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									                            MILK




Food Material Science 2010/11
Inneke Hantoro
Definition

   The normal secretion of the mammary
    glands of all mammals (Potter &
    Hotchiss, 1996).
   Milk is a complete food for the new
    born.
   High density of nutritious components.
MILK COMPOSITION &
STRUCTURE
The average composition of milk




                     Source: Walstra et al. (2006)
Principal components
   Lactose or milk sugar is the distinctiveACIDS
                                      FATTY
    carbohydrate of milk. It is a disaccharide
    composed of glucose and galactose.

   The fat is largely made up of triglycerides,
    constituting a very complicated mixture. The
    component fatty acids vary widely in chain
    length (2 to 20 carbon atoms) and in saturation
    (0 to 4 double bonds). Other lipids that are
    present include phospholipids, cholesterol, free
           TRIGLISERIDA
    fatty acids, monoglycerides, and diglycerides.
Principal components
Protein
   About four fifths of the protein consists of
    casein, actually a mixture of four proteins: αS1-,
    αS2-, β-, and κ-casein. The caseins are typical
    for milk.
   The remainder consists, for the most part, of
    the milk serum proteins, the main one being β-
    lactoglobulin.
   Moreover, milk contains numerous minor
    proteins, including a wide range of enzymes.
Principal components
   The mineral substances — primarily K, Na,
    Ca, Mg, Cl, and phosphate — are not
    equivalent to the salts. Milk contains
    numerous other elements in trace
    quantities. The salts are only partly ionized.
   The organic acids occur largely as ions or
    as salts; citrate is the principle one.
   Furthermore, milk has many
    miscellaneous components, often in trace
    amounts.
        Water            Organic acids    Proteins
        Carbohydrates     citrate          casein
         lactose          formate          -lactoglobuline
         glucose          acetate          -lactalbumine
         others           lactate          serum albumin
SERUM   Minerals          oxalate          immunoglobulines
         Ca, bound        others           proteose pepton
         Ca, ions        Gases            NPN
         Mg               oxygen           peptides
         K                nitrogen         amino acids
         Na              Lipids            urea
         Cl               glycerides       ammonia
         phosphate        fatty acids     Enzymes
         sulfate          phospholipids    acid phosphatase
         bicarbonate      cerebrosides     peroxidases
        Trace elements    sterols          many others
         Zn/Fe/Cu and    Vitamins         Phosphoric
        many              B vitamines     esthers
         others          ascorbic acid    Others
Compotition
    and
 Structure
(A) Uniform liquid. However, the liquid is turbid
and thus cannot be homogeneous.

(B) Spherical droplets, consisting of fat. These
globules float in a liquid (plasma).

(C) The plasma contains proteinaceous
particles, which are casein micelles. The
remaining liquid (serum) is still opalescent, so it
must contain other particles. The fat globules
have a thin outer layer (membrane) of different
constitution.
Fat Globules
   The surface layer or membrane of the fat globule is
    not an adsorption layer of one single substance but
    consists of many components; its structure is
    complicated.
   The dry mass of the membrane is about 2.5% of
    that of the fat.
   A small part of the lipids of milk is found outside the
    fat globules.
   At temperatures below 35°C, part of the fat in the
    globules can crystallize.
   Milk minus fat globules is called milk plasma, i.e.,
    the liquid in which the fat globules float.
   Composition and structure of milk fat
                                MEMBRAN
                                Water
FAT
GLOBULE                         Protein

          Glycerides            Phospholipids

               triglycerides    Cerebrosides

               diglycerides     Glycerides

               monoglycerides   Fatty acids

          Fatty acids           Sterols

          Sterols               Other lipids

          Carotenoids           Enzymes

          Vitamins A,D,E,K        alkaline phosphatase

                    Water         xanthine oxidase
                                  many others
                                Cu and Fe
Casein Micelles
   Casein micelles consist of water, protein, and
    salts. The protein is casein.
   Casein is present as a caseinate, which means
    that it binds cations, primarily calcium and
    magnesium.
   The other salts in the micelles occur as a
    calcium phosphate, varying somewhat in
    composition and also containing a small
    amount of citrate. This is often called colloidal
    phosphate. The whole may be called calcium-
    caseinate/calcium-phosphate complex.
Casein Micelles

   The casein micelles are just ‘small particles.’
   The micelles have an open structure and,
    accordingly, contain much water, a few
    grams per gram of casein.
   Milk serum, i.e., the liquid in which the
    micelles are dispersed, is milk minus fat
    globules and casein micelles.
CASEIN
          Protein
MICELLE
              Casein
              Proteose pepton
          Salts
              Ca
              Phosphate
              Citrate
              K, Mg, Na
          Water
          Enzymes (lipase, plasmine)
Other Milk Constituents

   Serum proteins are largely present in milk
    in molecular form or as very small
    aggregates.
   Lipoprotein particles, sometimes called
    milk microsomes, vary in quantity and
    shape. Presumably, they consist of
    remnants of mammary secretory cell
    membranes. Few definitive data on
    lipoprotein particles have been published.
Other Milk Constituents

   Cells, i.e., leukocytes, are always present in
    milk. They account for about 0.01% of the
    volume of milk of healthy cows. Of course,
    the cells contain all cytoplasmic components
    such as enzymes. They are rich in catalase.
Other milk constituents
LEUKOCYTE            LIPOPROTEIN
                     PARTICLE


  Many enzymes             Lipids
    e.g. katalase         Protein
   Nucleic acids          Enzymes
      Water                Water
   Properties of the main structural elements
   of milk
                             fat           casein         globular     lipoprotein
                                           micelles       proteins     paricles
Main component(s)                   Fat      Casein,         Serum        Lipids,
                                            water salts     proteins     proteins
To be considered as          Emulsion      Fine           Colloidal    Colloidal
                                           dispersion     solution     dispersion
Content (% dry matter)       4             2.8            0.6          0.01
Volume fraction              0.04          0.1            0.006        10-4

Particle diameter            0.1 – 10 m   20 – 300 nm    3 – 6 nm     10 nm

Number per ml                1010          1014           1017         1014

Surface area (cm2/ml milk) 700             40.000         50.000       100

Density (20 0C; kg/m3)       900           1100           1300         1100

Diffusion rate (mm in 1 h)   0.0           0.1 – 0.3      0.6          0.4

Isoelectric pH               ~3.8          ~4.6           4–5          ~4
MILK FORMATION
Digestion-1
   Milk components are for the most part formed in
    the mammary gland (the udder) of a cow, from
    precursors that are the results of digestion.
   In ruminants like the cow, considerable
    predigestion occurs by means of microbial
    fermentation, which occurs for the most part in
    the first stomach or rumen.
   It contains numerous bacteria that can digest
    cellulose, thereby breaking down plant cell
    walls, providing energy and liberating the cell
    contents.
Digestion-2
   From cellulose and other carbohydrates, acetic,
    propionic, butyric and lactic acid are formed,
    which are taken up in the blood. The
    composition of the organic acid mixture
    depends on the composition of the feed.
   Proteins are broken down into amino acids. The
    rumen flora uses these to make proteins but
    can also synthesize amino acids from low-
    molar-mass nitrogenous components. Further
    on in the digestive tract the microbes are
    digested, liberating amino acids.
Digestion-3

   Also, food lipids are hydrolyzed in the rumen
    and partly metabolized by the
    microorganisms.
   All these precursors can reach the
    mammary gland.
     Milk Synthesis-1

   The synthesis of
    milk components
    occurs for the
    greater part in the
    secretory cells of
    the mammary
    gland.
Milk Synthesis-2
   At the basal end precursors of milk components
    are taken up from the blood, and at the apical
    end milk components are secreted into the
    lumen.
   Proteins are formed in the endoplasmic
    reticulum and transported to the Golgi vesicles,
    in which most of the soluble milk components
    are collected.
   The vesicles grow in size while being
    transported through the cell and then open up
    to release their contents in the lumen.
Milk Synthesis-3

   Triglycerides are synthesized in the
    cytoplasm, forming small globules, which
    grow while they are transported to the apical
    end of the cell.
   They become enrobed by the outer cell
    membrane (or plasmalemma) while being
    pinched off into the lumen.
   This type of secretion is called merocrine,
    which means that the cell remains intact.
Excretion-1
   The glandular epithelium, consisting of layers of
    secretory cells, form spherical bodies called
    alveoli.
   Each of these has a central lumen into which
    the freshly formed milk is secreted.
   From there, the milk can flow through small
    ducts into larger and still larger ones until it
    reaches a cavity called the cistern.
   From the cistern, the milk can be released via
    the teat.
Excretion-2
   Excretion of the milk does not happen
    spontaneously. The alveoli have to contract,
    which can be achieved by the contraction of
    muscle tissue around the alveoli.
   Contraction is induced by the hormone
    oxytocin. This is released into the blood by
    stimulation of the teats of the animal, be it by
    the suckling young or by the milker.
   The udder is not fully emptied.
Lactation
   When a calf is born, lactation — i.e., the
    formation and secretion of milk — starts.
    The first secretion greatly differs in
    composition from milk.
   Within a few days the milk has become
    normal and milk yield increases for some
    months, after which it declines.
   The yield greatly varies among cows and
    with the amount and the quality of the feed
    taken by the cow.
    Colostrum
   Colostrum is the secretion produced over the first few
    days after parturition. The components of colostrum are
    synthesised in the mammary gland over several days
    prior to parturition.

   Colostrum is rich in special nutrients for the newborn.

   Colostrum contains more mineral salts and protein and
    less ash than later milk. Ca, Na, Mg, P, and chloride are
    higher in colostrum but K is lower.

   The most remarkable difference between colostrum and
    milk is the high concentration of immunoglobulins (Ig’s) in
    colostrum. Ig’s are related to passive immunity against
    gut pathogens.
Colostrum
   Colostrum has a higher level of -carotene, imparting
    an intense yellow colour, and a high level of
    somatic cells.

   Recently there has been a lot of commercial
    interest in colostrum because of its elevated levels
    of bioactives, especially growth factors, and there is
    a wide range of literature supporting the health
    benefits of colostrum

   Colostrum is 10 times more expensive than milk
    powder.
MILK ATTRIBUTES
Milk quality
   Factors that determine the quality of fresh milk
    (standard indicators) are:
     Total solid contents, including protein (min.
        2.7%), fat (min. 3%), solid non fat (min. 8%).
        Raw milk is purchased by weight, but processed
        milk is sold by volume.
     Freezing point

     Density
Milk quality
   Some factors can influence the quality of milk,
    including:
     Feed

     Genetic

     Climate

     The health status of cattle

     Milking process and storage

     Post harvest handling
Fresh Milk Deterioration

   Milk can deteriorate fast since milk contains
    high nutrient contents such as carbohydrate, fat
    and protein which required by bacteria to grow.
   Moreover, pH of milk is close to neutral pH.
    This is very suitable for the growth of
    microorganisms.
   Lastly, since most of microorganism (mesophilic
    and psychotrophic bacteria) can grow very well
    at room temperature, fresh milk stored in room
    temperature is susceptible to microbial
    deterioration.
Fresh Milk Deterioration

   Many of the psychrotrophic bacteria isolated from
    milk produce extracellular enzymes that degrade
    milk fat and protein (proteolysis and lypolysis).
   Bacterial lipase causes serious degradation of milk
    fat.
   Beside microbial degradation, fresh milk also
    susceptible to enzymatic degradation. Raw milk has
    an abundance of lipoprotein lipase, an enzyme that
    will rapidly hydrolyse milk fat to free fatty acids
    (FFAs).
   Some of these FFAs have low organoleptic
    thresholds and produce odors and flavors (rancid,
    bitter, soapy or unclean).
UHT vs Pasteurized Milk

   Generally, there are two heat treatment given to
    fresh milk, i.e. pasteurization and sterilization using
    ultra high temperature (UHT).
   Pasteurization is done at 63oC for 30 min or 72-
    75oC for 15-20 s (high temperature short time -
    HTST). Pasteurization is used mostly to kill Gram-
    negative psychrotrophs bacteria, but only has little
    effect on extracellular degradative enzymes.
   While UHT is done at 135 - 140oC for a few
    seconds. It can kill both pathogen and spoilage
    microorganisms. The most heat resistant
    pathogenic spore – C. botulinum and some
    enzymes also can be inactivated.
UHT vs Pasteurized Milk

   UHT products are commonly stored at room
    (ambient) temperature and good quality
    products should be microbiologically stable.
   Nevertheless, chemical reactions and physical
    changes will take place which will change the
    quality of the product. These include oxidation
    reactions, Maillard browning and chemical &
    physical changes which may give rise to age-
    thickening and gelation.
UHT vs Pasteurized Milk
   In pasteurization, thermoduric bacteria and spore
    forming bacteria can survive. Bacillus cereus
    spores are relevant here, being the main pathogen
    which will survive pasteurization and grow at low
    temperature. It will certainly cause spoilage in heat-
    treated milk.
   Enzymes in raw milk may give rise to problems in
    pasteurized milk. For example, indigenous lipases
    may give rise to soapy off-flavors. However, it is
    unlikely that bacterial lipases and proteases, which
    are very heat resistant, will cause problems in
    pasteurized milks because of their relatively short
    shelf-life and refrigerated storage conditions.
Milk & Dairy Products Adulteration

   Watering of milk
   Milk of different species
   Addition of non-dairy protein
   Altering the casein/whey protein ratio
   Addition of buttermilk or whey powder to
    milk powder
   Addition of vegetable or animal fats to milk
    fat
   Addition of reconstituted milk to fluid milk
   Non-authorized preservatives.
Milk Coagulation

   Desirable coagulation of milk can be seen in
    dairy products processing such as cheese,
    yoghurt, etc.
   Undesirable coagulation occur in liquid milk.
    It can caused by lactic acid (produced by
    bacteria) --- the reduction of pH or by
    physical separation (due to density
    difference) such as creaming, flocculation or
    coalescence --- see emulsion chapter).
Milk Coagulation
   Milk protein, such as whey protein and casein
    have important role in coagulation.
   The example of desirable coagulation:
     Acidification forms the basis of production of
       all fermented milks. The gels of fermented
       milks, such as yoghurt, are formed by
       acidification of milk. As the pH is reduced,
       the casein precipitates selectively. The first
       signs of aggregation occur around pH 5 and
       once the pH falls to 4.6 all the casein
       becomes insoluble.
Milk Coagulation

   Some factors influence coagulation,
    including:
     pH

     Temperature

     Heat treatment

     Casein concentration

     The presence of salt
Emulsion

   Milk proteins have excellent emulsifying
    properties.
   Milk is categorized as o/w emulsion, since
    the oil part is dispersed in the water.
   Milk proteins, both caseinates and whey
    proteins, are surface active, they are
    absorbed rapidly to the oil-water interface,
    forming stable emulsions.
Emulsion

   The primary processes leading to emulsion
    instability are:
     Creaming – refers to the gravitational separation
        of emulsified droplets to form a densely packed
        phase without change in droplet size.
     Flocculation – denotes the aggregation of
        droplets via interactions between adsorbed
        proteins.
     Coalescence – an increase in droplet size,
        gradually results in separation of the oil and
        aqueous phases.
                                     2 layers
                                    formation


                                    Coalescence




                                    Creaming


Flocculation



               Kinetically stable
                  emulsion
Creaming
   Since the specific gravity of lipids and skim
    milk is 0.9 and 1.036, respectively, the fat
    globules in milk held under quiescent
    conditions will rise to the surface under the
    influence of gravity, a process referred to as
    creaming.

   The rapid rate of creaming is due to the
    strong tendency of the fat globules to cluster
    due to the effect of indigenous
    immunoglobulin M which precipitates onto
    the fat globules when milk is cooled
    (cryoglobulins).
Creaming
   Large globules rise faster than smaller ones,
    collide with them and form aggregates. The
    clusters of globules rise rapidly and
    therefore the creaming process is
    accelerated as the globules rise and clump.

   Creaming is inhibited by reduction of the fat
    globule size by homogenisation. The milk fat
    globules are reduced in size by pumping at
    very high pressure (up to 400 bar) through a
    small slit. The size reduction results in an
    increase in specific surface area .
Whipping & Foaming
   As milk proteins are surface active, they have
    the ability to adsorb to the air-water interface
    during foam formation.
   Foams are most commonly formed by
    mechanically dispersing air into a solution
    containing surface-active agents. A rapid
    diffusion of the protein to the air-water interface
    to reduce surface tension, followed by partial
    unfolding of the protein is essential for the
    formation of protein-based foams.
Whipping & Foaming
   Caseinates generally give higher foam overruns but
    produce less stable foams than whey protein
    concentrates (WPC).
   The foaming properties are influenced by many
    factors, including:
     protein concentration,

     level of denaturation,

     ionic strength,

     preheat treatment and

     presence of lipids.
The Changes of Milk Flavor
   Deterioration of milk flavor can be caused by
    degradation milk fat and protein.
   Rancidity is a common indicator of the forming
    of undesirable flavor.
   Factors stimulating the off-flavor in fresh milk:
       Light
       Ion metals
       Transferred from cow to milk
       Microorganisms
       Enzymatic reactions
References

   Walstra, P., J.T.M. Wouters & T. J.
    Geurts. 2006. Dairy Science and
    Technology 2nd Edition. Taylor and
    Francis Group. Boca Raton.
Thank You….

								
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