Rennet coagulation properties of UHT-treated phosphocasein dispersions as a function of casein and NaCl concentrations

Abstract

The renneting properties of whey protein-free, UHT-heated (140 ºC/10 s) casein dispersions were investigated as a function of casein and NaCl concetration. It was found that the rennet coagulation time and gel firmes can be optimised when the whey protein-free casein concentration is increased, while the added NaCl concentration is kept low. The strongest gel firmness occurs at 0.05 and 0.08 M NaCl addition anda t a micellar casein concentration between 6.0 and 6.9 g/100 mL. Weak rennet gels were formed at 3.0–3.6 g/100 mL casein at all NaCl concentrations tested.   

Introduction 

Calcium chloride (CaCl2) is commonly added to the cheese milk during cheese production to promote aggregation of casein micelles (thereby decreasing the coagulation time). The addition of 0.001 M CaCl2 has been reported to increase the yield of Swiss-type cheese (Wolfschoon-Pombo, 1997). The aggregation of casein micelles occurs at a lower level of κ-casein hydrolysis with increasing added calcium concentration (Van Hooydonk et al., 1986). According to Yamauchi and Yoneda (1977) and Huppertz and Fox (2006), the rennet coagulation of the mil kis affected not only by the addition of CaCl2, but also by the addition of phosphate, an adjustment of pH or ionic strength and temperature. The decreased ionic strenght of the milk serum due to heating at high temperatures leads to insuficiente avaiable calcium content for the network formation of a rennet gel (Wolfschoon-Pombo, 1977). Poor rennetability of heated milk was related to an impaired aggregation of casein partially covered with denatured whey proteins (Van Hooydonk et al., 1987). Changes in Zeta potential, hydrophobicity and casein supplementation to skim milk (Philippe et al., 2003).

Besides calcium chloride, sodium chloride might be also added to the cheese milk. The addition of NaCl reduces the pH of milk (Grufferty; Fox, 1985; Van Hooydonk et al., 1986; Gaucheron et al., 2000) and increases micellar hydration (Creamer, 1985; Grufferty; Fox, 1985; Van Hooydonk et al., 1986). In industrial practice, NaCl is an essential standard aditive in many cheese varieties. Salting can be performed either with dry salto r by immersion of the cheese in brine. The amount of salt added depends on the type of cheese or dry matter of the curd. However, NaCl is also added into the cheese milk prior to renneting to produce Dominati cheese, also known as White salted cheese, in Egypt, Sudan and Other Middle Eastern countries (Gouda et al., 1985; Le Ray et al., 1998).

NaCl-induced changes in physicochemical properties of milk are widely reported and mosto f investigations refer to the renneting properties of milk (including the interaction of casein micelles and whey proteins), of phosphocaseinate solutions (Green, 1982; Grufferty; Fox, 1985; Patel, Reuter, 1986; Famelart et al., 1999) oro f UF concentrated milk. It can be assumed that a threshold exists for destabilising the casein micelles at higher Nacl concentrations and also that ionic interactions of NaCl with the surface of globular whey proteins are responsible for casin-whey protein interactions. However, little or no informastion is avaiable about the renneting properties of whey protein-free casein solutions in combination with the effect of varying ionic strengths as a result of NaCl addition.

According to many studies NaCl addition in milk cases a solubilisation of the calcium and phosphate from the colloidal phase into the serum phase (Grufferty; Fox, 1985; Van Hooydonk et al., 1986; Casiraghi et al., 1989; Zoon et al., 1989; Gatti; Pires, 1995; Aoki et al., 1999; Gaucheron et al., 2000). The reported research work have shown that the changes in the soluble calcium and inorganic phosphate (Pi) concentration caused by the addition of NaCl to milk are dependente on the NaCl concentration, albeit some diferences in soluble Pi concentration were reported by Gaucheron et al. (2000) and Huppertz and Fox (2006). Recently, Zhao and Corredig (2015) reported that the addition of NaCl (0–0.5M), besides affecting the mineral equilibrium, decreased the pH and the Zeta potencial and increased the viscosity of milk and of concentrated milk proteins.

Several authors have reported that differences (reduction or increase) in the rennet coagulation times of milk and UF concentrate are also dependent on the added NaCl concentration (Hamdy; Edelsten, 1970; Alais; Lagrance, 1972; Qvist, 1979; Grufferty; Fox, 1985; Famelart et al., 1999). Abd El-Salam et al. (1993) reported that the rennet coagulation time of cows’ and buffalos’ milk increases with increasing the amount of NaCl added up to 0.171 M, and decrease slightly with further increase in added salt.

The addition of NaCl to milk was reported to decrease the rate of the enzymatic reaction (Van Hooydonk et al., 1986) and also the coagulation of renneted micelles (Dalgleish, 1983). Zoon et al. (1989) found that above 0.1 M added NaCl the onset of gelation was retarded and the modulus increased at a slower rate compared to lower concentrations of added NaCl. In the same study, an addition of NaCl above 0.1 M decreased the curd tension greatly. Ramet et al. (1983), Gouda et al. (1985) and Awad (2007) found in their investigations that the rennet gel formation and gel firmness decreased When the concentration of the NaCl in milk was increased. In contrast, Jen and Ashworth (1970) reported that an addition of 0.1 M NaCl increased the curd tension, but Grufferty and Fox (1985) observed that the curd tension after 2.5 times the clotting time was not affected by added NaCl up to 0.5 M.            

As results from the state of knowledge, some of the studies report an effect of NaCl additionon the enzymatic phase of rennet coagulation time, while the aggregation phase appears to be unaffected (Van Hooydonk et al., 1986). However, it can be concluded that controversial statements about the effects os added NaCl concentration to milk on the rennet coagulation time, rennet gel firmness and properties of curd exist. Some of the diversity of the results regarding whether or not NaCl affects the renneting properties may be related to various states of the casein and whey proteins, for example as a consequence of heating such as whey protein denaturation and aggregation on casein micelle surfaces. Whether or not these results also apply for whey protein-depleted mil kis an open question.

Hence, it is the purpose of this study to assess the impact of NaCl concentration exclusively on whey protein-free casein micelles. In absence of whey proteins, casein micelles are known to be relatively heat stable and an UHT process can be applied without fundamentally affecting the renneting properties of the casein micelles (Bulca; Kulozik, 2004; Bulca et al., 2004; Bulca, 2007; Hinrichs et al., 2007; Wang et al., 2007). In the presente study, the renneting conditions and resulting cheesemaking properties of whey protein-depletedmilk concentrates, that is phosphocasein micelle dispersions in milk serum obtained via membrane technology, were investigated as a function of NaCl (0.05–0.2 M) and casein (2.5–6.5 g/100 mL) concentration. Thus, whey protein/casein interactions were avoided. Furthermore, the coagulation experiments of the UHT-treated dispersions were carried out in the absence of added calcium chloride.

Conclusion

These news results for the coagulation time and gel firmness of whey protein-depleted phosphocasein dispersions can be compared to skimmed milk, which have been often reviewed in the literature. As shown in many studies, supplementation of milk with sodium chloride has profound effects on physical-chemical characteristics of milk systems; ammong others, the coagulation time and the gel firmness are affected, dependingon both the concentration of casein and salt added. We have found that this is also valid in the system investigated in our work, Where a nonlinear dependency was found (see, e.g., Figure 3 for NaCl x casein concetration and gel firmness). The question of how the composition of a milk protein system influences the threshold above which an incresead NaCl concentration leads to a lower gel firmness if the protein concentrations is kept constant remains to be answered. The reported changes in the aqueous phase (e.g. mineral equilibria, nonsedimentable caseins, pH, ionic strength, viscosity) by many authors and also in the caseins per se (e.g. voluminosity and Zeta potential) must be part of a more comprehensive study using our milk system, keeping in mind that in the presente study calcium chloride was not added during the coagulation experiments.

Furthermore, a comparative experimente using the UHT-heated whey protein-free casein dispersion system is needed to elucidate the effect of sodium chloride vs calcium chloride on the rennet gel formation or shortest RCT of heated casein dispersions can be achieved if the casein concentrattion and therefore the colloidal calcium concentration are increased, but only if the NaCl concentration is kept under a certain very low value. This result suggests that a low ionic strength favours the renneting properties of UHT-heated whey protein-free casein dispersion systems. UHT treatment of milk leads to a decrease in ionic strength of milk especially due to the precipitation of calcium with the phosphate as insoluble mineral complexes. A low ionic into the aqueous phase. Therefore, a high casein concentration in combination with a low or no NaCl addition was found optimal for the rennet gel formationof UHT whey protein-depleted phosphocasein system.

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 [119] BULCA, S.; WOLFSCHOON-POMBO, A.; KULOZIK, U. Rennet coagulation properties of UHT-treated phosphocasein dispersions as a function of casein and NaCl concentrations. International Journal of Dairy Technology, v. 69, p. 1-9, 2016.