title: Solubilization of rehydrated frozen highly concentrated micellar casein for use in liquid food applications.
abstract: 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.
authors.affiliation: Western Dairy Center, Utah State University, Logan 84322.
authors.forename: Y
authors.initials: Y
authors.lastname: Lu
authors.affiliation: Western Dairy Center, Utah State University, Logan 84322. Electronic address: donald.mcmahon@usu.edu.
authors.forename: D J
authors.initials: DJ
authors.lastname: McMahon
authors.affiliation: Midwest Dairy Foods Research Center, South Dakota State University, Brookings 57007.
authors.forename: L E
authors.initials: LE
authors.lastname: Metzger
authors.affiliation: Midwest Dairy Foods Research Center, South Dakota State University, Brookings 57007.
authors.forename: A
authors.initials: A
authors.lastname: Kommineni
authors.affiliation: Western Dairy Center, Utah State University, Logan 84322.
authors.forename: A H
authors.initials: AH
authors.lastname: Vollmer
mesh_terms: D000818:Animals; D002364:Caseins / Q000737:chemistry*; D003080:Cold Temperature; D004041:Dietary Fats / Q000032:analysis; D005527:Food, Fortified; D005782:Gels / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007785:Lactose / Q000032:analysis; D008823:Micelles; D008892:Milk / Q000737:chemistry; D008894:Milk Proteins / Q000032:analysis; D012212:Rheology; D012995:Solubility
chemical_list: D002364:Caseins; D004041:Dietary Fats; D005782:Gels; D008823:Micelles; D008894:Milk Proteins; D007785:Lactose
keywords: micellar casein; microfiltration; solubility