Effect of chitosan on Bacillus cereus inhibition and quality of cooked rice during storage

Improper cooling of cooked rice at an inappropriate temperature or leaving cooked rice at room temperature can cause food poisoning attributed to Bacillus cereus. Natural food preservative of either squid or crab polymer chitosan solution was added to examine their antibacterial properties against Bacillus cereus in cooked rice during storage at 37 and 4 °C. Both types of chitosan could retard the growth of B. cereus and total aerobic counts in cooked rice stored at 37 °C up to 1 day. In addition, the effect of chitosans on the physical and textural properties of cooked rice during storage was studied. Both chitosans slightly increased the moisture content of cooked rice. However, chitosans had no effect on the whiteness and hardness of cooked rice during storage (P > 0.05). Therefore, both chitosans have a potential to be used as food preservative for cooked rice with no negative effects on rice quality.     

Effect of harvesting time and fruit size on titratable acidity, soluble solid and distribution of limonin in Thai tangerine juice

The effect of fruit size and harvesting of Thai tangerine fruit ( Citrus reticulata , Blanco) at different growth stages, 8–12 months after fruit set, on the distribution of limonin in whole fruit as well as in the individual fruit parts and extracted juice was investigated. The highest limonin concentration was observed in seed, followed by albedo, flavedo, segment membrane and juice sacs in decreasing order. The limonin concentration in juice as well as in whole fruit was decreased when fruit was harvested 8–12 months of fruit set. Increasing harvesting time from 8 to 12 months showed corresponding decreases in the amount of limonin in flavedo, albedo and seed. There was a decrease in titrable acidity and increase in total soluble solid and total soluble solid/titrable acidity ratio of juice with a later than normal harvesting time. However, fruit size showed no effect on limonin content and other properties of juice extracted from tangerine.     

Effect of Calcium Maltobionate on the Glass Transition Temperature of Model and Hand-made Hard Candies

Glass transition temperature (T_g) is an important parameter for the physical quality control of hard candies. In order to understand the applicability of calcium maltobionate to hard candy, effect of calcium maltobionate addition on the T_g of model and hand-made hard candies was investigated. Freeze-dried calcium maltobionate-sugar (sucrose containing a small amount of glucose-fructose mixture) and calcium maltobionate-reduced isomaltulose mixtures were prepared as model candies, and their anhydrous T_g was evaluated using a differential scanning calorimetry. The anhydrous T_g increased linearly with the molar fraction of calcium maltobionate. From these results, it was expected that calcium maltobionate can improve the physical stability of normal and sugarless candies. For comparison, various commercial candies were employed, and their T_g was evaluated using a thermal rheological analysis. The T_g values were in the range of 28–49 °C. The T_g values were higher than 25 °C, which is significant with respect to the physical stability of the candies. Calcium maltobionate-sugar and calcium maltobionate-reduced isomaltulose candies were prepared as hand-made candies. The calcium maltobionate-reduced isomaltulose candies had higher T_g than the calcium maltobionate-sugar candies at each calcium maltobionate content, although reduced isomaltulose has a lower T_g than sugar. At a high calcium maltobionate content, calcium maltobionate-reduced isomaltulose candy had an equivalent T_g to the commercial sugarless candies, and thus practically acceptable stability was expected. In the case of calcium maltobionate-sugar candies, there was a possibility that the hydrolysis of sugar reduced their T_g. Vacuum-concentration will be useful to improve the T_g of the candies.     

Ethanol production by repeated batch and continuous fermentations of blackstrap molasses using immobilized yeast cells on thin-shell silk cocoons

A thin-shell silk cocoon (TSC), a residual from the silk industry, is used as a support material for the immobilization of Saccharomyces cerevisiae M30 in ethanol fermentation because of its properties such as high mechanical strength, light weight, biocompatibility and high surface area. In batch fermentation with blackstrap molasses as the main fermentation substrate, an optimal ethanol concentration of 98.6 g/L was obtained using a TSC-immobilized cell system at an initial reducing sugar concentration of 240 g/L. The ethanol concentration produced by the immobilized cells was 11.5% higher than that produced by the free cells. Ethanol production in five-cycle repeated batch fermentation demonstrated the enhanced stability of the immobilized yeast cells. Under continuous fermentation in a packed-bed reactor, a maximum ethanol productivity of 19.0 g/(L h) with an ethanol concentration of 52.8 g/L was observed at a 0.36 h⁻¹ dilution rate.     

Alginate-loofa as carrier matrix for ethanol production

An alginate-loofa matrix was developed as a cell carrier for ethanol fermentation owing to its porous structure and strong fibrous nature. The matrix was effective for cell immobilization and had good mechanical strength and stability for long-term use. After a storage period of 4 months, yeast cells remained firmly immobilized and active.     

Identification of auxotrophic mutants of the yeast Kluyveromyces marxianus by non‐homologous end joining‐mediated integrative transformation with genes from Saccharomyces cerevisiae

The isolation and application of auxotrophic mutants for gene manipulations, such as genetic transformation, mating selection and tetrad analysis, form the basis of yeast genetics. For the development of these genetic methods in the thermotolerant fermentative yeast Kluyveromyces marxianus, we isolated a series of auxotrophic mutants with defects in amino acid or nucleic acid metabolism. To identify the mutated genes, linear DNA fragments of nutrient biosynthetic pathway genes were amplified from Saccharomyces cerevisiae chromosomal DNA and used to directly transform the K. marxianus auxotrophic mutants by random integration into chromosomes through non‐homologous end joining (NHEJ). The appearance of transformant colonies indicated that the specific S. cerevisiae gene complemented the K. marxianus mutant. Using this interspecific complementation approach with linear PCR‐amplified DNA, we identified auxotrophic mutations of ADE2, ADE5,7, ADE6, HIS2, HIS3, HIS4, HIS5, HIS6, HIS7, LYS1, LYS2, LYS4, LYS9, LEU1, LEU2, MET2, MET6, MET17, TRP3, TRP4 and TRP5 without the labour‐intensive requirement of plasmid construction. Mating, sporulation and tetrad analysis techniques for K. marxianus were also established. With the identified auxotrophic mutant strains and S. cerevisiae genes as selective markers, NHEJ‐mediated integrative transformation with PCR‐amplified DNA is an attractive system for facilitating genetic analyses in the yeast K. marxianus. Copyright © 2013 John Wiley & Sons, Ltd.     

Delta-9 fatty acid desaturase overexpression enhanced lipid production and oleic acid content in Rhodosporidium toruloides for preferable yeast lipid production

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Effect of starch and non-starch components on water migration,microstructure, starch retrogradation and texture of flat rice noodles made from different rice varieties

Effects of starch and non-starch components on rice noodle quality, water migration during rehydration, texture, microstructure and starch retrogradation of dried rice flour and rice starch noodles made from five rice varieties with similar high amylose contents were examined. Rice noodle qualities depended mainly on starch properties and to a lesser extent on the presence of non-starch components. Elongation of rice flour noodles was lower than rice starch noodles due to the presence of non-starch components that interrupted the starch network. Non-starch components reduced cooking loss of flour noodles in some varieties. Water migration in rice flour noodles with a looser microstructure was faster than in rice starch noodles. Nuclear magnetic resonance imaging revealed different water migration speeds in rice flour noodles among varieties, related to noodle microstructure and retrogradation properties. For production of good quality rice noodles, manufacturers should consider not only amylose content but also non-starch components.     

Quinoprotein alcohol dehydrogenase is involved in catabolic acetate production, while NAD-dependent alcohol dehydrogenase in ethanol assimilation in Acetobacter pasteurianus SKU1108

The relationship between quinoprotein alcohol dehydrogenase (ADH) and NAD-dependent ADH was studied by constructing quinoprotein ADH-deficient mutants. Quinoprotein ADH-deficient mutants were successfully constructed from Acetobacter pasteurianus SKU1108 by N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutagenesis and also by adhA gene disruption with a kanamycin cassette. The NTG mutant exhibited a complete loss of its acetate-producing ability and acetic acid resistance, while the disruptant also exhibited a loss of its acetic acid resistance but retained a weak ADH activity. The immunoblot analysis of quinoprotein ADH indicated that there are no appreciable ADH subunits in the membranes of both mutant strains. The NTG mutant grew better than the wild-type strain in ethanol-containing medium, despite the absence of quinoprotein ADH. In the mutant, the activities of two NAD-dependent ADHs, present in a small amount in the wild-type strain, markedly increased in the cytoplasm when cultured in a medium containing ethanol, concomitant to the increase in the activities of the key enzymes in TCA and glyoxylate cycles. The disruptant showed a poorer growth than the wild-type strain, producing a lower amount of acetic acid in ethanol culture, and it induced one of the two NAD-dependent ADHs and some of the acetate-assimilating enzymes induced in the NTG mutant. This study clearly showed that quinoprotein ADH is extensively involved in acetic acid production, while NAD-dependent ADH only in ethanol assimilation through the TCA and glyoxylate cycles in acetic acid bacteria. The differences between the NTG mutant and the disruptant are also discussed.