Dispersibility, Suspension Ability, Solubility, and Gelation Properties of Rehydrated Frozen Highly Concentrated Micellar Casein

Abstract Text: Highly concentrated micellar casein (HCMC), a potential ingredient of protein fortified food, is a gel at cold temperature containing ~20% casein with whey proteins and lactose removed by microfiltration (and diafiltration) of skim milk. Dispersibility, suspension ability, solubility, and gelation properties of rehydrated frozen HCMC were characterized. Thawed HCMC was reconstituted to 3% protein with water or trisodium citrate buffer and adjusted to pH 7 then mixed using 1 min of high shear (7500 rpm) at 4, 12, 20 and 50°C, or 30 min of low shear (800 rpm) at 4, 20, and 50°C, followed by 18 h storage at 4°C. Dispersibility was defined as percentage by dry weight of HCMC that did not pass through a 250-µm sieve. Material that was 100% dispersible was centrifuged for 5 min and tested for suspension ability (using 80 g) and solubility (using 20,000 g). Protein in the supernatant was measured and suspension ability and solubility calculated as percent protein that was not sedimented during centrifugation. The HCMC was also rehydrated with cream to obtain casein-to-fat ratio of 0.8 with casein levels from 9.3% to 12.5%, adjusted to pH 7.0, then stirred at 800 rpm for 30 min at 50°C, Gel modulus (G’, G’’) was measured at 50°C followed by cooling at 1°C/min to 5°C. At temperatures ≤20°C, HCMC was only partially dispersible in water (e.g., 60% dispersibility using high shear at 4°C), while at 50°C it was 100% dispersible. Mixing at ≤20°C followed by overnight hydration at 4°C also produced 100% dispersibility. Suspension ability at 50°C was ~90%, while mixing HCMC at ≤20°C followed by overnight hydration yielded only 50% to 60% suspension ability. Solubility followed a similar trend with HCMC having ~85% solubility at 50°C and only ~30% solubility at ≤20°C. Mixing HCMC in 60 mM trisodium citrate increased dispersibility, suspension ability and solubility of HCMC at 4°C to 97%, 75% and 75% respectively. Gelation temperature of the HCMC-cream mixture, defined as temperature at which G’=G’’, was positively correlated (R²=0.71) with casein level. Gelation occurred at ~35, ~25 and ~15°C with 12.5%, 10.5% and 9.5% casein, respectively. This process was reversible with a hysteresis effect observed depending on whether the mixture was being heated or cooled. With a 10.5% casein HCMC-cream mixture exhibiting G’ at 15°C of 30 Pa during cooling, and 160 Pa during warming.

Keywords: Casein Microfiltration Gelation