Due to its merits of drought tolerance and high yield, sweet potatoes are widely considered as a potential alterative feedstock for bioethanol production. Very high gravity (VHG) technology is an effective strategy for improving the efficiency of ethanol fermentation from starch materials. However, this technology has rarely been applied to sweet potatoes because of the high viscosity of their liquid mash. To overcome this problem, cellulase was added to reduce the high viscosity, and the optimal dosage and treatment time were 8 U/g (sweet potato powder) and 1 h, respectively. After pretreatment by cellulase, the viscosity of the VHG sweet potato mash (containing 284.2 g/L of carbohydrates) was reduced by 81%. After liquefaction and simultaneous saccharification and fermentation (SSF), the final ethanol concentration reached 15.5% (v/v), and the total sugar conversion and ethanol yields were 96.5% and 87.8%, respectively.

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The cell-density-dependent responses of Saccharomyces cerevisiae to inoculation sizes were explored by a proteomic approach. According to their gene ontology, 100 protein spots with differential expression, corresponding to 67 proteins, were identified and classed into 17 different functional groups. Upregulation of eight heat shock, oxidative response and amino acid biosynthesis-related proteins (e.g. Hsp78p, Ssa1p, Hsp60p, Ctt1p, Sod1p, Ahp1p, Met6p and Met17p), which may jointly maintain the cell redox homeostasis, was dependant on inoculation density. Significant increases in the levels of five proteins involved in glycolysis and alcohol biosynthesis pathways (e.g. Glk1p, Fba1p, Eno1p, Pdc1p and Adh1p) might play critical roles in improving ethanol productivity of the fermentation process and shortening the fermentation time when inoculation sizes were increased. Cell-density-dependent glycolytic variations of proteins involved in trehalose, glycerol biosynthesis and pentose phosphate pathway revealed shifts among metabolic pathways during fermentation with different inoculation sizes. Upregulation of three signal transduction proteins (Bmh1p, Bmh2p and Fpr1p) indicated that adequate cell-cell contacts improved cellular communication at high inoculation sizes. These findings provide insights into yeast responses to inoculation size and optimizing the direct inoculation of active dry yeast fermentation, so as to improve the ethanol production.

A high cell density strategy has been used in bioethanol production to shorten the fermentation period. To reveal the molecular basis of fermentative behavior in high cell density, the profiling of the phospholipids and sterols of Saccharomyces cerevisiae during fermentation at five different pitching rates (1, 5, 10, 20, and 40 g/L) was investigated. Using LC/ESI/MS(n) technology, 148 phospholipid species were detected, of which 91 species were quantified, and using the gas chromatography-time-of-flight mass spectrometry procedure, a total of 11 sterols were quantified. Phospholipid samples from different pitching rates were discriminated into three groups using principal component analysis (1, 5 g/L, and the others). The main changes in the lipid profile of yeast cells with higher pitching rates were as follows: (a) the relative contents of phosphatidylglycerol and phosphatidylserine were higher while phosphatidylinositol was lower compared with lower pitching rates, (b) the saturated and the relatively shorter fatty acyl chains of phospholipids decreased, and (c) the content of ergosterol was higher. These findings suggested a regulation of the property of the membrane at the situation of high cell density and a possible approach of self-protection of the yeast cells against the high density stresses.

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