Some abstracts do not have video files because ASAS was denied recording rights.
703
Gelation properties of micellar casein concentrate when recombined with cream
Skim milk can be concentrated using microfiltration and evaporation to produce a highly concentrated micellar casein concentrate (HC-MCC), containing ~20% casein with ~70% of serum proteins removed during diafiltration. Understanding the gelation properties of HC-MCC and when mixed with cream to form a recombined concentrated milk (RCM) is important for using RCM for cheese manufacture.
After concentration, HC-MCC forms a gel when cooled. Heating above the cold gelation temperature (up to 50°C) can break up the gel so that the individual casein micelles are solubilized. When examined using transmission electron microscopy, cold-gelled HC-MCC was observed to form a close-packed gel, which probably occurs when kinetic energy of the casein micelles is sufficiently reduced to inhibit their mobility in relation to adjacent casein micelles.
Similar observations of cold gelation were made when HC-MCC was mixed with cream in casein to fat ratios of 0.8 or 1.2 as would be used for the manufacture of cheddar or part skim mozzarella cheese. At pH 6.6, an RCM with high protein can gel at cheese making temperatures, while with 12% or less casein it does not gel above 12°C. In micrograghs of cold-gelled RCM, casein micelles were less closely packed together and were partially dissociated. It appears that the protein strands that have been partially released from the casein micelles still entangle, restrict the mobility of each other and form a fine stranded gel network.
To understand challenges related to cheese making using an RCM that contains 4 times the level of casein than normally found in milk, its coagulation properties (rennet coagulation time and curd firmness) were studied using a rheometer. Reducing rennet amount can lengthen coagulation time of RCM but it does not affect curd firmness or firming rate. Decreased coagulation temperature can lengthen coagulation time and slow curd firming rate, but it also increases initial viscosity of RCM. Lowering pH of RCM to pH 6.0 did not solve the problem of curd firming being too rapid. Microstructure of RCM and its rennet coagulum indicated that the increased curd firmness probably results from the highly inter-linked and longer protein strands in RCM curd. Overall, RCM with a casein level of 11 to 12% has potential for use in cheese making provided its higher viscosity compared to milk and its fast curd firming rate can be overcome. Reducing rennet amount can be used to slow coagulation and curd firming.
Keywords: rheology, microstructure, casein micelle