牛奶中醛脱氢酶的动力学

以新鲜牛奶作为研究对象,利用分光光度计,基于底物浓度、温度和pH值研究以乙醛为底物的乙醛脱氢酶动力学系统.结果表明,牛奶中的醛脱氢酶酶促反应动力学符合米氏方程所描述的单底物酶促反应动力学,相应的动力学参数Km=0.006 3mol/L,Vmax=0.036U/min.此外,醛脱氢酶不是耐热酶,它的最适温度在35℃左右,最适pH在7.0左右,底物浓度与酶活力的关系基本符合酶促反应动力学.同时研究了10种试剂对醛脱氢酶的抑制或激活作用,硝酸钙和硫脲是比较理想的抑制剂,硝酸钾和钼酸铵则是比较理想的激活剂.     

A KINETIC STUDY OF THE SUSCEPTIBILITY TO ENZYMIC HYDROLYSIS OF STARCH IN MALT

Finely-ground malt was held in suspension in water for various times at given temperatures and measurements were then made of the susceptibility of starch to enzymic hydrolysis. The results were used to determine first-order rate constants for the process of conversion of starch to the susceptible form. The variation of rate constant with temperature was satisfactorily described by the Arrhenius relationship with an activation energy of 8·06 × 10⁴ cal/mole and a frequency factor of 6·73 × 10⁵³ min^?1. The variation of the extent of conversion with temperature was adequately described by a cumulative normal relationship with a mean of 49·8°C and a standard deviation of 5·79°C.     

KINETIC STUDY of QUALITY LOSSES IN FROZEN SCALLOP MEATS

The rates of changes of texture, cook drip and free fatty acid during frozen storage of scallop meats at ?30, ?18, ?12, ?8 and ?5°C were studied. Both the physical and chemical changes were found to follow first order kinetics. the activation energies for toughness, cook drip and free fatty acid were 43.6, 69.3 and 37.9 kJ/mol, respectively.     

CONVERSION OF α-ACETOLACTATE AND REMOVAL OF DIACETYL A KINETIC STUDY

Brewers' yeast does not form diacetyl during fermentation, but α-acetolactate which spontaneously is converted to diacetyl. This non-enzymic reaction is mainly dependent upon pH, temperature and substrate concentration. The present kinetic studies demonstrate that temperature and pH have a dominating effect on the spontaneous conversion of α-acetolactate during primary fermentation. From the reported kinetic studies it is possible to predict the conversion rate of α-acetolactate present in the fermenting liquid, and to calculate the time necessary to convert a given amount of α-acetolactate to diacetyl. As long as yeast is present in the fermenting liquid, diacetyl can not be detected analytically due to the high diacetyl-reducing activity of the yeast cells. The specific activity is strain-dependent and decreases gradually during fermentation. The diacetyl-reducing activity of the yeast will, however, normally be high enough to compete with the diacetyl formed from α-acetolactate.     

IDENTIFICATION OF HAZARDOUS FUSARIUM SECONDARY METABOLITES OCCURRING IN FOOD RAW MATERIALS

Hazardous secondary metabolites produced by Fusarium molds were identified in the first step of a risk assessment procedure, the hazard identification. These mycotoxins are excreted, often simultaneously, by the fungus into plant tissue. A literature survey revealed the occurrence of 61 Fusarium species in agricultural produce (cereals, vegetables, fruits) of which 35 species were reported to produce a total of 137 secondary metabolites in laboratory experiments. This literature review shows that 41 (30%) of these secondary metabolites have been tested and found toxic in various assays and should be considered hazardous if present in food. Six of these were reported to be related to human intoxications. Four metabolites belonged to the trichothecenes (T-2 toxin, nivalenol, deoxynivalenol and acetyldeoxynivalenol), the others were zearalenone and fumonisin B₁. Twelve metabolites, including the previously mentioned six, were reported to have been tested in feeding trials using vertebrates. These twelve secondary metabolites should be regarded as most important and need to be evaluated in each risk assessment procedure for mycotoxins in food commodities .     

ACTINOMYCIN D INDUCTION OF TERPENE STRESS METABOLITES IN POTATO TUBER SLICES

Relatively large amounts of rishitin and lubimin accumulate in potato tuber slices following treatment with actinomycin D (AD). Optimum elicitation of these terpenes occurred at an (AD) concentration of μM. Terpene accumulation was evident within 24 h after treatment and progressed for up to 96 h at which time rishitin levels exceeded 100 μg/g fresh weight, discs. Slices prepared from tubers previously held at 25°C were less responsive to (AD) induction of terpenes than those from tubers previously stored at 4°C. Treatment of discs with 50 μM cycloheximide following (AD) treatment blocked further accumulation of terpenes indicating a requirement for continual protein synthesis. (AD) (20 μM) treatment caused a slight reduction in protein synthesis for the first 48 h following treatment after which it caused an increase in protein synthesis compared to controls. (AD) also affected respiration of discs in a manner which paralleled its effect on protein synthesis. Cyanide resistant respiration was more pronounced in (AD) treated discs than control discs. The data are discussed with reference to the mechanism by which (AD) elicits terpene biosynthesis in potato tubers discs.     

高产乳糖酶酵母菌生长模型的研究前景

酵母菌作为生产乳糖酶的一种主要工程菌,在实验及生产应用方面得到了许多学者的关注。在已筛选得到高产乳糖酶酵母菌基础上,分析可用于高产乳糖分解酶酵母菌的两种生长模型,以期针对高产乳糖酶筛得酵母菌建立一种生长动力学模型,指导其发酵和研究其生长机理,并为工业化生产提供指导。     

YEAST STRAIN AND KINETIC ASPECTS OF THE FORMATION OF FLAVOUR COMPONENTS IN CIDER

An apple juice was fermented at 8°C using twelve strains of Saccharomyces uvarum. Glucose, fructose, malic and L-lactic acids, isobutanol, 2,3 butanediol, isoamyl alcohol, ethyl acetate and titratable acidity were determined and monitored in the course of fermentation. It was observed that yeast strains differed from one another mainly in fermentation rates. When components were determined in ciders of the same remaining fructose concentration, rather than after the same fermentation time, the only significant effect of the yeast strain was on the amounts of glucose and ethanol in sweeter cider (fructose 34 g/litre), or on the amounts of glucose, acetic acid, isobutanol and amyl alcohols in dryer ciders (fructose 17 g/litre). A sensory analysis showed that standard deviations of concentrations of remaining or formed components in ciders fermented by different yeast strains, was sufficient to be detected by the taste panel. Added glucose in this range of concentration (1.5 g/litre) in cider reduced the perception of sourness while the addition of acetic acid (0.3 g/litre) reduced the perception of the scented flavour. Added Isobutnol (6 mg/litre) reduced the perception of sweetness. No significant effect of added isoamyl alcohol (30 mg/litre) could be detected. Chemical determinations were correlated by means of regression equations derived from earlier work with some organoleptic characteristics. No anticipated effect of the yeast strain could be detected on any studied flavour characteristic. It is concluded that in ciders of the same attenuation, the effect if it does exist, of the S.uvarum strain isolated from ciders, must be low.