D Donald Muir

Efficacy of Various Pasteurization Time-Temperature Conditions in Combination with Homogenization on Inactivation of Mycobacterium avium subsp. paratuberculosis in Milk

ABSTRACT The effect of various pasteurization time-temperature conditions with and without homogenization on the viability of Mycobacterium avium subsp. paratuberculosis was investigated using a pilot-scale commercial high-temperature, short-time (HTST) pasteurizer and raw milk spiked with 101 to 105M. avium subsp. paratuberculosis cells/ml. Viable M. avium subsp. paratuberculosis was cultured from 27 (3.3%) of 816 pasteurized milk samples overall, 5 on Herrolds egg yolk medium and 22 by BACTEC culture. Therefore, in 96.7% of samples, M. avium subsp. paratuberculosis had been completely inactivated by HTST pasteurization, alone or in combination with homogenization. Heat treatments incorporating homogenization at 2,500 lb/in2, applied upstream (as a separate process) or in hold (at the start of a holding section), resulted in significantly fewer culture-positive samples than pasteurization treatments without homogenization (P < 0.001 for those in hold and P < 0.05 for those upstream). Where colony counts were obtained, the number of surviving M. avium subsp. paratuberculosis cells was estimated to be 10 to 20 CFU/150 ml, and the reduction in numbers achieved by HTST pasteurization with or without homogenization was estimated to be 4.0 to 5.2 log10. The impact of homogenization on clump size distribution in M. avium subsp. paratuberculosis broth suspensions was subsequently assessed using a Mastersizer X spectrometer. These experiments demonstrated that large clumps of M. avium subsp. paratuberculosis cells were reduced to single-cell or “miniclump” status by homogenization at 2,500 lb/in2. Consequently, when HTST pasteurization was being applied to homogenized milk, the M. avium subsp. paratuberculosis cells would have been present as predominantly declumped cells, which may possibly explain the greater inactivation achieved by the combination of pasteurization and homogenization.

Enhancement of amino acid catabolism in Cheddar cheese using α-ketoglutarate : amino acid degradation in relation to volatile compounds and aroma character

The effectiveness of the transaminase acceptor α-ketoglutarate in enhancing amino acid catabolism and manipulating the aroma profile of Cheddar cheese has been studied. Utilisation of α-ketoglutarate, catabolism of amino acids, volatiles production, and aroma profile of the cheese were monitored after 6, 12 and 24 weeks ripening. Glutamate and GABA were considerably enhanced on addition of the transaminase acceptor while levels of phenylalanine, leucine, isoleucine, alanine, valine, methionine and threonine were reduced. Addition of α-ketoglutarate increased volatile components originating from the catabolism of branched chain and aromatic amino acids. These compounds included acetic, propanoic, 2-methylpropanoic and 3-methylbutanoic acids, 3-methylbutanol, phenylacetaldehyde and benzaldehyde. Additionally enhanced production of 3-OH-2-butanone was evident. Addition of α-ketoglutarate increased aroma intensity, creamy and fruity aromas. Effects obtained must be verified by tasting cheeses made with food grade α-ketoglutarate, but results suggest potential benefits in accelerated maturation, low fat systems and manipulation of flavour profiles.

Sensory evaluation of Cheddar cheese: Order of tasting and carryover effects

Abstract Order of tasting and carryover effects of Cheddar cheese have been evaluated during a sensory study of the cheese. These effects are of such magnitude as to cause serious biases in judgements on the acceptability of cheese samples. The influence of order of tasting and carryover effects can be minimised and their magnitude estimated by application of well-proven principles of experimental design.

Lactosylation of milk proteins during the manufacture and storage of skim milk powders

Abstract Extensive lactosylation of milk proteins in standard skim milk powder dried against air between 185 and 90°C (inlet and outlet temperatures of the air) was detected by capillary electrophoresis. Optimisation of the drying conditions included keeping the outlet temperature low (preferably

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.

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.

Sensory properties of Cheddar cheese: changes during maturation

Abstract The aroma, flavour and texture of 16 samples of commercial Cheddar cheese have been profiled after ripening at 10 °C for 3, 4, 6, 8, 10 and 12 months. Systematic changes in sensory character have been studied and the main changes during maturation identified. Although sensory character changed slowly during ripening, assessment early in the maturation period was an unreliable estimate of ultimate sensory character. Progressive changes in Cheddar aroma and flavour, creamy flavour, acid flavour and mouth-coating character were noted during ripening. Changes in minor components of aroma and flavour were also observed but, on average, were small. Two samples eventually developed marked rancid character and another became excessively bitter. The relation between gross composition of the cheese and sensory properties was investigated. In the early stages of ripening, the ratings for Cheddar flavour and mouth-coating character were associated with the salt content of the cheese and with the concentration of fat in dry matter. However, as the cheese matured these associations weakened.

Volatile components in steam distillates of Cheddar cheese as indicator indices of cheese maturity, flavour and odour

Abstract Chemical analyses were carried out on 12 samples of commercial Cheddar cheese differing widely in perceived maturity. Quantitative descriptive analysis of the cheeses has been previously reported elsewhere. Protein breakdown was estimated by selective fractionation of the water soluble components. Steam distillates of the cheese were also prepared. The volatiles in the steam distillates were fractionated by gas liquid chromatography and 31 peaks identified by mass spectrometry. Multivariate calibration was used to derive equations relating the sensory attributes to the chemical assay. Perceived maturity, intensity of cheddar cheese flavour and acid flavour were modelled with a high degree of fit. As a result, the chemical analyses employed offer considerable potential for objective assessment of key quality attributes of Cheddar cheese.

A comparison of the flavour and texture of Cheddar cheese of factory or farmhouse origin

Diversity in the flavour and texture of Cheddar cheese was studied in 34 samples of factory or farmhouse origin. Sensory profiles were measured using an integrated Design, Data capture and Sensory Profiling Protocol (DDASPP). The main differences in character of the cheese were between the sub-set of farmhouse cheeses manufactured from raw milk and cheese made from pasteurised milk. Raw milk cheese was more intensely flavoured than conventional product, but was notable for elevated ratings for atypical flavours such as rancid, bitter and unclean. In general, farmhouse cheese showed excessively wide variations in composition which were also associated with atypical flavour or texture. Within factory-made products there was little distinction between samples of Scottish, English, Irish and Canadian origin. However, mature samples of New Zealand Cheddar were of a slightly different character. No strong relations between the sensory properties and composition of the cheese could be deduced. Nevertheless, as expected, the extent of proteolysis and moisture in non-fat solids content of the samples were associated with differences in flavour and texture.

Genotypic effects on sensory quality of blackcurrant juice using descriptive sensory profiling

A range of 46 Ribes genotypes from the Eucoreosma subgenus, predominantly Ribes nigrum (blackcurrant) was analysed for sensory qualities by descriptive sensory profiling. Using an appropriate vocabulary against a branded product standard, significant genotypic variation was found in all the main sensory characters (appearance, flavour, aroma, mouthfeel and aftertaste), with by far the most important variation detectable in the flavour component. No correlation of sensory characters with biochemical characters was observed. An initial consideration of the inheritance of sensory characters in Ribes was made.