Milk Carbohydrates

Dr. Sonu K S
Scientist
Dairy Chemistry Division
 Milk Carbohydrates
❑ Lactose is the principal carbohydrate in the milk of most mammals, exceptions are
the California sea lion and the hooded seal, which are the only significant sources.

❑ Milk contains only trace amounts of other sugars, including glucose (50 mg/L)
and fructose and glucosamine, galactosamine and N-acetyl neuraminic acid as
components of glycoproteins and glycolipids.
❖The lactose content of cows' milk varies with the breed of cow, individual
animals, udder infection (mastitis) and stage of lactation.
❖The concentration of lactose decreases progressively and significantly during
lactation.
❖The concentration of lactose in milk is inversely related to the concentrations of
lipids and proteins.
Changes in the concentrations of fat
❖The inverse relationship between the concentrations of (closed triangle), protein (empty
lactose and lipids and protein reflects the fact that the square) and lactose (open circle) in
milk during lactation
synthesis of lactose draws water into the Golgi
vesicles, thereby diluting the concentrations of
proteins and lipids.
❖Mastitis causes an increased level of NaCl in milk and
depresses the secretion of lactose.
❖A Koestler Number <2 indicates normal milk
❖A Koestler Number 3 is considered abnormal.
❖Lactose, along with sodium, potassium and chloride ions, plays a major role in
maintaining the osmotic pressure in the mammary system.
❖Thus, any increase or decrease in lactose content (a secreted constituent, i.e., formed
within the mammary gland, which is isotonic with blood) is compensated for by an
increase or decrease in the soluble salt constituents.
❖Lactose plays an important role in milk and milk products:
It is an essential constituent in the production of fermented dairy products.
It contributes to the nutritive value of milk and its products.
limited or zero ability to digest lactose in adulthood, leading to Average concentration (%) of lactose and ash in the milk
lactose intolerance. of some mammals

It affects the texture of certain concentrated and frozen

products.
It is involved in heat-induced changes in the colour and flavour
of highly heated milk products.
Its changes in state (amorphous vs. crystalline) have major
implications for the production and stability of many
dehydrated milk products
 Chemical and Physical Properties of Lactose
Structure of Lactose
❖Lactose is a disaccharide consisting of galactose and glucose, linked by a β1-4
glycosidic bond.
❖ Its systematic name is 0-β-d-galactopyranosyl-(1-4)-αd-glucopyranose (α-
lactose) or 0-β-d-galactopyranosyl-(1-4)-β-d-glucopyranose (β-lactose)
Haworth projection

Open chain Structure / Fisher projection

Chemical and Physical Properties of Lactose
Structure of Lactose
 Chemical and Physical Properties of Lactose
Biosynthesis of Lactose
❖ Lactose is unique to mammary secretions. It is synthesized from glucose absorbed from
blood. One molecule of glucose is isomerized to UDP-galactose via the 4-enzyme
Leloir pathway.
❖ UDP-Gal is then linked to another molecule of glucose in a reaction catalysed by the
enzyme, lactose synthetase, a 2-component enzyme
✓ Component A is a non-specific galactosyl transferase
(EC 2.4.1.22) which transfers the galactose from
UDP-gal to a number of acceptors.
✓ In the presence of the B component, which is the
whey protein, α-lactalbumin, the transferase becomes
highly specific for glucose (its KM is decreased
1,000-fold), leading to the synthesis of lactose.
✓ Thus, α-lactalbumin is an enzyme modifier and its
concentration in milk is directly related to the
concentration of lactose
 Chemical and Physical Properties of Lactose
Lactose Equilibrium in Solution
❖The configuration around the C1 of glucose (i.e., the anomeric C) is not stable and
can readily change (mutarotate) from the α- to the β-form and vice versa when the
sugar is in solution as a consequence of the fact that the hemiacetal form is in
equilibrium with the open chain aldehyde form which can be converted into either
of the two isomeric forms.
✓ Thus, the equilibrium mixture at 20 °C is composed of 62.7 % β and
37.3 % α-lactose.
✓ The equilibrium constant, β/α, is 1.68 at 20 °C.
✓ The proportion of lactose in the α-form increases as the temperature is
increased and the equilibrium constant consequently decreases.
✓ The equilibrium constant is not influenced by pH, but the rate of
mutarotation is dependent on both temperature and pH.
✓ The rate of mutarotation is slowest at pH 5.0, increasing rapidly at
more acid or alkaline values; equilibrium is established in a few
minutes at pH 9.0
 Chemical and Physical Properties of Lactose
Significance of Mutarotation
❖The α and β forms of lactose differ with respect to:
➢Solubility
➢Crystal shape and size
➢Hydration of the crystalline form, which leads to hygroscopicity
➢Specific rotation
➢Sweetness
 Chemical and Physical Properties of Lactose
Solubility
❖When α-lactose is added in excess to water at 20 °C, about 7 g per 100 g water
dissolve immediately.
❖Some α-lactose mutarotates to the β anomer to establish the equilibrium ratio
62.7β:37.3α
❖Therefore, the solution becomes unsaturated with respect to α and more α-lactose
dissolves and some mutarotates to β-lactose.
❖These two processes (mutarotation and solubilization of α-lactose) continue until two
criteria are met:
❖~7 g α-lactose are in solution and the β/α ratio is 1.6:1.0.
❖Since the β/α ratio at equilibrium is about 1.6 at 20 °C, the final solubility is 7
g+(1.6×7) g=18.2 g per 100 g water.
❖The solubility of α-lactose is more temperature dependent than that of β-lactose and
the solubility curves intersect at 93.5 °C.
 Chemical and Physical Properties of Lactose
Solubility
❖When β-lactose is dissolved in water, the initial solubility is ~50 g per 100 g water at
20 °C.
❖Some β-lactose mutarotates to α to establish a ratio of 1.6:1.
❖At equilibrium, the solution would contain 30.8 g β and 19.2 g α/100 ml; therefore,
the solution is supersaturated with α-lactose, some of which crystallizes, upsetting the
equilibrium and leading to further mutarotation of β to α.
❖These two events, i.e., crystallization of α-lactose and mutarotation of β, continue
until the same two criteria are met, i.e., ~7 g of α-lactose in solution and a β/α ratio of
1.6:1.
❖Again, the final solubility is ~18.2 g lactose per 100 g water.
❖Since β-lactose is much more soluble than α and mutarotation is slow, it is possible to
form more highly concentrated solutions by dissolving β- rather than α-lactose.
❖In either case, the final solubility of lactose is the same (18.2 g/100 g of water).
 Chemical and Physical Properties of Lactose
Crystallization of Lactose

▪ In the absence of nuclei and agitation, solutions of lactose are capable of being highly supersaturated before spontaneous
crystallization occurs.
▪ Solubility curves for lactose are divided into unsaturated, metastable and labile zones.
▪ Cooling a saturated solution or continued concentration beyond the saturation point, leads to supersaturation and
produces a metastable area where crystallization does not occur readily.
▪ At higher levels of supersaturation, a labile area is observed where crystallization occurs readily
 Chemical and Physical Properties of Lactose
Crystallization of Lactose
1. Neither nucleation nor crystal growth occurs in the unsaturated region.
2. Growth of crystals can occur in both the metastable and labile areas.
3. Nucleation occurs in the metastable area only if seeds (centers for crystal growth) are
added.
4. Spontaneous crystallization can occur in the labile area without the addition of seeding
material
➢ α-Hydrate: α-Lactose crystallises as a monohydrate containing 5 % water of crystallization and can be
prepared by concentrating an aqueous lactose solutions to supersaturation and allowing crystallization to
occur below 93.5 °C.
➢ α-Anhydrous: Anhydrous α-lactose may be prepared by dehydrating α-hydrate in vacuo at a temperature
between 65 and 93.5 °C; it is stable only in the absence of moisture.
➢ β-Anhydride: Since β-lactose is less soluble than the α-isomer >93.5 °C, the crystals formed from aqueous
solutions at a temperature above 93.5 °C are β-lactose which are anhydrous
➢ Lactose glass: When a lactose solution is dried rapidly (e.g., spray drying lactose-containing concentrates),
viscosity increases so quickly that there is insufficient time for crystallization to occur. A non-crystalline
amorphous form is produced containing α- and β-forms in the ratio at which they exist in solution. Tomahawk-shape
 Chemical and Physical Properties of Lactose

Some physical properties of the two common forms of lactose

 Derivatives of Lactose
❖Enzymatic Modification of Lactose: Lactose may be hydrolysed to glucose and galactose by enzymes (β-
galactosidases, commonly called lactase)
❖Chemical Modifications of Lactose :
1. Lactulose:
✓ Lactulose is an epimer of lactose in which the glucose moiety is isomerized to fructose.
✓ The sugar does not occur naturally and was first synthesized by Montgomery and Hudson in 1930.
✓ It can be produced under mild alkaline conditions via the Lobry de Bruyn-Alberda van Ekenstein
reaction
✓ It is produced on heating milk to sterilizing conditions and is a commonly used index of the severity of
the heat treatment to which milk has been subjected (e.g., to differentiate in-container sterilized milk
from UHT milk. It is not present in raw or HTST pasteurized milk.)
✓ It is not metabolized by oral bacteria and hence is not cariogenic.
✓ It is not hydrolysed by intestinal β-galactosidase and hence reaches the large intestine where it can be
metabolised by lactic acid bacteria, including Bifidobacterium spp. and serves as a bifidus factor
✓ It is now commonly added to infant formulae to simulate the bifidogenic properties of human milk
 Derivatives of Lactose
Chemical Modifications of Lactose
2. Lactitol (4-O-β-d-galactopyranosyl-d-sorbitol):
❖It is a sugar alcohol produced on reduction of lactose usually using Raney nickel;
❖it does not occur naturally.
❖It can be crystallized as a mono- or di-hydrate.
❖Lactitol is not metabolized by higher animals;
❖ it is relatively sweet and hence has potential as a non-nutritive sweetener.
❖It is claimed that lactitol reduces the absorption of sucrose, reduces blood and liver
cholesterol levels and is anti-cariogenic.
❖It has applications in low calorie foods (jams, marmalade, chocolate, baked goods)
❖it is non-hygroscopic and can be used to coat moisture-sensitive foods, e.g., candies.
❖It can be esterified with 1 or more fatty acids to yield a family of food emulsifiers,
analogous to the sorbitans produced from sorbitol.
 Derivatives of Lactose
Chemical Modifications of Lactose
3. Lactobionic Acid:
• This derivative is produced by oxidation of the free carbonyl group of lactose, chemically (Pt, Pd or
Bi), electrolytically, enzymatically or by fermentation.
• It has a sweet taste, which is very unusual for an acid.
• Its lactone crystallizes readily.
• Lactobionic acid has found only limited application; its lactone could be used as an acidogen but it is
probably not cost-competitive with gluconic acid-δlactone.
• It is used in preservation solutions for organs (to prevent swelling) prior to transplantation, and in skin-
care products.
4. Lactosyl Urea:
• Reaction of urea with lactose yields lactosyl urea, from which NH3 is released more slowly.
• Urea can serve as a cheap source of nitrogen for cattle but its use is limited because NH3 is released too
quickly, leading to a toxic level of NH3 in the blood
 Problems related to Lactose Crystallization
❖The tendency of lactose to form supersaturated solutions that do not crystallize readily causes
problems in many dairy products.
❖The problems are due primarily to the formation of large crystals, which cause sandiness, or to
the formation of a lactose glass, which leads to hygroscopicity and caking
Dried Milk and Whey
❖Lactose is the major component of dried milk products: whole milk powder, skim milk powder
and whey powder contain ~30, 50 and ~70 % lactose, respectively. Protein, fat and air are
dispersed in a continuous phase of amorphous solid lactose.
❖In freshly-made powder, lactose is in an amorphous state with an α:β ratio of 1:1.6. This
amorphous lactose glass is a highly concentrated syrup since there is not sufficient time during
drying for crystallization to proceed normally.
❖The glass has a low vapour pressure and is hygroscopic, taking up moisture very rapidly when
exposed to the atmosphere
❖On the uptake of moisture, dilution of the lactose occurs and the molecules acquire sufficient
mobility and space to arrange themselves into crystals of α-lactose monohydrate. These
crystals are small, usually with dimensions of <1 μm
 Problems related to Lactose Crystallization

• Lactose crystallization in dried milk can cause powder caking.

• However, if a significant portion of lactose is already crystalline in the freshly-dried
product, caking is prevented, maintaining dispersibility.
• Crystallization is achieved by rehydrating the powder to ~10% H2O, exposing it to
moisture-saturated air, and then redrying, or by completing drying in a fluidized bed.
This process is used commercially to produce instantized milk powders.
• Problems from lactose crystallization in milk and whey powders can be controlled by
pre-crystallizing the lactose. This involves adding finely-divided lactose powder,
which acts as nuclei for crystallization.
• Adding 0.5 kg of finely-ground lactose to 1 tonne of concentrated product induces
~10⁶ crystals/ml, with ~95% being <10 μm and 100% <15 μm, too small to cause
textural defects.
 Problems related to Lactose Crystallization
• During the drying of whey or other lactose-rich solutions, hot, semi-dry powder can
adhere to metal surfaces, forming deposits—a phenomenon known as
thermoplasticity
• The sticking temperature is influenced by the concentrations of lactic acid,
amorphous lactose, and moisture.
• Increasing lactic acid concentration from 0 to 16% lowers the sticking temperature.
• Pre-crystallization of lactose also affects this temperature; for example, 45% pre-
crystallized lactose sticks at 60°C, while 80% pre-crystallization raises it to 78°C.
• Pre-crystallization allows for higher feed concentrations and drying temperatures,
and is commonly used in drying high-lactose products like whey powder.
• Pulverized α-lactose, or preferably lactose “glass”, is used as seed.
• Continuous vacuum cooling, combined with seeding, gives the best product
 Problems related to Lactose Crystallization
Sweetened Condensed Milk
• Crystallization of lactose occurs in SCM and crystal size must be controlled if a
product with a desirable texture is to be produced.
• As it comes from the evaporators, SCM is almost saturated with lactose.
• When cooled to 15–20 °C, 40–60 % of the lactose will eventually crystallize as α-
lactose hydrate.
• There are 40–47 parts of lactose per 100 parts of water in SCM, consisting of about
40 % α- and 60 % β- (ex-evaporator).
• To obtain a smooth texture, crystals with dimensions <10 μm are desirable.
• The optimum temperature for crystallization is 26–36 °C.
 Problems related to Lactose Crystallization
Ice Cream
• Crystallization of lactose in ice cream causes a sandy texture.
• In freshly hardened ice cream, the equilibrium mixture of α- and β-lactose is in the
“glass” state and is stable as long as the temperature remains low and constant.
• During the freezing of ice cream, the lactose solution passes through the labile zone
so rapidly and at such a low temperature that little lactose crystallization occurs.
• When ice cream is warmed or experiences temperature fluctuations, some ice melts,
leading to varying lactose concentrations. These concentrations may enter zones
where spontaneous or induced crystallization can occur, though extensive
crystallization is unlikely at low temperatures. However, any nuclei formed can
slowly grow over time, eventually causing a sandy texture.
 Maillard Browning
• As a reducing sugar, lactose can participate in the Maillard reaction, leading to non-
enzymatic browning.
• The Maillard reaction involves interaction between a carbonyl (in this case, lactose) and
an amino group (in foods, principally the ε-NH2 group of lysine in proteins) to form a
glycosamine (lactosamine)
• The glycosamine may undergo an Amadori rearrangement to form a 1-amino-2-keto sugar
(Amadori compound)
• The reaction is base-catalysed and is first order.
• The Amadori compound may be degraded via either of two pathways, depending on pH,
to a variety of active alcohol, carbonyl and dicarbonyl compounds and ultimately to
brown-coloured polymers called melanoidins.
• Many of the intermediates are (off-) flavoured.
• The dicarbonyls can react with amino acids via the Strecker degradation pathway to yield
another family of highly flavoured compounds
 Maillard Browning
• The Maillard reaction has desirable consequences in many foods, e.g., coffee,
bread crust, toast, french fried potato products
• Its consequences in milk products are negative, e.g., brown colour, off-flavours,
slight loss of nutritive value (lysine), loss of solubility in milk powders (although it
appears to prevent or retard age-gelation in UHT milk products).
• Maillard reaction products (MRP) have antioxidant properties.