Changes in Milk Protein Functionality at Low Temperatures and Rennet Concentrations

This study aimed to evaluate the influence of low-concentration rennet on the chemical, rheological characteristics, and protein fractions of skim milk (SM) at 4 ± 1 °C. Skimmed milk (SM) was divided into four lots of 500 mL, and diluted rennet (1:10,000) was added at different levels at 4 ± 1 °C. The treatments included control (no rennet), T1 (0.001 mL/rennet), T2 (0.01 mL rennet), and T3 (0.1 mL rennet) treatments, which were incubated for 24 h. The sampling was performed at 0, 1, 2, 6, 12, and 24 h, and the SM after incubation time was heated to 73 °C/16 s to denature the rennet enzyme. Skim milk samples (SMS) (control and rennet-added samples) were evaluated for proximate composition, capillary gel electrophoresis (CGE), hydrodynamic diameter, zeta potential, and rheology at 0, 1, 2, 6, 12, and 24 h. Foaming ability, foaming stability, water-holding capacity (WHC), oil emulsifying activity (OEA), and emulsion stability (ES) were performed at 0, 12, and 24 h of incubation time. There was a significant (p < 0.05) increase in non-proteins by 0.50% and in non-casein nitrogen by 0.81% as incubation progressed. The results showed that aggregation or curd was not formed during storage time. The CGE data indicated that increasing the rennet concentration had a significant (p < 0.05) effect on decreasing κ-CN, and breakdown increased at higher levels of rennet usage. There was a significant (p < 0.05) increase in the hydrodynamic diameter and a decrease in the zeta potential values in rennet-added samples at the end of the incubation time (24 h). The rheological results showed no changes in the storage modulus (G'), loss modulus (G″), or viscosity values. Increasing the rennet amount and storage time led to a significant (p < 0.05) decrease in the foaming ability and foaming stability and a significant (p < 0.05) increase in the oil emulsifying activity and emulsion stability of rennet-added SMS. This study concluded that milk protein functionality can be changed without aggregating or curd formation, and rennet milk can be processed.

Comparison of milk protein concentrate, micellar casein, and whey protein isolate in loading astaxanthin after the treatment of ultrasound-assisted pH shifting

Milk proteins can be used as encapsulation walls to increase the bioavailability of active compounds because they can bind hydrophobic, hydrophilic, and charged compounds. The objective of this study was to investigate the effects of astaxanthin (ASTA) encapsulation and the functional properties of milk protein and ASTA nanocomposites by an ultrasound-assisted pH-shifting treatment of different milk proteins, including milk protein concentrate (MPC), micellar casein (MCC), and whey protein isolate (WPI). The ultrasound-assisted pH-shifting treatment of milk protein helped to improve the encapsulation rate of ASTA. Therein, MCC showed great improvement of encapsulating ASTA after co-treatment with the raised encapsulated rate of 5.11%, followed by WPI and MPC. Furthermore, the nanocomposites of ASTA with milk protein exhibit improved bioavailability, antioxidant capacity, and storage stability. By comparison, MCC-encapsulated ASTA has the best storage stability, followed by MPC, and WPI-encapsulated ASTA has the least stability over a 28-d storage period. The results of intrinsic fluorescence and surface hydrophobicity showed that milk protein underwent fluorescence quenching after binding to ASTA, which was due to the hydrophobic sites of the protein being occupied by ASTA. In general, the nanocomposites of milk protein and ASTA fabricated by using an ultrasound-assisted pH-shifting treatment have the potential to be better nano-delivery systems for ASTA in functional foods, especially MCC, which showed excellent performance in encapsulation after treatment technique.

Effect of temperature and protein concentration on the protein types within the ultracentrifugation supernatant of liquid micellar casein concentrate

Liquid micellar casein concentrate (MCC) is an ideal milk-based protein ingredient for neutral-pH ready-to-drink beverages. The texture and mouthfeel of liquid MCC-based beverages depend on the beverage protein content, as well as the composition of soluble proteins in the aqueous phase around the casein micelle. The objective of this study was to determine the composition of soluble proteins in the aqueous phase around the casein micelles in skim milk and liquid MCC containing 7.0% and 11.6% protein content. Skim milk was pasteurized and concentrated to 7% protein content by microfiltration and then to 18% protein content by ultrafiltration. The 18% MCC was then serially diluted with distilled water to produce 11.6% and 7.0% protein MCC. Skim milk, 7.0% MCC, and 11.6% MCC representing starting materials with different protein concentrations were each ultracentrifuged at 100,605 × g for 2 h. The ultracentrifugation for each of the starting materials was performed at 3 different temperatures: 4°C, 20°C, and 37°C. The ultracentrifugation supernatants were collected to represent the aqueous phase around the casein micelle in MCC solutions. The supernatants were analyzed by Kjeldahl to determine the crude protein, casein, and casein as a percentage of crude protein content, and by sodium dodecyl sulfate PAGE to determine the composition of the individual proteins. Most of the proteins in MCC supernatant (about 45%) were casein proteolysis products. The remaining proteins in the MCC supernatant consisted of a combination of intact αS-, β-, and κ-caseins (about 40%) and serum proteins (14-18%). Concentrations of αS-casein and β-casein in the supernatant increased with decreasing temperature, especially at higher protein concentrations. Temperature and interaction between temperature and protein explained about 80% of the variation in concentration of supernatant αS- and β-caseins. Concentration of supernatant κ-casein, casein proteolysis products, and serum protein increased with increasing MCC protein concentration, and MCC protein concentration explained most of the variation in supernatant κ-casein, casein proteolysis products, and serum protein concentrations. Predicted MCC apparent viscosity was positively associated with the dissociation of αS- and β-caseins. Optimal beverage viscosity could be achieved by controlling the dissociation of these proteins in MCC.

The effect of calcium removal from skim milk by ion exchange on the properties of the ultrafiltration retentate

New processes are needed to produce concentrated milk feedstocks with tailored calcium content, due to the direct link between calcium concentration and final product texture and functionality. Skim milk treatment with cation exchange resin 1% (w/v) or 2% (w/v) prior to ultrafiltration to a volumetric concentration factor (VCF) of 2.5 or 5 successfully decreased the calcium concentration by 20-30% and produced concentrates with solids content at ∼22-24 g 100 g-1 at a VCF of 5. Calcium reduction partially solubilized the casein micelles, increasing the concentration of soluble protein and individual caseins, leading to decreased turbidity but increased protein hydration and hydrophobicity. Decalcification (2% (w/v) resin treatment) reduced thermal stability, significantly decreasing the denaturation temperature of α-lactalbumin and β-lactoglobulin in the milk by ∼3 °C and ∼1 °C respectively. Filtration was also altered, reducing permeation flux and the gel concentration and increased filtration time. When combined, calcium reduction and filtration altered functional properties including soluble calcium, soluble protein and sedimentable solids, with increased milk protein hydration also contributing to increased viscosity. This study provides a route to produce calcium-reduced milk concentrates with potential for use in retentate-based dairy products with tailored functionality.

Production and storage stability of concentrated micellar casein

Concentrated micellar casein (CMC) is a high-protein ingredient that can be used in process cheese product formulations. The objectives of this study were to develop a process to produce CMC and to evaluate the effect of sodium chloride and sodium citrate on its storage stability. Skim milk was pasteurized at 76°C for 16 s and cooled to ≤4°C. The skim milk was heated to 50°C using a plate heat exchanger and microfiltered with a graded permeability (GP) ceramic microfiltration (MF) membrane system (0.1 μm) in a continuous feed-and-bleed mode (flux of 71.43 L/m² per hour) using a 3× concentration factor (CF) to produce a 3× MF retentate. Subsequently, the retentate of the first stage was diluted 2× with soft water (2 kg of water: 1 kg of retentate) and again MF at 50°C using a 3× CF. The retentate of the second stage was then cooled to 4°C and stored overnight. The following day, the retentate was heated to 63°C and MF in a recirculation mode until the total solids (TS) reached approximately 22% (wt/wt). Subsequently, the MF system temperature was increased to 74°C and MF until the permeate flux was <3 L/m² per hour. The CMC was then divided into 3 aliquots (approximately 10 kg each) at 74°C. The first portion was a control, whereas 1% of sodium chloride was added to the second portion (T1), and 1% of sodium chloride plus 1% of sodium citrate were added to the third portion (T2). The CMC retentates were transferred hot to sterilized vials and stored at 4°C. This trial was repeated 3 times using separate lots of skim milk. The CMC at d 0 (immediately after manufacturing) contained 25.41% TS, 21.65% true protein (TP), 0.09% nonprotein nitrogen (NPN), and 0.55% noncasein nitrogen (NCN). Mean total aerobic bacterial counts (TBC) in control, T1, and T2 at d 0 were 2.6, 2.5, and 2.8 log cfu/mL, respectively. The level of proteolysis (NCN and NPN values) increased with increasing TBC during 60 d of storage at 4°C. This study determined that CMC with >25% TS and >95% casein as percentage of TP can be manufactured using GP MF ceramic membranes and could be stored up to 60 d at 4°C. The effects of the small increase in NCN and NPN, as well as the addition of sodium chloride or sodium citrate in CMC during 60 d of storage on process cheese characteristics, will be evaluated in subsequent studies.

Prevention of low-temperature gelation in milk protein concentrates by calcium-binding salts

The objective of this study was to determine the effect of adding low concentrations of calcium-binding salts on the prevention of low-temperature gelation in milk protein concentrates (MPC). The MPC were created by a combination of ultrafiltration and diafiltration, standardized from 14 to 17% (wt/vol) protein content and mixed with one of 5 calcium-binding salts (sodium citrate, sodium hexametaphosphate, sodium polyphosphate, sodium pyrophosphate, and sodium monophosphate) adjusted to a pH of 6.75. The flow properties, apparent viscosity, and gel strength were determined for MPC containing a wide range of calcium-binding salt concentrations. Low-temperature gelation occurred in MPC with 16.0% and higher protein content. Low-temperature gelation at 16.0% protein content was prevented by the addition of any of the 5 salts tested at low concentrations (0.30 mM or less; sodium citrate, sodium hexametaphosphate, sodium polyphosphate, sodium pyrophosphate or sodium monophosphate), with sodium polyphosphate and sodium monophosphate being the most consistent in preventing low-temperature gels. All MPC samples exhibited shear-thinning behavior (n = 0.52-0.72), which increased (lower n values) as the protein content increased and decreased by addition of salt. At concentrations of salt above 1.00 mM, thermally irreversible gels were observed with relative strength dependent on the salt and protein content.

Viscosity changes and gel formation during storage of liquid micellar casein concentrates

Our objective was to determine the effects of temperature and protein concentration on viscosity increase and gelation of liquid micellar casein concentrate (MCC) at protein concentrations from 6 to 20% during refrigerated storage. Skim milk (~350 kg) was pasteurized (72°C for 16 s) and filtered through a ceramic microfiltration system to make MCC and replicated 3 times. The liquid MCC was immediately concentrated via a plate ultrafiltration system to 18% protein (wt/wt). The MCC was then diluted to various protein concentrations (6-18%, wt/wt). The highest protein concentrations of MCC formed gels almost immediately on cooling to 4°C, whereas lower concentrations of MCC were viscous liquids. Apparent viscosity (AV) determination using a rotational viscometer, gel strength using a compression test, and protein analysis of supernatants from ultracentrifugation by the Kjeldahl method were performed. The AV data were collected from MCC (6.54, 8.75, 10.66, and 13.21% protein) at 4, 20, and 37°C, and compression force test data were collected for MCC (15.6, 17.9, and 20.3% protein) over a period of 2-wk storage at 4°C. The maximum compressive load was compared at each time point to determine the changes in gel strength over time. Supernatants from MCC of 6.96 and 11.61% protein were collected after ultracentrifugation (100,605 × g for 2 h at 4, 20, and 37°C) and the nitrogen distributions (total, noncasein, casein, and nonprotein nitrogen) were determined. The protein and casein as a percent of true protein concentration in the liquid phase around casein micelles in MCC increased with increasing total MCC protein concentration and with decreasing temperature. Casein as a percent of true protein at 4°C in the liquid phase around casein micelles increased from about 16% for skim milk to about 78% for an MCC containing 11.6% protein. This increase was larger than expected, and this may promote increased viscosity. The AV of MCC solutions in the range of 6 to 13% casein increased with increasing casein concentration and decreasing temperature. We observed a temperature by protein concentration interaction, with AV increasing more rapidly with decreasing temperature at high protein concentration. The increase in AV with decreasing temperature may be due to the increase in protein concentration in the aqueous phase around the casein micelles. The MCC containing about 16 and 18% casein gelled upon cooling to form a gel that was likely a particle jamming gel. These gels increased in strength over 10 d of storage at 4°C, likely due either to the migration of casein (CN) out of the micelles and interaction of the nonmicellar CN to form a network that further strengthened the random loose jamming gel structure or to a gradual increase in voluminosity of the casein micelles during storage at 4°C.

The relationship between ultra-small-angle X-ray scattering and viscosity measurements of casein micelles in skim milk concentrates

Skim milk concentrates have important applications in the dairy industry, often as intermediate ingredients. Concentration of skim milk by reverse osmosis membrane filtration induces water removal, which reduces the free volume between the colloidal components, in particular the casein micelles. Thermal treatment before or after concentration impacts the morphology of casein micelles. These changes affect the flow behavior and viscosity, but the consequences for supermicellar structure have not been elucidated. In the present study, skim milk concentrates with different total solid contents from 8.7% (control) up to 22.8% (w/w), prepared by reverse osmosis membrane filtration of non-heated and pasteurized skim milk, were heat treated at 75 °C for 18 s, and compared with non-heated concentrates. The structure of the concentrates was studied using Ultra Small Angle X-ray Scattering (USAXS), and the viscosity of concentrates was measured. The USAXS intensity I(q) was fitted at small and intermediate q-regions (0.0005 < q < 0.003 Å^-1 and 0.0035 < q < 0.03 Å^-1, respectively) with a power law. The value of the power law exponent was used to assess the heat- and concentration-induced aggregation of the milk solids and correlate it with the apparent viscosity. The results showed that increased viscosity of skim milk concentrates, due to water removal and heat-load, can be explained by increased aggregation of the casein micelles into elongated aggregates and increased smoothening of the casein micelle surface.

Progress in micellar casein concentrate: Production and applications

Micellar casein concentrate (MCC) is a novel ingredient with high casein content. Over the past decade, MCC has emerged as one of the most promising dairy ingredients having applications in beverages, yogurt, cheese, and process cheese products. Industrially, MCC is manufactured by microfiltration (MF) of skim milk and is commercially available as a liquid, concentrated, or dried containing ≥9, ≥22, and ≥80% total protein, respectively. As an ingredient, MCC not only imparts a bland flavor but also offers unique functionalities such as foaming, emulsifying, wetting, dispersibility, heat stability, and water-binding ability. The high protein content of MCC represents a valuable source of fortification in a number of food formulations. For the last 20 years, MCC is utilized in many applications due to the unique physiochemical and functional characteristics. It also has promising applications to eliminate the cost of drying by producing concentrated MCC. This work aims at providing a succinct overview of the historical progress of the MCC, a review on the manufacturing methods, a discussion of MCC properties, varieties, and applications.

Updating insights into the rehydration of dairy-based powder and the achievement of functionality

Dairy-based powder had considerable development in the recent decade. Meanwhile, the increased variety of dairy-based powder led to the complex difficulties of rehydrating dairy-based powder, which could be the poor wetting or dissolution of powder. To solve these various difficulties, previous studies investigated the rehydration of powder by mechanical and chemical methods on facilitating rehydration, while strategies were designed to improve the rate-limiting rehydration steps of different powder. In this review, special emphasis is paid to the surface and structure of the dairy-based powder, which was accountable for understanding rehydration and the rate-limiting step. Besides, the advantage and disadvantage of methods employed in rehydration were described and compared. The achievement of the powder functionality was finally discussed and correlated with the rehydration methods. It was found that the surface and structure of dairy-based powder were decided by the components and production of powder. Post-drying methods like agglomeration and coating can tailor the surface and structure of powder afterwards to obtain better rehydration. The merit of the mechanical method is that it can be applied to rehydrate dairy-based powder without any addition of chemicals. Regarding chemical methods, calcium chelation is proved to be an effective chemical in rehydration casein-based powder.