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Membrane proteins may denature on heating milk above 70

°C,

resulting in exposure of

reactive thiol groups. Therefore, thiol-disulphide interchange reactions may occur between membrane proteins, and whey proteins may also participate in these reactions. Dalgleish and Banks ( 1 99 1 ) and Houlihan et

al.

( 1 992a) found that whey proteins,

especially �-lactoglobulin, became associated with the membranes of natural milkfat

globules at temperatures

>

70 °C. Houlihan

et al.

( 1992a) found that K-casein also

became incorporated into the membrane with increase in heating time from 2.5 to 20 min

at 80 °C. However, no K-:casein and only a small amount of �-casein were detected by

Dalgleish and Banks ( 1991 ) using temperatures between 65 and 90

oc

and holding time 0-

20 min; the presence of �-casein was not caused by heating. It was concluded that whey

proteins alone, rather than whey protein/K-casein complexes, bind to the fat globules

during heating (Dalgleish and Banks, 199 1 ) . Dalgleish and Banks ( 199 1 ) suggested that a

layer of whey proteins, especially �-lactoglobulin, binds to the membrane proteins on

heating through intermolecular disulfide bonds. Houlihan

et al.

( 1992a) found losses of

some original MFGM proteins (polypeptides) in milk on heating at 80

oc for 2.5 to 20 min

and suggested whey proteins may have displaced these proteins. However,

Kim

and

Jimenez-Flores ( 1 995) claimed that �-lactoglobulin and a-lactalbumin were involved in

heat-induced interactions with MFGM components; direct disulfide binding between whey

proteins and MFGM proteins could not explain their results, suggesting that interaction

between these two groups of proteins are more complex than direct disulfide bond

formation.

Rather than direct disulfide bond formation, whey proteins (particularly �­

lactoglobulin) may possibly deposit on the MFGM with the displacement from the

membrane of polypeptides such as the cysteine-containing protein ( 49 kDa protein) of the

MFGM (Kim and Jimenez-Flores (1995)). Corredig and Dalgleish ( 1996b) found that

both a-lactalbumin and �-lactoglobulin bind via intermolecular disulfide bridges to the

surface of the milk fat globule on heating milk from 65-85

oc.

Whey proteins in whole

milk have more affinity for the MFGM than for the casein micelle surface (Corredig and

Dalgleish, 1996b).

Membrane proteins and lactose may participate in the Maillard reaction when milk

is heated above 100 °C, but the recent work of Berg and van Boekel ( 1 994) has shown that

the Maillard reaction is not very important in milk from a quantitative point of view;

therefore its importance for the milk fat globule membrane is difficult to estimate. More

important may be the formation of dehydroalanine (an intermediate product in the

Maillard reaction) from cysteine residues present in the MFGM proteins (W alstra and

Jenness, 1984); dehydroalanine is quite reactive and lysinoalanine and lanthionine residues

may be formed which cause inter- and intra-molecular crosslinks between membrane

proteins which can be expected only when the heat treatment is severe.

Phospholipids migrate from the fat globules to the aqueous phase during heating,

to some extent (Houlihan

et al.

1992a). The zeta-potential (or electrophoretic mobility) of

fat globules changes as a result of heating (Fink and Kessler, 1985a; Dalgleish and Banks,

1 99 1 ), indicating that changes occur on the surfaces of the globules, probably owing to the

binding of whey proteins.

The changes in the membrane composition and structure, as a result of heating,

would be expected to affect the stability of the fat globules. Owing to the absence of cold

agglutination, creaming would be slower, and the cream layer formed would be more

closely packed, which could result in partial coalescence on cooling when fat

crystallization starts (Walstra, 1983). van Boekel and Folkerts ( 199 1 ) found that batch

heating of 30% fat cream for up to 40 min at 1 30 °C caused no change in the size

distribution of the natural milkfat globules, suggesting that the changes in membrane

composition discussed above do not impair coalescence stability. Also, indirect UHT

heating at up to 150 °C for 30 s of milk with 4% fat or cream containing 20 or 30% fat

caused no changes in globule size distribution (Streuper and van Hooydonk, 1 986; van

Boekel and Folkerts, 1 99 1 ). Hence, even the turbulence that occurs in combination with

heating during indirect UHT treatment does not appear to cause coalescence (or

disruption).

These results are in contrast to those of Pink and Kessler ( 1985b) who observed a

small increase in fat globule size in the temperature range 90-1 25 °C for 0.9 to 63 s (from

4.25 Jlm to 4.55 Jlm), followed by a decrease in the temperature range 1 25-1 50 °C (from

4.55 J.Lm back to 4.25 J.Lm); these changes in particle size were accompanied by similar

changes in "free fat" (actually extracted fat) and electrophoretic mobility. Fat globule

coalescence and disruption, and aggregation were also studied by van Boekel and Folkerts

( 1 99 1), but with natural milkfat globules no aggregation was observed after either indirect

UHT or batch heating.

Pink and Kessler ( 1985a) studied UHT heating of non-homogenized milk and cream. They reported that the fat globule membrane becomes progressively more "permeable" to fat on heating milk between 105 and 1 25 oc (because of loss of membrane forming constituents, in particular phospholipids and lipoproteins which are responsible for the stabilisation of the fat globule membrane), while the membrane becomes increasingly dense at temperatures above 1 25 oc (because of the heat induced deposition and polymerization of whey proteins on the membrane similar to the deposition of whey proteins on the casein micelles). Their finding that coalescence does occur to some extent on heating milk between 105 and 125 oc has been refuted by later work (van Boekel and Walstra, 1989, Streuper and van Hooydonk, 1986, Melsen and Walstra, 1989; van Boekel and Polkerts, 199 1 , Dalgleish and Banks, 1991). van Boekel and Polkerts ( 1 991) stated that fat as such cannot permeate through a membrane, and that it will never be free as milk has abundant surface active compounds that will cover denuded fat instantaneously. The explanation of Pink and Kessler ( 1985a) that polymerization of whey proteins causes a more dense membrane at temperatures > 125 oc is at variance with the fact that the effect does not depend at all on heating time (van Boekel and Polkerts, 1991). Dalgleish and Banks ( 1 991) found that heating at 85 oc for 4 min has a maximum effect on the association of whey proteins with fat globules. A possible explanation for the discrepancy between the findings of Pink and Kessler ( 1 985a) and those of other workers mentioned above may be due to a possible difference in the equipment used by Pink and Kessler which might have resulted in the induction of partial coalescence at some processing stage. With direct UHT heating, disruption of fat globules has been observed (Zadow, 1969; Ramsey and Swartel, 1984; van Boekel and Polkerts, 199 1 ; Corredig and Dalgleish, 1996a), probably because steam injection causes severe turbulence and flash cooling causes cavitation. The disruption in such cases is, for a milk containing 4% fat, more or less comparable with homogenization at 100 bar. Incidentally, this effect is not limited to natural milkfat globules, but also occurs with recombined milkfat globules (Melsen and W alstra, 1989).

Heating of milk is always accompanied by some agitation, which is of importance for physical stability as it may lead to changes in globule size as a result of coalescence or disruption (Mulder and Walstra, 1974). Consequently, changes in the surface layers of the

MFGM may occur. Coalescence causes desorption of membrane material whereas disruption results in the adsorption of surface active material from the milk plasma. When foaming occurs, fat globules may come in contact with air bubbles still uncovered by plasma proteins; hence spreading of membrane material over bubbles may occur, and when such bubbles disappear this material is released into the plasma; meanwhile the plasma proteins adsorb on to the partly denuded fat globules (Mulder and Walstra, 1974). These changes are thus not the result of heating per se, but of the agitation during heating. Whether and to what extent coalescence or disruption occurs depends on the type of apparatus used. In the case of direct UHT heating by steam injection, for example, disruption is very pronounced and has nearly the same effect as homogenization (Zadow, 1969; Ramsey and Swartzel, 1984; van Boekel and Folkerts, 1 99 1 ; Correidg and Dalgleish, 1996a).

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