Starch addition in renneted milk gels: partitioning between curd and whey and effect on curd syneresis and gel microstructure.

Milk gels were made by renneting and acidifying skim milk containing 5 different starches, and then compressed by centrifugation to express whey and simulate curd syneresis during the manufacture of low-fat cheese. A series of 17 starches were examined, with 5 starches being selected for in-depth analysis: a modified waxy corn starch (WC), a waxy rice starch (WR), an instant tapioca starch (IT), a modified tapioca starch (MT), and dextrin (DX). Milks containing WC, WR, and DX were given a 72°C heat treatment, whereas those containing IT and MT had a 30-min treatment at 66°C that matched their optimum gelatinization treatments. Curd yields were calculated by weight, estimated starch content in whey was measured gravimetrically by alcohol precipitation, and starch retention in curd was calculated. Curd yields were 13.1% for the control milk (no added starch) and 18.4, 20.7, 21.5, 23.5, and 13.2% for the gels containing starches WC, WR, IT, MT, and DX, respectively. Estimated starch retentions in the curd were, respectively, 71, 90, 90, 21, and 1%. Laser scanning confocal microscopy was used to determine the location of the starches in the curd and their interaction with the protein matrix. Waxy corn, WR, and IT starches have potential to improve texture of low-fat cheese because they had high retention in the curd and they generated interruptions in the protein matrix network that may have helped limit extensive protein-protein interactions. Modified tapioca starch interfered with formation of the protein structure of the curd and produced a soft noncohesive gel, even though most (79%) of the MT starch was lost in the whey. Few distinct starch particles were present in the MT curd network. Dextrin was not retained in the curd and did not disrupt the protein network, making it unsuitable for use in low-fat cheese.

Effect of insoluble calcium concentration on rennet coagulation properties of milk.

Rennet-induced gels were made from milk acidified to various pH values or milk at pH 6.0 that had added EDTA. The objective was to examine the effect of removing insoluble Ca (INS Ca) from casein micelles (CM) on rennet gelation properties. For the pH trial, diluted lactic acid was added to reconstituted skim milk to decrease the pH to 6.4, 6.0, 5.8, 5.6, and 5.4. For the EDTA trial, EDTA was slowly added (0, 2, 4, and 6 mM) to reconstituted skim milk, and the final pH values were subsequently adjusted to pH 6.0. Dynamic low amplitude oscillatory rheology was used to monitor gel development. The Ca content of CM and rennet wheys made from these milks was measured using inductively coupled plasma spectroscopy. The INS Ca content of milk was altered by the acidification pH values or level of EDTA added. In all samples, the storage modulus (G' ) exhibited a maximum (GM), with a decrease in G' during longer aging times. Gels made at pH 6.4 had higher GM compared with gels made at pH 6.7 probably due to the reduction in electrostatic repulsion, whereas the INS Ca content only slightly decreased. The highest GM value of gels was observed at pH 6.4 and the GM value decreased with decreasing pH from 6.4 to 5.4. This was due to an excessive loss of INS Ca from CM. There was a decrease in GM with the increase in the concentration of added EDTA, which was probably due to the loss of colloidal calcium phosphate, which weakens the integrity of CM. Loss tangent (LT) values at GM increased with a reduction in milk pH and the addition of EDTA to milk. Rennet gels at the point of the GM were subjected to constant low shearing to fracture the gels. With a reduction in INS Ca content, the yield stress decreased, whereas LT values increased indicating a weaker, more flexible casein network. Microstructure of rennet-induced gels near the GM point and 2 to 10 h after this point was studied using fluorescence microscopy. At GM, gels made from milk acidified to pH 6.4 exhibited more branched, interconnected networks, whereas strands and clusters became larger with a reduction in milk pH to 5.4. Gels made from milk with EDTA added had more finely dispersed protein clusters compared with gels made from milk with no EDTA added. These microscopic observations supported the effect of loss of INS Ca on GM and LT. There was a decrease in apparent interconnectivity between strands in gel microstructure during aging, which agreed with the decrease in G' after GM. It can be concluded that low levels of solubilization of INS Ca and the decrease in milk pH resulted in an increase in GM. With greater losses of INS Ca there was excessive reduction in cross-linking within CM, which resulted in weaker, more flexible rennet gels. This complex behavior cannot be explained by adhesive hard sphere models for CM or rennet gels made from these CM.

Influence of transglutaminase treatment on some physico-chemical properties of milk.

Transglutaminase (TGase) is an enzyme that cross-links many proteins, including milk proteins. In this study, the effects of TGase on some physico-chemical properties of milk were studied. TGase-treated milk was not coagulable by rennet, which was due to failure of the primary (enzymic) stage of rennet action rather than the non-enzymic secondary phase. Dissociation of TGase-treated casein micelles by urea or sodium citrate or removal of colloidal calcium phosphate by acidification and dialysis was reduced, presumably due to the formation of cross-links between the caseins. Casein micelles in TGase-treated milks were also resistant to high pressure treatment and to hydrolysis by plasmin. Results of the present study show that milk proteins are fundamentally modified by the action of TGase, which may have applications in the manufacture of functional proteins for use as novel food ingredients.