Gelation of proteins is important to give food products, such as yoghurt-desserts, desirable textures. On an industrial scale, gelation is often induced by heating the final product. This has three main disadvantages. Firstly, sub-optimal uses of protein-ingredients since not all proteins (usually between 50 and 80 per cent) form aggregates that contribute to the gel strength. Secondly, the final texture is hard to predict and control and finally, delicate flavours are heated and sometimes destroyed. The cold gelation method that is based on separating the protein aggregation (heat) and gelation steps that are intertwined in traditional heat-set gels.
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Aggregation is induced in a pre-heating step that makes it possible to fully denature the protein ingredient before gelation sets in. This ensures that all of the protein - more than 95 per cent - contributes to the gel. Gradual acidification of the solution of aggregates to form a gel at ambient temperature comprises the second step of the cold gelation method. A large part of the research has focused on the relations between the properties of the protein-aggregates and the mechanical properties of the gels. One important conclusion, , that allow formation of disulphide bonds during the second acidification step, is of crucial importance for the final gel hardness. Finally, heat-sensitive flavour compounds can now be added to a solution that does not require additional heating to form product in which the final gel properties can be precisely controlled. Food manufacturers can use the cold gelation method for dairy products such as yoghurt desserts, or processed fish such as surimi, and for the encapsulation of probiotics.
PH-Induced cold gelatin of whey proteins is a two-step process. After protein aggregates have been prepared by heat treatment, gelatin is established at ambient temperature by gradually lowering the pH. To demonstrate the importance of electrostatic interactions between aggregates during this latter process; -lacto globulin aggregates with a decreased is-electric point were prepared via succinylation of primary amino groups. The kinetics of pH-induced gelatin was affected significantly, with the pH gelatin curves shifting to lower pH after succinylation. With increasing modification, the pH of gelatin decreased to about 2.5. In contrast, unmodified aggregates gel around pH 5. Increasing the is-electric point of -lacto globulin via methylation of carboxylic acid groups resulted in gelatin at more alkaline pH values. Comparable results were obtained with whey protein isolate. At low pH disulfide cross-links between modified aggregates were not formed after gelatin and the gels displayed both syneresis and spontaneous gel fracture, in this way resembling the morphology of previously characterized thiol-blocked whey protein isolate gels Our results clearly demonstrate the importance of the net electric charge of the aggregates during pH-induced gelatin. In addition, the absence of disulfide bond formation between aggregates during low-pH gelatin was demonstrated with the modified aggregates. Keywords: -lacto globulin; whey protein isolate; chemical modification; aggregation/gelatin; electrostatic interactions; disulfide bonds
The mechanism of pectin gelatin depends on the degree of methoxylation. High methoxyl pectin gels due to hydrophobic interactions and hydrogen bonding between pectin molecules. Low methoxyl pectin forms gels in the presence of di- and polyvalent cations which cross link and neutralise the negative charges of the pectin molecule. Monovalent cations normally do not lead to gel formation with high methoxyl pectin solutions free of divalent cations, especially Ca. The present study found that alkali (NaOH or KOH) added to high methoxyl pectin leads to gel formation in a concentration-depended manner. It was also found that monovalent cations (Na and K) induce gelation of low methoxyl pectin and the time required for gel formation (setting time) depends on the cation concentration. The results indicate that a combined charge neutralisation and ionic strength effect is responsible for the monovalent cation-induced gelation of pectin.
This investigated the effect of a number of commercial-grade ingredients, either polysaccharides (wheat starch, corn starch and dextrin's) or with high protein content (gluten and dried egg), on the viscous and dynamic viscoelastic behaviour of a batter containing methylcellulose (MC) employed in a process that does not require a pre-frying step. Replacing part of the wheat flour present in a reference batter formula with either gluten or dried egg yielded an increase in the actual concentration of protein material, which resulted in batters that exhibited more marked shear-thinning behaviour at 15 °C. Conversely, the consistency of batters dropped when wheat flour was partially replaced with either wheat starch or modified corn starch, probably due to the ‘dilution’ effect of the polysaccharidic material on the wheat flour proteins which, in practice, form the network responsible for viscosity development in the batters studied at 15 °C.
A similar approach is used to explain the dynamic viscoelastic results at 15 °C. The storage and loss module both increased upon adding protein ingredients but decreased when some of the polysaccharide ingredients partially replaced the wheat flour. However, at 60 °C the opposite tendency was observed, since both viscoelastic module decreased in protein-enriched batters as a consequence of the disruption of starch gelatinization. On the other hand, both storage and loss module were observed to be higher in batters where wheat flour was partially replaced by native wheat starch or corn starch, which were already beginning to gelatinize at 60 °C.
The characteristic gelling pattern of MC, which is the key component to avoid the pre-frying step, was practically unaffected by the ingredients studied.
Finally, the thermo reversibility of the gelation process after a sudden increase in temperature up to 45, 60 or 80 °C, was progressively less evident as the final temperature increased. This is attributed to the fact that gelatinized starch reinforced the batter structure to a greater degree as the temperature was raised.
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