Oxygen addition and sterol synthesis in Saccharomyces cerevisiae during enological fermentation

Under anaerobic conditions, yeast growth normally requires oxygen in order to favour the synthesis of sterols and unsaturated fatty acids. However, in such conditions, superfluous oxygen consumption by yeast cells is observed. The superfluous oxygen consumed by the yeast cells appears to be not related to classical respiration, but mainly to the operation of several alternative oxygen consumption pathways. In this study, the potential relationship between this superfluous oxygen consumption and the yeast sterol synthesis pathway was investigated during enological fermentation. Additions of small (7 mg l(-1)) and excess (37 mg l(-1)) amounts of oxygen at the end of cell growth phase were used as a method of comparing oxygen consumption by normal synthetic pathways with that by alternative respiration pathways. The superfluous oxygen consumption by yeast cells during fermentation seemed not to alter and strongly favoured fermentation kinetics and cell biomass formation. However, a marked decrease of the orderliness of the membrane phospholipids is observed, which is not related to the drop of cell viability. After oxygen additions, squalene contents of the cells decreased, while the relative proportions of ergosterol or its precursors in the total sterol fraction did not correlatively increase. It was further found that an oxygen-dependent sterol degradation occurred when oxygen was added in excess amounts with respect to the cellular requirements for sterol synthesis. At present, this modification of the sterol contents of yeast membranes has not been related to any physiological parameters.     

MIDN: a spacecraft microdosimeter mission

MIDN (MIcroDosimetry iNstrument) is a payload on the MidSTAR-I spacecraft (Midshipman Space Technology Applications Research) under development at the United States Naval Academy. MIDN is a solid-state system being designed and constructed to measure microdosimetric spectra to determine radiation quality factors for space environments. Radiation is a critical threat to the health of astronauts and to the success of missions in low-Earth orbit and space exploration. The system will consist of three separate sensors, one external to the spacecraft, one internal and one embedded in polyethylene. Design goals are mass <3 kg and power <2 W. The MidSTAR-I mission in 2006 will provide an opportunity to evaluate a preliminary version of this system. Its low power and mass makes it useful for the International Space Station and manned and unmanned interplanetary missions as a real-time system to assess and alert astronauts to enhanced radiation environments.     

Peptides released from acid goat whey by a yeast-lactobacillus association isolated from cheese microflora

Seven lactobacilli and a variety of microflora extracted from twenty five commercial cheeses were grown on unsupplemented acid goat whey and screened for their capacity to hydrolyse whey proteins [alpha-lactalbumin (alpha-la) and beta-lactoglobulin (beta-lg)] and to generate peptides. Fermentations were performed aerobically or anaerobically at 37 degrees C using crude or pre-heated whey (10 min at 65, 75 or 85 degrees C). Under aerobic conditions, growth of lactobacilli was poor and protein hydrolysis did not occur. Anaerobic conditions slightly increased lactobacilli growth but neither beta-lg hydrolysis nor peptide generation were observed. More than 50% of alpha-la was digested into a truncated form of alpha-la (+/- 12 kDa) in crude whey and whey pre-heated at 65 degrees C. Twenty-five microflora extracted from raw milk cheeses were screened for their proteolytic activities on acid goat whey under the conditions previously described. Eight of them were able to hydrolyse up to 50% of alpha-la mainly during aerobic growth on crude or pre-heated whey. The corresponding hydrolysates were enriched in peptides. The hydrolysate involving microflora extracted from Comté cheese after or at 18 months ripening was the only one to exhibit hydrolysis of both alpha-la and beta-lg. Microbiological analysis showed that microorganisms originating from Comté cheese and capable of growth on unsupplemented whey consisted of Candida parapsilosis and Lactobacillus paracasei. Fermentation kinetic profiles suggested that peptides were released from alpha-la hydrolysis. The co-culture of both microorganisms was required for alpha-la hydrolysis that occurred concomitantly with the pH decrease. During whey fermentation, Cand. parapsilosis excrete at least one protease responsible for alpha-la hydrolysis, and Lb. paracasei is responsible for medium acidification that is required for protease activation.