproic CH3(CH2)
4
COOH
2.0 6
-2
Caprylic
Ct12(Cl12)
6
COOH
1.6 8
16
Capric Cl13(CH2)8COOIl
2.6
10
31.5
Lauric
CHt(CH2)IoCOOH
3.3
12 44
Myristic
CI3(Ctl2)12COOH
8.7 14
58
Palmitic CH3(CHt2)I4COOI
27.0 16
64
Stearic
CH3(CH
2
)t6COOH
10.0
18 70
Oleic
Ct4I(CH2)7CH=Ctt(CH2)7COOH
35.0
18 13
Linoleic
CH3(CI12)4(CH=CH.CH2)2(CH2)6COOH
4.5
18 -6
Linolenic
C113.CH12(CH=CH.CH2)3(CI
2
)6COOH
0.6
18
-13
Arichidic
CttI(CIl))IsCOOH
1.0
20 77
The
most
importantfatty
acids
found inmilk
triglyceridesare
shown
inTable5.
Fatty
acids
areesterified
with
glycerol
as
follows:
H
2
_C-OH HOOC-RI
H
2
_C-OOCRI
Ht-C-OH
+
HOOC-R2 >
H-C-OOCR
2
+
3H
2
0
H2-C--OH IIOOC-R
3
H2-C-OOCR
3
Glycerol
+
fatty acids-
.
triglyceride
(fat) +
water
The
melting point and
hardness ofthe fattyacid isaffected
by
the length
ofthe
carbon
chainand
the
degreeof
unsaturation.
As chain
length
increases, meltingpoint
increases.
As
the
degree
of
unsaturation
increases,
the
melting pointdecreases.
Fatscomposed
ofshort-chain
or
unsaturated
fatty acids
have lowmelting
pointsand
are
liquid
at
room
temperature,
i.e.oils. Fats
high
in
long-chain
saturated
fatty
acids
have highmelting
pointsandare
solid
at
room
temperature.
Butterfat
is
a
mixture
of
fatty
acidswithdifferent
meltingpoints and therefore
does
not
have
a
distinct melting
point. Sincebutterfat
melts gradually
over
atemperature
range
of
0-40'C,
someof
the
fat is
liquid
and some solidat temperatures
between16 and 25"C.
The ratio
ofsolidto liquid fat at
the
time
of
churning
influences
the
rate of
chuming and the
yieldandquality
ofbutter.
Fats readilyabsorb
flavours,
e.g.
buttermade
in
asmokedgourd
has
a
smoky flavour.
Lipids in foods
are
subject
to two
forms
of
deterioration
that
affect the
flavour
offoodproducts:
Hydrolytic
rancidity
Lipolysis,
which
isthe breakingdown
of
milk
fat intocomponent fatty
acids, increases
theconcentrationof
free
fattyacids.Lipolysis
is
inducedby
theaction ofnatu~ally
occurring lipase
in milkwhich
hydrolyses
the
13
-
8/15/2019 Rural Dairy Technology
23/123
-
8/15/2019 Rural Dairy Technology
24/123
-
8/15/2019 Rural Dairy Technology
25/123
Wheyproteins
When
milk
isbrought
to
pH
4.6,
the
caseins
precipitate.
Thesupernatant
contains
fourprincipal
proteins
in
the whey
fraction,B-lactoglobulin,
a-lactalbumin,
blood
serum
albumin,
immunoglobulins
and
a
number
of
minor
proteins, e.g. lactoferrin
and enzymes.
Most ofthe whey
proteins
are
denatured
byheat,
i.e. they
become
less
soluble
ifmilk
isheated.
B-Iactoglobulin
is the
principal
whey
protein
of the
cow,
goat
and
sheep,
although
there
are slight interspecies
differences.
B-lactoglobulin
accounts
forabout
50%
of
the
total
wheyproteins
or
about
11%
of
the Iotal
protein in
milk.
Relatedbut
substantiallydifferent proteins
occur
in
porcine milk.
No
13-lactoglobulin
has
been identified
inhuman,camel
or horsemilk
in
which a-lactalbumin
is the principal
whey protein.
Denaturation
ofwhey
proteins
andB3-lactoglohulin,
in
particular,is
of
majortechnological
significance.
0-lactoglobulin
interactswith
k-casein
duringheating
and this
reduces the
heatstability
ofmilk,
slows
down
rennet
clotting
duringcheese
manufacture and gives
asoft
curdwhich
tends
to retain water.
(x-lactalbumin
represents
about
20% of
theprotein
of
bovinewhey
(3.5% of
thetotal milk
protein)
and
isa
relativelyminor
proteinin
terms
of
quantity.
Itfunctions
as part
of
theenzyme
system
involved
in lactose
synthesis.
The
immunoglobulins
are
antibodies
which
are
present
in
high
concentrations
in
colostrum.
Infants
and
mammals
areborn
withoutcirculating
antibodies
andthe
main wayin
which
theyacquire
these
isbyingestion
of
colostrum.
Minorproteinconstituents
About
50
enzymes
have
been detected in
bovine
milk.The
concentration
of
milk
enzymes
varies
greatly
among
species.
Some milkenzymes
act
on
substrates present
as normal
constituentsofmilk
andmay
play
eitherbeneficialor
deleterious roles
duringmilk
processing.
Catalase.
This
enzymecatalyses
thedecompositionof
hydrogen
peroxide
(1-1202) to
1-120
and
02.Its
activity
ishigher
in mastitic
milk
andcolostruin
than in
normalmilk and
increases
with
increase
in
bacterial
numbers.
latoperoxidase.
This
enzyme catalyses
oxidation
ofa
range
of
substrates
by
11202.
Theenzymecatalyses
oxidation
of
thiocyanatc
to
products
that
inhibit
certain
bacteria.
It is
relativelyheat
stable: it
isnot inactivated
by
pasteurisation
(72'C x 15
seconds) but
is destroyed
when
milk
is heated above
80'C. The
absence of
lactoperoxidase
in milk
indicates
that the
milk
has
beenheated
toat
least 80'C.The
test for
the
presence
of
lactoperoxidase
isbased
on the
oxidationof
the
substrate para-phenylenediarnine
in the
presenceof
11202.
Phosphatase.
Phosphatase
enzymes
catalyse
the hydrolysis
of
phosphate
esters.
Milkcontains
an acid
and
alkalinephosphatase.
Alkaline
phosphatase
hasap-I
optimum
near
9 and
is inactivated
byheating milk
to
72"C for 15
seconds. Its
absence
indicates
that milk has
been properlypasteurised.
Ifmilk
is inadequately
pasteurised,
the
residual
enzyme
willcatalyse
thehydrolysis
of
added
disodiumpara-nitro-phenol
phosphate
liberating
para-nitro-phenol
which is
yellow
in
alkalinesolution.Acid
phosphatase
which
has apH
optimum
of 4,
ismore heat stable
than
alkaline
phosphatase.
Other
milk
enzymes.
Milk
alsocontains
lipases
(discussed
earlierunder hydrolytic
rancidity)
proteases,
amylases,
xanthine
oxidase,
carbonic
anhydrase and lysozyme.
4.3.3
Carbohydrates
Lactose
is
tie
majorcarbohydrate
fraction in milk.
It is
a
disaccharide
composed
of
two sugars,glucose
and
galactose
(F;gure
6).
The
average
lactosecontentof
milk
varies
between
4.7
and
4.9%,although
milk
from
individual
cowsmay
vary more.
Mvastitis
reduces
lactose
secretion.
Lactose
is
a
source of
energy for
the young
calfand
provides
4calories/g
of
lactose
metabolised,
It
is
less
soluble
in
waterthan
sucrose
and
is
also
lesssweet.It can
be broken
down
to
glucose
and
galactose
by
bacteria
that have
the
enzyme
LB-galactosidase.
Theglucose
aidgalactose
can then be
fermented to
lactic
acid,
Thisoccurswhen milk
goessour.Under
controlled
conditions
theycanalso
befermented
to
otheracids
to
give adesired
flavour,
such
as
propionic
acidfermentation
in Swiss-cieese
manufacture.
16
-
8/15/2019 Rural Dairy Technology
26/123
-
8/15/2019 Rural Dairy Technology
27/123
-
8/15/2019 Rural Dairy Technology
28/123
5.
Microbiology
Micro-organism
is
the term applied
to
all microscopic
living
organisms.
Micro-organisms
tend
to
be
associated
with disease; those
that cause
disease
are
called
pathogens.
However,
few
micro-organisms
are
pathogens
and
micro-organisms play
a
crucial
part
in
life
on ourplanet.Forexample theyprovide
food
for
fish,they occur
in soil
where they
provide
nutrients
forplants
andtheyplay
animportant
role
in
ruminant
digestion.
In dairying
some
micro-organisms
are
harmful,
e.g.
spoilage
organisms
andpathogens
while
othersare
beneficial,
e.g.
cheese and yoghurt
starters and
yeasts and
moulds
used
in
controlled
fermentations and
cheesemaking.
The micro-organisms
principally
encountered
inthe dairy
industry
are bacteria,
yeasts, moulds
and
viruses.
5.1
Bacteria
Bacteria
are
microscopic single-celled organisms
that are
present
inair,
water
and
onmostsolidmaterials.
When
observed
under a
microscope
the
cells
can be
seen to
differ
inshape
and
in
conformation
ofgroups
of cells.
Cells
are either
spherical orrod-shaped
(Figure
7);
spherical
bacteriaare called
cocciwhile
those
that
are rod-shaped are called
bacilli. Thisis the
firstbasis fordifferentiating
betweenbacterial
cells.
Figure
7. Rod-shaped
bacilli)and
spherical
(cocci)
bacteria.
6
9
~
0
0
0
&Q
00
Diplococci Micrococci
With
endospores Staphylococci
Streptococci
With flagella
:01
Rodbacteria
(bacilli)
and
spiralbacteria
Spherical
bacteria(cocci)
Bacteria
are alsoclassified
according
tocell-cluster
formation:
*
Diplococci: paired
cocci
cells.
*
Staphylococci: a
number
ofcells
clustered
together.
*
Streptococci:
a
number
ofcellsarranged
ina
chain.
Somebacteria
aremotile.
They move
using
flagellac-long,
hair-like
appendages growing
outof the
cell. Some
rod-shaped
bacteria
may
formspores
whenthe
cells
are
facedwith
adverse
conditions
such
as
high
temperature.
Once
suitableconditions
are
re-established
the spores
germinate
to
formnewcells.
19
-
8/15/2019 Rural Dairy Technology
29/123
Close
examination
of
the
simplecell revealsthat
it iscomposed
of thefollowing
components
(Figure
8):
* cell wall
which
gives thecell
its shape and
retains
the
constituents
*
cell
membrane
for filtering
in foodconstituents
anddischarging
waste products
* nucleus
where
the
geneticmaterial
of
the
cell
is
stored.
The
cytoplasm
is a
semi-liquid
proteinaccous
substance which
contains
starch,
fat
andenzymes.
Figure
8. Schematic illustration
of
bacterial structure.
Nucleus
Cytoplasm
Cytoplasmic
membrane
Cell wall
-Capsule
The
cell
membrane is
semi-permeable
andallows the
cell to
feed
by osmosis,
i.e.
the
exchange of
nutrientsbetweenthe
cytoplasm
ofalivingcell and
thesurrounding
aqueous
material.Only small molecules
can
pass
in
and out of
the
cell, e.g. with
asugar
solution
on
one
side
of a
semi-pemeable
membrane and
water
onthe
other,water
will
diffuse
in,
diluting
the
sugar
solution. The
sugarmolecules
cannot
passout
so
a hydrostatic
pressure,
knownas
osmoticpressure, develops.
Bacteria
feedby
selective intake
of
nutrients
dissolved
in water.They
can also
take in
nutrientsagainst
the
normal
osmotic
flow,
aprocess called active transport.
5.1.1 Bacterial
growth
Bacterial
growth
refers to
an increase
in
cell numbers
rather
than an
increase
in cell size. The
process
by
which
bacterial
cells
di,,
ide
to reproduce
themselves
is
known
as binary
transverse fission. The
time
taken
from
cell formation
tocell
division
is
calledthe generation
time
whichcan
he definedas the
time taken for
the cell
count todouble.
Figure
9 shows the
phases
of bacterial
growth following
inoculation
of
bacteria into
a new growth
medium.
20
-
8/15/2019 Rural Dairy Technology
30/123
Figure
9. e fourphases of
bacterial
growth.
Number
ofbacteria
(log)
d
a
b
a
Lagphase c
Stationaryphase
bLogphase d
Death
phase
Thefollowing
phases can
be
identified:
I. Lag phase: There isusually some
delay ingrowthafter
inoculation of
bacteria
intoanew
medium.During
this time
the bacteria adapt to the medium
and enzymes neededynthesise '.he
to break down the
sub.t
,nces in
it.
2. Log phase: Or'-e the bacteria
haveadapted to thenew medium they
start toreproduce quicklyandtheir
numbers multiply
evenly
foreach increment oftime.Plotting
the
lognumber
ofcells
against
time
gives
a linearrelationship; this is
therefore
called
the
logphase.
The
cells
are
attheir greatest
activityin
this
phase. Transferring cultures to
a fresh medium
at
regular intervals
can maintain thecells in an active
state. An active culture
can
rapidly
dominate any
new
environment.
Thisphase can
be
prolonged
by
removing
toxicwaste,
addingmorenutrients
or
both.
3. Stationary phase:
As the
bacteria
dominate
the growth
medium
they deplete
theavailable nutrients
and
toxicwaste productsaccumulate,
slowing the rateofreproduction.
Atthe
sametime,
cellsare dyingoff.
A state
of
equilibrium
is reached between the
death ofoldcells andformation
ofnewones resulting
in
no netchange
incell
numbers.
4. Death phase:
In
this
phase
the formation
of
new
cellsceasesand the existing cells
graduallydieoff.
5.1.2 Factorsaffecting
bacte..al growth
Bacterial growth
isaffected
by temperature,
nutrient
availability,
water
supply,
oxygen supply,and acidity
of
the
medium.
Temperature
Theoretically,bacteria
can
grow
atall
temperaturesbetween
thefreezing
pointofwater
and
the
temperature
at which
protein or protoplasm
coagulates.
Somewhere
between
these
maximumand
minimum
points
lies
theoptimum
temperature at
which
the bacteria
grov best.
Temperatures
below
the minimum
stopbacterial
growth
butdo not
killthe
organism,
however, ifthe
temperature
is
raised above
the
maximum,
bacteria
are
soon
killed.M ostcells
die
after
exposure
to
heat
treatments of70'C
for
15 seconds,
although spore-forimngorganisms require moresevere
heattreatment,
e.g.
livesteamat 120'C
for30 minutes.
Bacteria
can be
classified
according
to
temperature
preference. Psychrotrophic
bacteria
grow
at
temperatures
below 16"C, mesophilic
bacteria
grow
best
at
temperatures
between
16 and
40"C,
and
thermophilic bacteriagrow
bestat
temperatures
above
40'C.
21
-
8/15/2019 Rural Dairy Technology
31/123
Nutrients
Bacteria
neednutrients
fortheirgrowth
and
some
need
more nutrients
thanothers.
Lactobacilli live
in milk
andhave losttheir
abilitytosynthesise
many
compounds,
while
Pseudomonas
can
synthesisenutrients
from
verybasicingredients.
Bacteria normally feed on organic
matter
which
contains
both material
for
cell
formation
and
the
necessary
energy.
The
organic
mattermust
be soluble
inwater
andof
low molecular
weight to
be able to
pass
through
the
cell
membrane.
Bacteria
therefore need
waterto
transport
nutrients
intothe
cell.
If
the nutrient
material
isnotsufficiently
broken
down, the
micro-organismcan
produceexo-enzymes
which
split the nutrients
into smaller,
simpler
components
so
they can
enter
the
cell.
Inside the cell
the
nutrients
arebroken
down further
byother
enzymes,
releasing
energy
which
isusedby the
cell.
Water
Bactcria
cannot
grow
without
water.
Many
bacteriaarequickly
killed
bydryconditions,
although
otherscan
tolerate
such conditions
for
months;
bacterial
spores can survive
dry conditions
for
years.
Water
activity
(Aw) isusedas
an
indicator
of
theavailability o.
water
for
bacterial
growth.
Distilledwater
has
anAwof
1.
Addition
of
solute, e.g.salt reduces the
availabilityof
water tothe cell
and the Aw
drops;
at Aw less
than
0.8
cell
growthisreduced.
Cells
that
cangrowat low
Aw
arecalledosmophiles.
Oxygen
Animals
require oxygen
tosurvivebut
bacteriadiffer in
theirrequirements
forand
in theirability
to
utilise
oxygen.
Aerobic
bacteria
need
oxygen
for growth,
however,
it is
toxic to
anaerobic
bacteria.
Anaerobic
organisms
are responsilie
for
reactionssuch
as methane
production
inbiogasplants
and
spoilage
incanned
foods
and cheeses.
Some bacteria
can
live
either
with or
without
oxygen
and
are
known
as facultative
anaerobic
bacteria.
Acidity
Theacidity
of
a nutrient
substrate
ismost
simply expressed
as
itspH
value. Sensitivity
to
pH varies
from
onespecies
of
bacteria
toanother.
The terms
pHoptimum
and
pH
maximum
are used.
Mostbacteria
prefer
a
growthenvironment
witha pH
of
about
7,
i.e.
neutrality.
Bacteria
that
can
tolerate
low
pHare referred to
as aciduric.
Lacticacidbacteria
in
milk produce
acid
and
continue
todo
sountilthe
pHofthemilk
falls below4.6,
at which
pointthey
gradually
die
off.
5.1.3
Bacteria
in
milk
Milk
fresh from
ahealthycow
contains
few
bacteria,
butcontamination
during
hAndlingcan
rapidly
increase
bacterial
numbers.
Milk
isan ideal food
andmany
bacteria
growreadily
init.
Some
bacteria
(lactic
acidbacteria)
are
useful
inmilk
processing,causing
milk
to
sournaturally,
leading
to
fermented
products
suchasirgo.
However,
milk
canalsocontain
pathogenic
bacteria,
such
as
Salmonella,
Mycobacteriun
tuberculosis,
Listeria
and
Brucella,
andcan
thus
transmit
disease. Other
bacteria
cancause
spoilage
of
the
milk, andspoilage
and
pooryields
ofproducts.
5.2 Moulds
Moulds
are a heterogeneous
group
of multicelled
organisms which
reproduce
asexually
either
by spore
formation
or
by fragmentation.
They
can growon
.widevariety
ofsubstrates
and
aregenerally
regarded
as
spoilage
organisms. However,
moulds
are
used
inthe
productionof antibiotics
and
in
certain
cheese
varieties.
Moulds
are
aerobic
organisms
and
theirgrowth
onfoods
can
be retarded
by excluding
air
through
careful
packaging.They
can
be
killed
by
jelativelymildheattreatments,
butmould spores
aremoreresistant
toheat.
Thestructure
ofmoulds
isshown
inFigure
10.
22
-
8/15/2019 Rural Dairy Technology
32/123
Figure 10. Structure
of
moulds.
Aspergillus
Penicillium
5.3
Yeasts
Yeasts
are
unicellular
organisms
which
reproduce
asexually
by
budding. They
are used industrially
to
ferment carbohydrates
to
such
products
as
alcohol
and citric
acid.
Yeasts are not
usually
used
in
milk
processing
and
are
normally
regarded
as
spoilage
organisms
in
dairy products.
The structure
of
yeasts
is
shown
in
Figure
11.
Figure
11. Structure
ofa
yeast
cell.
Fatglobules
Nucleus
Cell wall
Cytoplasmic
Cytoplasm membrane
Vacuole
5.4Viruses
Viruses
are extremely
small
organisms
comprising
a spherical
head containing
thegenetic
material,
and
a
cylindrical tail.
They
must invade
other
cells to
reproduce.
Viruses
that
attackbacterial
cellsare
known
as
bacteriophages.
Bacteriophages
that
attackacid-producing
bacteria
inhibit acid
production
in milk
thereby
causing
problems
in the manufacture
offermented
milks,
yoghurtand
cheese.
23
-
8/15/2019 Rural Dairy Technology
33/123
5.5
Milk
microbiology
In
addition
to
b
top related