Binary mixtures with significant size ratios are scarcely studied. Yet, contaminants of chromatographic columns or ion-exchange resins have size ratios of δ < 0.1. Binary mixtures of glass beads with δ ) 0.1-0.0375 were used experimentally to measure packing porosity. Simultaneously, a significant number of published data was analyzed. A linear mixing model was adopted to predict the porosity of each particle fraction in the binary mixture. Deviations from the model may be caused by wedging of small particles between the large ones. Large particles may disturb the porous medium properties by inducing a wall effect over the small particles. Wedging analysis led to the conclusion that, for δ < 0.01, its effect is insignificant. The wall effect yields an additional void around the large particles as long as δ > 0.0035. For δ < 0.0035, the small particles form a monosized dense packing and both wedging and wall effects become negligible.

Flocculation onset, the time during the fermentative cycle at which the strains of Saccharomyces cerevisiae become flocculent, is an important factor in the brewing industry. The flocculation ability of Flo 1 phenotype (strain NCYC 869) remained practically unchanged throughout the growth and seems to be insensitive to the presence of nutrients of the culture medium. On the contrary, the flocculation of NewFlo phenotype (strain NCYC 1195) exhibited a cyclic behaviour. It was found that the loss of flocculation in the early growth was the result of two combined effects: the dismantling of the flocculation mechanism of the cells coming from the inoculum and the nonflocculent state of the new cells produced after growth has started. The onset of flocculation of strain NCYC 1195 in the cultural conditions used in this work coincided with the end of the exponential growth, when the minimum glucose level in the culture medium was attained. It was demonstrated that it is possible to manipulate the flocculation onset by changing the initial glucose concentration in the culture medium.

An expeditious and accurate simplification of Stone and Scallan's solute exclusion technique was developed, thereby avoiding several sources of experimental error coupled with the determination of cellulose pore volume. Using this method, it is shown that cellulolytic enzymes do not enter into the micropores of five studied celluloses. These results suggestes that hydrolysis occurs initially at the external surface of the fibers. This surface area was calculated with the help of adsorption isotherms of bovine serum albumin. The obtained values for the different samples agree with the microscopically observed cellulose morphology. The correlation obtained by several authors relating cellulose porosity and its digestibility is explained as a consequence of the lower crystallinity and easier fragmentation of the more porous celluloses during hydrolysis. (c) 1994 John Wiley & Sons, Inc.

This study describes the characterisation of whey protein hydrolysates obtained from tryptic hydrolysis to assess their application as ingredients with angiotensin-converting-enzyme (ACE) inhibitory action. The levels of a-lactalbumin (a-la) and b-lactoglobulin (b-lg) remaining after hydrolysis were quantified. Peptides were separated by RP-HPLC, and Ala-Leu-Pro-Met-His-Ile-Arg (ALPMHIR), the most potent b-lg-derived ACE-inhibitory peptide was monitored. A correlation curve was established for the production of this peptide as a function of hydrolysis time. Heat-induced gelation of hydrolysates was studied by small-deformation rheology. The gelation times and the strength of the final gels were highly dependent on the degree of hydrolysis. Smaller peptides liberated by hydrolysis contributed to the inability of whey protein hydrolysates to gel. r

The effect of cultural (temperature and pH) and nutritional conditions (nitrogen and carbon source) on the flocculation expression of three strains was studied. The strains' flocculation ability was determined by placing the cells in a stationary phase of growth in standard flocculation conditions. The flocculation ability of strain NCYC 1195, recently classified in the literature as the NewFlo phenotype, was more sensitive to growth temperature than Flo1 phenotype strains (NCYC 869 and NRRL Y265). The initial pH of the culture medium did not affect the flocculation ability of Flo1 phenotype strains but in the case of strain NCYC 1195 flocculation was repressed when the initial pH of the culture medium was below 3.5. Flocculation in strain NCYC 1195 was also repressed in defined culture medium; this inhibition was not related to a deficiency in any particular nitrogen source, but rather to the poor buffering capacity of the defined medium. All strains showed strong flocculation when grown in glucose, but were nonflocculent in glycerol. It was clearly demonstrated that the phenotypic expression of flocculation could be induced or repressed by changing cultural and nutritional conditions. Two distinct behaviours were also displayed with regard to the effect of the cultural conditions upon flocculation, namely the effect of pH. These different behaviours can be used to distinguish the two flocculation phenotypes.

No abstract

A sedimentation method based in the Helm's flocculation test was improved. In this method, all steps were standardised: initial cell concentration, agitation, sampling and determination off settled cells. The results obtained with the sedimentation test do not differ significantly (P = 0.05) from those obtained with the Stratford test; besides, the precision of the two methods do not differ significantly (P o 0.05). The results presented clearly demonstrated that the method improved in this work is rigorous for quantification of yeast flocculation and allows the differentiation of flocculation ability of the strains.The respective micro-flocculation tests were also compared; Stratford micro-flocculation test was selected as producing the most meaningful results.

Hydrolysis of whey protein concentrates (WPCs) was performed using trypsin under different combinations of temperatures and pH values in a total of three experiments, namely experiment A at 37°C and pH 8, experiment B at 37°C and pH 9, and experiment C at 50°C and pH 8. Monitorization of the degradation of native whey proteins and the peptide formation throughout hydrolysis was performed by reverse phase HPLC/UV. Seven main peptides were separated according to their polarity and numbered from T1 to T7. In general a difference was observed between rate of hydrolysis of a-lactalbumin and b-lactoglobulin; in the former case hydrolysis was complete by 15 min in experiments B and C and by 120 min in experiment A, whereas in the later case the rate of hydrolysis was much slower. ANOVA analysis, performed to assess whether the average values obtained for the three experiments conducted to statistically different results for each variable (% of b-lactoglobulin degradation and % of peptides T1 to T7), showed significant differences between experiments A and C. However, between A and B and between B and C no significant differences were observed. Principal Component Analysis evidenced the time period at which similarities and/or differences between experiments were observed. Additionally, a mathematical equation for the degradation of b-lactoglobulin as a function of hydrolysis time was established and some peptides were correlated with their parental protein using linear regression analysis.