Rheological properties of chymosin-induced skim milk gels as affected by milk storage time and temperature

ABSTRACTFundamental dynamic rheological tests were used to mea-sure the rigidity (|G*|) and viscoelasticity (tan d ) of gels madefrom skim milk samples that were (1) never cooled, (2) storedat 48 C, or 3) stored at 48 C and then rewarmed to 258 C. Milkcooled and held at 48 C prior to gel formation produced gelswith lower |G*| and higher tan( d ) than fresh milk gels. Chilledmilk samples that were rewarmed to 25 8 C for 16h prior to gelformation produced gels with rheological properties similarto fresh milk gels. The solubilization of b -casein during coldstorage was hypothesized to be the cause of observed dif-ferences in gel rheology.Key words: milk gels, rheology, b -casein, rigidity, vis-coelasticity INTRODUCTION T HE STORAGE HISTORY OF MILK IS VITAL TO THE SUCCESS mL) was added to some samples to inhibit plasmin activity. OF cheese making. Storing milk for extended times at 48C decreasescheese yield, increases coagulation time, and in some cases, producescheese with less desirable texture attributes (Ali et al.,1980; Pyne andMcGann, 1960). Such changes result in increased costs to processorsand consumers.Milk is typically cooled to 4 8C immediately after milking and storedat or near 48C until further processing. Downey and Murphy (1970)reported on the dissociation of b-casein from casein milk micellesbetween 48 and 308C using high-speed centrifugation and gel filtra-tion. They found that soluble casein increased after lowering the tem-perature of milk. Murphy and Fox (1991) successfully fractionated b-casein from sodium caseinate for further processing using ultrafiltra-tion at 48C. Ali et al. (1980) investigated the partial dissociation ofmicellular caseins into the soluble phase during storage at 4 to 7 8C. Amaximum dissociation of casein was found after storing milk for 48hat 4 8C. b-Casein accounted for most of the increase in soluble casein.In the same study, using empirical measurements, they reported thatcurd structure was weaker and cheese yields were lower in milk withelevated soluble phase casein.Curd firmness is influenced by many factors in addition to milkstorage time and temperature. Other important parameters include pHand heat treatment as well as calcium, phosphate and protein concen-tration (Kowalchyk and Olson, 1979; McMahon et al., 1984; Ustunoland Hicks, 1990). Enriching milk with up to 10 mM calcium also wasreported to increase gel strength (Schultz and Ashworth, 1974; Rametet al., 1981; Marshall et al., 1982; Storry and Ford, 1982; McMahonet al., 1984; Zoon et al., 1988). However, adding high amounts ofcalcium ( .10 mM) decreased gel strength (Murphy and Fox, 1991).Studies on the structure and mechanical properties of milk gelshave been conducted using instruments that measured dynamic rheo-logical properties. Dejmek (1987) used a Bohlin universal viscometerto study aged chymosin gels at various temperatures, strains and strainrates. Halim and Shoemaker (1990) used controlled stress rheometryto study the firmness of gels from milk enriched with purified caseins.Our objective was to determine the influence of milk storage time andtemperature on the firmness and viscoelasticity of chymosin-inducedskim milk gels.