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Proteolysis and changes in the rheological properties of Cheddar cheese during ripening were compared in cheeses containing active (recombinant chymosin or porcine pepsin) or inactivated (porcine pepsin) coagulant. Modification to the cheesemaking process extensively denatured porcine pepsin. Very little proteolysis occurred in the modified pepsin cheese although some α s1 -casein was hydrolysed to α s1 -I-casein ( α s1 f24-199) and other peptides during the later stages of ripening, perhaps due to the action of the indigenous milk acid proteinase, cathepsin D, or to some residual pepsin activity in the cheese. In the control cheeses, porcine pepsin was less proteolytic than chymosin, as indicated by polyacrylamide gel electrophoresis or the formation of water-soluble nitrogen, but there was little difference in the concentration of free amino acids between these two cheeses. Some differences were observed between the peptides produced by chymosin or pepsin. As the cheeses matured, cohesiveness, the force required to fracture the cheeses and distance to fracture decreased. Modification to the cheesemaking process affected all the rheological parameters measured after 60 and/or 180 days ripening. The type of coagulant had no effect on hardness or the force required to fracture the cheeses but significant differences were found between the chymosin and pepsin cheeses with regard to cohesiveness and distance to fracture.

Acid-set gel formation is an important first step in the production of dairy products such as yoghurt and cottage cheese. Acid-set gels prepared from pressure treated skim milk have increased rigidity and resistance to breaking. Water binding capacity, protein hydration index and resistance to syneresis (separation of an aqueous phase) were also increased. As stirred-type yoghurt has a larger share of the market it is of greater commercial interest. The effects of a range of pressure-time treatments of the milk on the characteristics of stirred type yoghurts prepared at pH 4.1 have been investigated. The viscosity was improved with increasing pressures in the range 100–600 MPa and with the duration of the treatment up to 1 h.

SummaryAcid-set milk gels containing up to 30 mM added hexametaphosphate, oxalate, citrate, EDTA or orthophosphate were prepared using gluconic acid-δ-lactone and their properties investigated. Increases in the apparent shear modulus were observed in the gels containing hexametaphosphate, oxalate, citrate and EDTA. The effect was maximum for hexametaphosphate-containing gels at ˜ 8 mM. Increases were also observed in the force required to break the gels, and for added hexametaphosphate the effect peaked at ˜ 5 niM. The ability of the gels to resist syneresis was investigated by a cut, tip and drain type test and by a centrifugation method. In both tests, resistance to syneresis was increased by added hexametaphosphate, oxalate, citrate and EDTA. The additions of anions were shown to cause disintegration of the micelles and release caseins to the serum. It is suggested that the improved homogeneity of casein distribution that results allows greater opportunity for protein-protein interaction during gel formation and leads to improved properties. Controlled micelle disintegration would therefore appear to offer promise for the development of improved or novel products.

SummaryThe effects of thermization (a sub-pasteurization heat treatment), followed by cold storage at 7 °C for up to 3 d on the post-gelation behaviour of rennet-induced gels from alternate-day collected milks, pasteurized before testing, were investigated. Observations of the time course of gel rigidity modulus development and of the syneresis of cut curds were fitted to a general exponential relationship to aid interpretation. Milk performance was assessed by measurements of rennet coagulation time at pH 6·5, various gel rigidity and syneresis parameters and losses of protein and Ca to the whey. A small but significant reduction in protein losses to the whey (mean 0·05%) was the only effect resulting from thermization. Cold storage gave rise to significantly weaker gels than those from unstored controls. The gels from cold-stored milks attained 50% of their maximum rigidity modulus in significantly less time and lost significantly more protein (mean 0·11%) to the whey. Samples cold stored for 1, 2 or 3 d were not significantly different from each other and no other parameters were affected. No significant interactions between thermization and cold storage were found.

Changes in the chemical composition of three popular Sudanese varieties of date (Jawa, Bentamoda and Mishrig Wad Laggai) during ripening are described and are related to changes in texture and in the activities of various degradative enzymes in the fruit. Total invertase levels were high throughout maturation of the three varieties. Although the proportion of soluble enzyme increased during ripening solubilisation did not appear to influence the relative amounts of sucrose and reducing sugars in the ripening fruit. Both cellulase and polygalacturonase were absent or at low levels in the green fruit but displayed large increases in activity during ripening. This enhancement was reflected in a reduction in fruit firmness although little change in cellulose or pectin levels was apparent analytically until the final stage of ripening. Despite a considerable rise in pectinesterase activity during ripening no significant trend could be discerned in the degree of esterification of pectin suggesting that the enzyme is of minor importance in softening of these fruit.

SummaryA general theory of gel formation by cross-linking of polymer molecules, orginally developed for the study of rubber vulcanization, is proposed as a model for milk gel formation. The model divides gel formation and development into 4 stages, an enzymic and a non-enzymic stage preceding gelation, then a stage of material transition from the sol to the gelled state and a stage of increase in gel strength due to internal cross-link formations. The 2 post-gelation stages overlap to a variable degree depending upon particular circumstances. Gel formation and properties are interpreted in terms of the number of initial units which combine to form the gel and the number of cross-links eventually formed by each unit. When combined with a kinetic expression for the rate of cross-link formation the time course of gel shear modulus development or of the incorporation of material into the gel may be predicted. The classical asymmetric sigmoid shape of gel shear modulus development curves results from the third stage of material transition into the gel, which in turn depends upon the number of cross-links eventually formed by each initial unit. The modulus of mature gels is found to depend upon the square of material concentration, and gel breaking strength is found to be proportional to the modulus, in agreement with the predictions of the theory.

Differences in the extent of loss of pectic substances from potato (Solanum tuberosum L.) after boiling, steaming and pressure cooking were investigated. Losses were found to be significantly greater (P< 0.001) after boiling than after pressure cooking or steaming. The pectic substance content (g 100 g−1 dry matter) of potatoes (cv. Pentland Crown) was 2.98 for raw peeled tubers, 0.97 after boiling for 35 min, 1.22 after steaming for 35 min and 1.24 after pressure cooking at 103.4 kPa (15 psi) for 15 min. The roles of cooking time, cooking temperature and heat transfer medium are discussed. The effects of calcium sulphate (at levels typical of ‘hard’ and ‘soft’ water supplies) in the cooking water on losses of pectic substances from potato (Solanum tuberosum L.), carrot (Daucus carota L.), and swede (Brassica napus L. var. napobrassica Peterm.) after boiling and pressure cooking were also investigated. Water hardness did not influence losses during pressure cooking but samples boiled in hard water had decreased losses of pectic substances compared to those boiled in soft water. The increase in retention of pectic substances due to hard water was about 8 % for potatoes (cv. Pentland Dell), about 33 % for sliced carrots (cv. Berlicum, approved maintenance Perfecta) and about 23% for diced swedes (cv. Acme). The interaction of water hardness with cooking method was significant for potato (P< 0.05) and swede (P