J. M. Banks

Proteolytic and other hydrolytic enzyme activities in non-starter lactic acid bacteria (NSLAB) isolated from Cheddar cheese manufactured in the United Kingdom

The populations of non-starter lactic acid bacteria (NSLAB) in a selection of 15 good-quality UK-manufactured Cheddar cheeses that had been matured for 6–9 months ranged from 105 to 107 bacteria g-1. Fifteen different species of lactic acid bacteria were identified using commercially-available identification systems. The species isolated most frequently were Lactobacillus paracasei subsp. paracasei and Lb. plantarum; 10 other species were isolated from two or more cheeses and three species were recovered from only a single cheese. There were marked differences in the NSLAB populations of the cheeses produced by different UK manufacturers, and differences were also apparent in the populations of two cheeses produced on different occasions at the same creamery. Forty-one isolates, selected to include all the species identified and the dominant strains present in cheeses produced at several different creameries, were screened for activities of 34 proteolytic, five glycoside hydrolase and five esterolytic enzymes. All the NSLAB possessed a wide range of hydrolytic enzymes and therefore had the potential to contribute at some stage to the development of cheese flavour during the maturation and ripening period. Inter-species and strain differences in enzyme profiles and levels of activity were apparent and were determinants for the non-random selection of NSLAB for use as adjunct cultures in subsequent cheese-making trials. The breakdown of diagnostic substrates was indicative of the presence of multiple proteinase, tripeptidase, dipeptidase (including prolinase-and prolidase-like), dipeptidyl peptidase, prolyl, proline, aspartyl, pyroglutamyl (pyrrolidonecarboxyl) and general aminopeptidase activities.

Sensory properties of cheddar cheese: Effect of starter type and adjunct

Abstract The effect of commercially available types of starter and starter adjunct on the sensory properties of Cheddar cheese has been studied during controlled ripening at 10 °C. Cheese was manufactured under very closely controlled conditions to ensure that initial composition did not influence maturation. Differences in aroma, flavour and texture were associated strongly with starter type and, to a lesser extent, with starter adjunct. The development of perceived maturity was strongly influenced by starter and secondary aspects of sensory character were determined by the starter and adjunct used. Interactive effects between starter and adjunct were noted.

Influence of other whey proteins on the heat-induced aggregation of α-lactalbumin

Abstract The thermally induced aggregation of α-lactalbumin in solution and in the presence of serum protein-free casein micelles has been studied using gel permeation chromatography. No aggregation of α-lactalbumin was detected after heating at 90°C for 24 min. However, addition of β-lactoglobulin or serum albumin to the serum protein-free casein micelles + α-lactalbumin system caused aggregation of the α-lactalbumin, the rate and extent of this aggregation being dependent upon the concentration of free sulphydryl groups present in the other whey protein. It is therefore the sulphydryl group which is important and which appears to function by inducing cleavage of intramolecular disulphide bonds in the α-lactalbumin. This leads to the formation of intermolecular disulphide bridges and hence to aggregation of the α-lactalbumin.

Sensory properties of hard cheese: Identification of key attributes

Abstract The sensory properties of 16 samples of hard cheese encompassing the main types on sale in the United Kingdom have been studied. A panel of 16 assessors rated the cheese according to five odour, ten flavour and five textural attributes. The data were analysed by the Residual Maximum Likelihood technique and estimates of effects of sample, order of tasting, carryover and assessor were computed. Significant sample differences were found for 19 of the 20 attributes. Generalized Procrustes Analysis was also applied to the individual assessor matrices for odour, flavour and texture as a method of data simplification that allows for differences between assessors. Consensus models were fitted which accounted for 41–58% of the total variation after rotation and scaling. These models were of low dimensionality and were used to construct perceptual space maps. Interpretation of the individual dimensions in terms of the original attributes was achieved by correlation and projection methods. This allowed the key elements of the sensory properties of hard cheese to be identified.

Application of a mass spectrometry sequencing technique for identifying peptides present in Cheddar cheese

Abstract Peptides were isolated from a water-soluble fraction of a commercial mature Cheddar cheese by reversed phase-high performance liquid chromatography. Characterisation of 16 peptides was performed using a matrix assisted laser desorption/ionisation time-of-flight mass spectrometer, by determining their masses and applying N-terminal sequencing using a volatile degradation reagent, trifluoroethylisothiocyanate. The experimental approach allowed the sequencing of overlapping peptide mixtures. Of the 16 peptides identified, eight came from αs1-casein, seven from β-casein and one from αs2-casein.

Influence of heating regime and pH on the primary phase of renneting of whole milk

Abstract The rate and extent of the chymosin-catalysed hydrolysis of κ-casein, together with the extent of denaturation of individual and total whey proteins, has been determined in whole milk heated on a pilot plant-scale apparatus for periods of 15 s to 5 min at temperatures between 72 and 140 °C. Denaturation of about 10% of the total whey protein resulted in partial inhibition of the reaction. Additional denaturation (up to about 60% of the total whey proteins) had no further effect on the reaction until the heating conditions were sufficiently severe to cause chemical damage to the casein(s). Inhibition of the chymosin-catalysed hydrolysis milk heated at pH values between 6.2 and 8.5 and subsequently hydrolysed at pH 6.7, was greater at pH values above about 7.0. This may be due to the solubilization of micellar κ-casein which has been reported to occur under these conditions.