Short communication: Effect of pH on the heat stability of reconstituted reduced calcium milk protein concentrate dispersions.

This study aimed to investigate the heat stability of dispersions from reconstituted reduced-calcium milk protein concentrate (RCMPC) with 80% protein or more. The tested RCMPC powders were produced from skim milk subjected to CO2 treatment before and during the process of ultrafiltration. The CO2 injection was controlled to obtain 0 (control, no CO2 injection), 20, 30, and 40% reduction in calcium levels in the RCMPC powders. The RCMPC powders were reconstituted to 10% (wt/wt) protein in deionized water. These dispersions were tested for heat stability in a rocking oil bath at 140°C at unadjusted, 6.5, 6.7, 6.9, and 7.1 pH. Calcium ion activity (CIA) and ionic strength measurements were carried out using a Ca ion-selective electrode and conductivity meter. Unadjusted pH of the dispersions varied from 6.8 in control to 5.96 in 40% RCMPC dispersions. The CIA of unadjusted dispersions ranged from 1.31 mM in control to 2.83 mM in 40% RCMPC. Heat stability, expressed as heat coagulation time (HCT) of unadjusted dispersions decreased as the level of Ca removal in powders increased (from 13.81 min in control to 0.46 min in 40% RCMPC) and was negatively correlated with the CIA of the dispersions. For control RCMPC dispersions, the minimum and maximum heat stability were observed at dispersion pH of 6.5 and 6.9, respectively, followed by a decrease at pH 7.1 (CIA was the lowest). Dispersions from 40% RCMPC and pH 7.1 had the maximum HCT of 30.94 min among all RCMPC dispersions at all pH values. From this study, it can be concluded that improved heat stability in high protein formulation beverages subjected to UHT processing could be achieved through calcium reduction in milk protein concentrates using CO2 injection.

Solubilization of rehydrated frozen highly concentrated micellar casein for use in liquid food applications.

Highly concentrated micellar casein concentrate (HC-MCC), a potential ingredient of protein-fortified food, is a gel at cold temperature. It contains ~17 to 21% casein, with most serum proteins and lactose removed by microfiltration and diafiltration, and it is then further concentrated using vacuum evaporation. The HC-MCC can be stored frozen, and our objective was to determine the conditions needed to obtain complete solubility of thawed HC-MCC in water and to understand its gelation upon cooling. Dispersibility (ability to pass through a 250-µm mesh sieve), suspendability (percentage of protein not sedimented at 80 × g within 5min), and solubility (percentage of protein not sedimented at 20,000 × g within 5min) were measured at 4, 12, or 20°C after various mixing conditions. Gelation upon cooling from 50 to 5°C was monitored based on storage (G') and loss (G'') moduli. The gelled HC-MCC was also examined by transmission electron microscopy. Thawed HC-MCC was added to water to reach a protein concentration of 3% and mixed using high shear (7,500rpm) for 1min or low shear (800rpm) for 30min at 4, 12, 20, or 50°C and at pH 6.4 to 7.2. The HC-MCC completely dispersed at 50°C, or at ≤20°C followed by overnight storage at 4°C. Suspendability at 50°C was ~90% whereas mixing at ≤20°C followed by overnight storage resulted in only ~57% suspendability. Solubility followed a similar trend with ~83% at 50°C and only ~29% at ≤20°C. Mixing HC-MCC with 60mM trisodium citrate increased dispersibility to 99% and suspendability and solubility to 81% at 20°C. Cold-gelling temperature, defined as the temperature at which G'=G'' when cooling from 50 to 5°C, was positively correlated with protein level in HC-MCC. Gelation occurred at 38, 28, and 7°C with 23, 20, and 17% of protein, respectively. Gelation was reversible upon heating, although after a second cooling cycle the HC-MCC gel had lower G'. In micrographs of gelled HC-MCC, the casein micelles were observed to be within the normal size range but packed very closely together, with only ~20 to 50 nm of space between them. We proposed that cold-gelation of HC-MCC occurs when the kinetic energy of the casein micelles is sufficiently reduced to inhibit their mobility in relation to adjacent casein micelles. Understanding solubilization of rehydrated frozen HC-MCC and its rheological properties can help in designing process systems for using HC-MCC as a potential ingredient in liquid food.

Manufacture of modified milk protein concentrate utilizing injection of carbon dioxide.

Dried milk protein concentrate is produced from skim milk using a combination of processes such as ultrafiltration (UF), evaporation or nanofiltration, and spray drying. It is well established that dried milk protein concentrate (MPC) that contains 80% (MPC80) and greater protein content (relative to dry matter) can lose solubility during storage as a result of protein-protein interactions and formation of insoluble complexes. Previous studies have shown that partial replacement of calcium with sodium improves MPC80 functionality and prevents the loss in solubility during storage. Those studies have used pH adjustment with the addition of acids, addition of monovalent salts, or ion exchange treatment of UF retentate. The objective of this study was to use carbon dioxide to produce MPC80 with improved functionality. In this study, reduced-calcium MPC80 (RCMPC) was produced from skim milk that was subjected to injection of 2,200 ppm of CO2 before UF, along with additional CO2 injection at a flow rate of 1.5 to 2 L/min during UF. A control MPC80 (CtrlMPC) was also produced from the same lot of skim milk without injection of CO2. The above processes were replicated 3 times, using different lots of skim milk for each replication. All the UF retentates were spray dried using a pilot-scale dryer. Skim milk and UF retentates were tested for ζ-potential (net negative charge), particle size, and viscosity. All the MPC were stored at room (22±1°C) and elevated (40°C) temperatures for 6 mo. Solubility was measured by dissolving the dried MPC in water at 22°C and at 10°C (cold solubility). Injection of CO2 and the resultant solubilization of calcium phosphate had a significant effect on UF performance, resulting in 10 and 20% loss in initial and average flux, respectively. Processing of skim milk with injection of CO2 also resulted in higher irreversible fouling resistances. Compared with control, the reduced-calcium MPC had 28 and 34% less ash and calcium, respectively. Injection of CO2 resulted in a significant decrease in ζ-potential and a significant increase in the size of the casein micelle. Moreover, RCMPC had a significantly higher solubility after storage at room temperature and at elevated temperature. This study demonstrates that MPC80 with a reduced calcium and mineral content can be produced with injection of CO2 before and during UF of skim milk.

Effect of xylitol on the functional properties of low-fat process cheese.

Process cheese (PC) is a dairy food prepared by blending natural cheese, salt, emulsifying salts, and other dairy and nondairy ingredients, and heating with continuous agitation to produce a homogeneous product. Fat is a critical component of PC and plays an important role in its functional characteristics. The health concerns associated with fat consumption have led to an increase in the demand for low-fat dairy products. Reducing the fat content of PC results in poor functional properties such as increased hardness and reduced melt characteristics. The objective of the current study was to evaluate the effect of xylitol on the functional properties of low-fat PC. Three different low-fat PC formulations were prepared with 0% (control), 2%, and 4% xylitol. All 3 PC formulations were formulated to contain 5% fat, and each treatment was manufactured in triplicate. Rheological characteristics including elastic modulus, viscous modulus, and temperature at Tanδ = 1 (melt temperature) were determined using dynamic stress rheometry (DSR). The DSR was carried out at a frequency of 1.5 Hz and stress levels of 400 Pa, using a temperature sweep from 20 to 90 °C. The hardness of the samples was determined by using texture profile analysis (TPA). Compositional analysis indicated that all treatments had similar fat, protein, and moisture contents. Elastic and viscous moduli results obtained with DSR showed a significant difference between 0% xylitol (control) and xylitol-containing treatments in the temperature range of 30 to 80 °C. The melt temperature was not significantly different between the 3 treatments. However, TPA demonstrated that the addition of xylitol significantly decreased the hardness of low-fat PC. Based on TPA and DSR data obtained in this study, we determined that xylitol addition improved the functional properties of low-fat PC.