Effect of sodium chloride and pH on the rennet coagulation and gel firmness

Influence of Sodium and Potassium Chloride on Rennet Coagulation and Curd Firmness in Bovine Milk

One of the salting methods in cheese production implies salting the milk before coagulation used in making Domiati-type cheeses and a variety of autochthonous "Lički Škripavac" cheese. The most used sodium replacer is potassium. This study investigated the influence of different added salt concentrations (1%, 1.5%, and 2%) and NaCl to KCl ratios (100%, 50:50%, 25:75%) on the rennet coagulation and curd firmness in bovine milk. The milk coagulation parameters were determined with a computerized renneting meter, Lactodinamograph. The results showed significant interactions between the salt concentrations and NaCl to KCl ratios (p < 0.0001, α = 0.05) by prolonging the beginning of coagulation (10-20 min) and curd firming rate (1-5 min) by an increase in salt concentration for all treatments. The 50:50 treatment values (RCT, k₂₀, a₃₀, a₆₀, a_max) were closest to the control (without salt) and had the best results among all treatments in the lower (1%) and medium (1.5%) salt concentration (p > 0.0001, α = 0.05) while in the highest salt concentration (2%) the treatment effect was nonsignificant (p > 0.05). These results should help future studies make a lower sodium product appealing to consumers without losing quality.

Effect of mild thermal and pH changes on the sol-gel transition in skim milk

The present work aimed to improve acid and rennet milk gelation properties with mild thermal and pH changes to skim milk, with emphasis on heating temperatures below the denaturation temperature of whey proteins. We hypothesized the heat-induced, pH-dependent micellar changes, namely the shifts in casein and calcium equilibria between the micellar (or colloidal) and serum phases, result in firmer acid and rennet milk gels and reduced gelation time. Homogenized, pasteurized skim milk was adjusted to pH values in the range of 6.4 to 7.3, heated at temperatures in the range of 50 to 80°C, cooled to refrigeration temperature, and restored to native pH (pH 6.7). Then, acid and rennet gels were made by the addition of glucono-δ-lactone and chymosin, respectively. We monitored the storage modulus (G', Pa) during gel formation with small-amplitude oscillatory shear and the gelation time and maximum G' (G'max, Pa) of acid and rennet gels, were measured at 3 and 2 h, respectively. When skim milk was heated at 50°C for 15 min, there was a 58 and 163% increase in the G'max of acid and rennet gels, respectively, as the pH at heating was raised from pH 6.7 to 7.3. Increases in gel strength were greater for skim milk heated at 60°C for 15 min. There was a positive correlation between G'max of acid gels and the heat-induced casein protein exchanges between the micellar and serum phases on heating milk at pH in the range from 6.4 to 7.3 (r = 0.78). We also found positive correlations between the variation in G'max of rennet gels with the heat-induced, pH-dependent migration of casein (r = 0.83) and calcium (r = 0.80) from the micelle into the serum phase, as determined by PAGE and atomic emission spectroscopy. Under these mild heating temperatures (50 and 60°C), rennet coagulation time was significantly reduced from 45 ± 5 to 27 ± 3 min when the pH at heating was raised from pH 6.7 to 7.3. The ability to enhance milk gelation properties with a scalable pretreatment allows for the expression of novel functionality of casein.

The mineral profile affects the coagulation pattern and cheese-making efficiency of bovine milk

Natural variations in milk minerals, their relationships, and their associations with the coagulation process and cheese-making traits present an opportunity for the differentiation of milk destined for high-quality natural products, such as traditional specialties or Protected Designation of Origin (PDO) cheeses. The aim of this study was to quantify the effects of the native contents of Ca, P, Na, K, and Mg on 18 traits describing traditional milk coagulation properties (MCP), curd firming over time (CF_t) equation parameters, cheese yield (CY) measures, and nutrient recoveries in the curd (REC) using models that either included or omitted the simultaneous effects of milk fat and casein contents. The results showed that, by including milk fat and casein and the minerals in the statistical model, we were able to determine the specific effects of each mineral on coagulation and cheese-making efficiency. In general, about two-thirds of the apparent effects of the minerals on MCP and the CF_t equation parameters are actually mediated by their association with milk composition, especially casein content, whereas only one-third of the effects are direct and independent of milk composition. In the case of cheese-making traits, the effects of the minerals were mediated only negligibly by their association with milk composition. High Ca content had a positive effect on the coagulation pattern and cheese-making traits, favoring water retention in the curd in particular. Phosphorus positively affected the cheese-making traits in that it was associated with an increase in CY in terms of curd solids, and in all the nutrient recovery traits. However, a very high P content in milk was associated with lower fat recovery in the curd. The variation in the Na content in milk only mildly affected coagulation, whereas with regard to cheese-making, protein recovery was negatively associated with high concentrations of this mineral. Potassium seemed not to be actively involved in coagulation and the cheese-making process. Magnesium content tended to slow coagulation and reduce CY measures. Further studies on the relationships of minerals with casein and protein fractions could deepen our knowledge of the role of all minerals in coagulation and the cheese-making process.

Addition of buttermilk powder improved the rheological and storage properties of low-fat yogurt

Buttermilk is used widely in dairy products due to its good emulsifying and nutritional properties. In the present study, 0%-4.0% (w/w) buttermilk powder was added to low-fat yogurt with a constant protein content to investigate its efficacy on the rheological and storage properties of low-fat yogurt. Buttermilk increased the final titration acidity. Addition of buttermilk decreased the pH at the gelation point, shortened the gelation time, and thus shortened the fermentation period. Storage modulus G', yield stress, yield strain, and compact cross-links of the microstructure were enhanced greatly with addition of 1.0%-2.0% (w/w) buttermilk powder. In addition, addition of buttermilk decreased whey separation and increased the viscosity and firmness of low-fat yogurt during storage. Our findings suggest that the addition of an appropriate amount of buttermilk altered the rheological characteristics and improved the textural and storage properties of low-fat yogurt.

Understanding the differences in cheese-making properties between reverse osmosis and ultrafiltration concentrates

Concentrating milk by reverse osmosis (RO) has the potential to increase cheese yield but is known to impair cheese-making properties. The main compositional differences between ultrafiltration (UF) and RO concentrates are the high lactose and mineral contents of the latter. The objective of this work was to determine the distinct effects of high lactose and high minerals on the cheese-making properties of RO concentrate, by supplementing UF concentrate with lactose. The soluble colloidal equilibria of concentrates were studied as well as several other properties: rennet gelation behavior, cheese mass balance, composition, and microstructure. Rennet coagulation time was longer and gel firming rate was lower for RO concentrate than for UF concentrate. Lactose was mainly responsible for these differences. Lactose in RO concentrate was also responsible for the 7% increase of moisture-adjusted cheese yield, relative to UF concentrate. Compared with cheese made from UF concentrate, cheese made from RO concentrate showed higher moisture content, which could not be attributed to lactose but to the high mineral concentration. This study showed the potential of using RO instead of UF concentrate to maximize cheese yield. The approach is, however, limited to applications where post-acidification can be controlled, and will require appropriate strategies to reduce the negative effects of high mineral content in RO concentrate.

Effects of indirect indicators of udder health on nutrient recovery and cheese yield traits in goat milk

In dairy goats, very little is known about the effect of the 2 most important indirect indicators of udder health [somatic cell count (SCC) and total bacterial count (TBC)] on milk composition and cheese yield, and no information is available regarding the effects of lactose levels, pH, and NaCl content on the recovery of nutrients in the curd, cheese yield traits, and daily cheese yields. Because large differences exist among dairy species, conclusions from the most studied species (i.e., bovine) cannot be drawn for all types of dairy-producing animals. The aims of this study were to quantify, using milk samples from 560 dairy goats, the contemporary effects of a pool of udder health indirect indicators (lactose level, pH, SCC, TBC, and NaCl content) on the recovery of nutrients in the curd (%REC), cheese yield (%CY), and daily cheese yields (dCY). Cheese-making traits were analyzed using a mixed model, with parity, days in milk (DIM), lactose level, pH, SCC, TBC, and NaCl content as fixed effects, and farm, breed, glass tube, and animal as random effects. Results indicated that high levels of milk lactose were associated with reduced total solids recovery in the curd and lower cheese yields, because of the lower milk fat and protein contents in samples rich in lactose. Higher pH correlated with higher recovery of nutrients in the curd and higher cheese yield traits. These results may be explained by the positive correlation between pH and milk fat, protein, and casein in goat milk. High SCC were associated with higher recovery of solids and energy in the curd but lower recovery of protein. The higher cheese yield obtained from milk with high SCC was due to both increased recovery of lactose in the curd and water retention. Bacterial count proved to be the least important factor affecting cheese-making traits, but it decreased daily cheese yields, suggesting that, even if below the legal limits, TBC should be considered in order to monitor flock management and avoid economic losses. The effect of NaCl content on milk composition was linked with lower recovery of all nutrients in the curd during cheese-making. In addition, high milk NaCl content led to reductions in fresh cheese yield and cheese solids. The indirect indicators of the present study significantly affected the cheese-making process. Such information should be considered, to adjust the milk-to-cheese economic value and the milk payment system.

Effect of pH adjustment on the composition and rennet-gelation properties of milk concentrates made from ultrafiltration and reverse osmosis

The objective of this work was to investigate the effect of pH adjustment (initial pH vs. pH 6.50) on the rennet-gelation properties of concentrates made by ultrafiltration (UF) and reverse osmosis (RO). Rennet-gelation kinetics were followed by dynamic rheology and κ-casein hydrolysis by reverse-phase HPLC. At initial pH, RO concentrates had better rennet-coagulation behavior than UF concentrates and skim milk, whereas adjusting the pH to 6.50 produced the opposite results. The kinetics of κ-casein hydrolysis were similar in skim milk, and both concentrates and were not affected by pH adjustment. Differences in rennet coagulation were then related to the extent of hydrolysis required to trigger casein micelle aggregation. Small pH adjustments (<0.2 pH unit) enabled the use of RO concentrate with similar rennet-gelation behavior to UF concentrate, despite major compositional differences. This study shows that pH adjustment of RO concentrates can be a simple approach to improve their coagulation properties; however, the mechanisms behind these improvements remain to be elucidated.

Kinetic Modeling and Optimization of Milk Coagulation Affected by Several Prevalent Cheesemaking Factors and Essence Addition

The effect of Tarragon essence concentration (0–1,000 ppm), whey protein concentration (0.7–2.8 % milk), calcium chloride (0.15–0.35 g/kg), rennet concentration (0.01–0.03 g/kg), starter content (0.01–0.03 g/kg), and renneting temperature (25–45 °C) on kinetic parameters of milk coagulation was investigated using statistical approach. A central composite design was used to investigate the effect of studied factors on coagulation parameters such as coagulation rate, lag time and inflection time. In general, with a decrease in whey protein concentration or an increase in starter level, rennet content and renneting temperature, coagulation rate increased while the lag time and time at inflection point decreased. Essence addition and calcium chloride did not have significant effects on rennet coagulation kinetic. The optimum conditions of milk coagulation were: rennet content, 0.025 g/kg of milk; whey protein concentration, 0.93 %; and renneting temperature, 44.9 °C.