Food Biochemistry and Food Processing (2 edition)

BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come

24 Chemistry and Biochemistry of Milk Constituents 447

Lactose in Fermented Dairy Products

The fermentation of lactose to lactic acid, by lactic acid bacte-

ria (LAB) is a critical step in the manufacture of all fermented

dairy products (cheese, fermented milks and lactic butter). The

fermentation pathways are well established (see Cogan and Hill

1993, Poolman 2002). Lactose is not a limiting factor in the man-

ufacture of fermented dairy products; only approximately 20%

of the lactose is fermented in the production of these products.

Individuals suffering from lactose intolerance may be able to

consume fermented milks without ill-effects, possibly because

LAB produceβ-galactosidase and emptying of the stomach is

slower than that for fresh milk products, thus, releasing lactose

more slowly into the intestine.

In the manufacture of cheese, most (96–98%) of the lactose

is removed in the whey. The concentration of lactose in fresh

curd depends on its concentration in the milk and on the mois-

ture content of the curd and varies from approximately 1.7%,

w/w, in fresh Cheddar curd to approximately 2.4%, w/w, in fresh

Camembert. The metabolism of residual lactose in the curd to

lactic acid has a major effect on the quality of mature cheese

(Fox et al. 1990, 2000). The resultant lactic acid may remain es-

sentially unchanged in the cheese during ripening (e.g., Cheddar

cheese) or may be catabolised to other compounds, for example

CO 2 and H 2 O, by surface mould in Camembert, or to propionic

acid, acetic acid, H 2 O and CO 2 in Emmental-type cheeses. Ex-

cessive lactic acid in cheese curd may lead to a low pH and a

number of defects, such as a strong, acid, harsh taste, an increase

in brittleness and a decrease in firmness. The pH of full-fat Ched-

dar is inversely related to the lactose/lactic acid content of the

curd. Excess residual lactose may also be fermented by hetero-

fermentative lactobacilli, with the production of CO 2 leading to

an open texture.

In the manufacture of some cheese varieties, for example

Dutch cheese, the curds are washed to reduce the lactose content

and thereby regulate the pH of the pressed curd at approximately

5.3. For Emmental, the curd-whey mixture is diluted with water

by approximately 20%, again to reduce the lactose content of

the curd, maintain the pH at approximately 5.3, and keep the

calcium concentration high, which is important for the textural

properties of this cheese. For Cheddar, the level of lactose, and

hence lactic acid, in the curd is not controlled. Hence, changes

in the concentration of lactose in milk, such as those occurring

throughout lactation, can result in marked changes in the quality

of such cheeses. To overcome seasonal variations in the lactose

content of milk, the level of wash water used for Dutch-type

cheeses is related to the concentrations of lactose and casein

in the milk. Ideally, the lactose-to-protein ratio in any particular

variety should be standardised, for example by washing the curd,

to minimise variations in the concentration of lactic acid, pH and

the quality of cheese.

Oligosaccharides

Lactose is the principal sugar in milk but the milk of most, if not

all, species also contains oligosaccharides, up to hexasaccha-

rides, derived from lactose (the reducing end of the oligosaccha-

rides is lactose and many contain fucose andN-acetylneuraminic

acid). About 130 oligosaccharides have been identified in hu-

man milk; the milk of elephant, bears and marsupials also con-

tains high levels of oligosaccharides. The oligosaccharides are

considered to be important sources of certain monosaccharides,

especially fucose andN-acetylglucosamine, for neonatal devel-

opment, especially of the brain (Urashima et al. 2001, 2009,

2011).

MILK LIPIDS

Definition and Variability

The lipid fraction of milk is defined as those compounds that are

soluble in non-polar solvents (ethyl/petroleum ether or chloro-

form/methanol) and is comprised mainly of triglycerides (98%),

with approximately 1% phospholipids and small amounts of

diglycerides, monoglycerides, cholesterol, cholesterol esters and

traces of fat-soluble vitamins and other lipids. The lipids occur

as globules, 0.1–20μm in diameter, each surrounded by a mem-

brane, the milk fat globule membrane (MFGM), which serves

as an emulsifier. The concentration of total and individual lipids

varies with breed, individual animal, stage of lactation, mastitic

infection, plane of nutrition, interval between milking and point

during milking when the sample is taken. Among the principal

dairy breeds, Friesian/Holsteins produce milk with the lowest fat

content (∼3.5%) and Jersey/Guernsey the highest (∼6%). The

fat content varies considerably throughout lactation; when syn-

chronised calving is practised, the fat content of bulk Friesian

milk varies from approximately 3% in early lactation to>4.5%

in late lactation. Such large variations in lipid content obviously

affect the economics of milk production and the composition of

milk products, but can be modified readily by natural cream-

ing, centrifugal separation or addition of cream, and hence need

not affect product quality. Milk lipids also exhibit variability

in fatty acid composition and in the size and stability of the

globules. These variations, especially fatty acid profile, are es-

sentially impossible to standardise and hence are responsible

for considerable variations in the rheological properties, colour,

chemical stability and nutritional properties of fat-containing

dairy products.

Fatty Acid Profile

Ruminant milk fat contains a wider range of fatty acids than any

other lipid system – up to 400 fatty acids have been reported

in bovine milk fat; the principal fatty acids are the homologous

series of saturated fatty acids with an even number of C-atoms,

C4:0–C18:0,andC18:1. The outstanding features of the fatty acid

profile of bovine milk fat are a high concentration of short- and

medium -chain acids (ruminant milk fats are the only natural

lipids that contain butanoic acid, C4:0) and a low concentration

of polyunsaturated fatty acids.

In ruminants, the fatty acids for the synthesis of milk lipids

are obtained from triglycerides in chylomicrons in the blood

or synthesised de novo in the mammary gland from acetate or

β-hydroxybutyrate produced in the rumen. The triglycerides in