CAMBRIDGE PRIZE LECTURE DEVELOPING NEW STRAINS OF YEAST*

Genetic engineering or recombinant DNA technology is now routinely applied to the construction and development of new strains of brewing yeast. This is a direct consequence of the power of the technology which facilitates the modification, introduction and stable maintenance of specific genes in brewing yeast, without compromising the intrinsic brewing properties of the yeast itself. The way in which gene technology has been applied to the development of new strains of Bass yeast is briefly illustrated by the provision of plasmid-based systems for ensuring the stable maintenance of recombinant genes, the construction of amylolytic and β-glucanolytic yeast and the design and development of genetic systems for enhancing the value of waste brewers yeast. Commercial and regulatory issues are discussed.     

Unmalted triticale cultivars as brewing adjuncts: effects of enzyme activities and composition on beer wort quality

The applicability of three selected triticale cultivars (Trinidad, Lamberto, Fidelio) for use as brewing adjuncts was investigated in comparison with wheat adjunct and barley malt. Fermentable substance, crude protein and arabinoxylan levels of starchy materials were determined as well as their native potencies (amylolytic, proteolytic, pentosolytic) to solubilise and degrade grain components during mashing. Laboratory‐scale experiments were performed to evaluate the influence of the adjuncts (composition, enzyme potency) on beer wort quality by mashing mixed (1:1) grists of malt and adjunct. Barley malt was rated as the superior raw material, possessing considerably higher enzyme activities and yielding the lowest wort viscosity. Among the triticale cultivars cv Trinidad was identified as the most suitable to serve as a brewing adjunct due to its improved starch solubilisation properties and its ability to generate low wort viscosities. Compared with the potent malt enzymes, the enzyme activities of unmalted triticale (such as amylases, pentosanases and proteases) had little affect on the composition of the sweet worts. In contrast, the contents of crude protein and fermentable substance of the triticale varieties greatly affected wort quality. Furthermore, the adjunct moiety determined the level of wort viscosity when mashing a combination of malt and triticale. In general, the brewing properties of triticale cv Trinidad were comparable with those of wheat. Copyright © 2004 Society of Chemical Industry     

米饭回生研究(III)米饭回生抑制的原理与工艺

透彻地了解米淀粉的回生机理是有效抑制米饭回生的基本前提 ,有效地修饰米粒组织结构是得到可接受的米饭质构的必要保证。文章通过 β -淀粉酶处理工艺缩短米支链淀粉分子外支链长度 (ECL) ,显著减缓其回生结晶速率 ;并采用米粒表面干热处理及中性蛋白酶液浸泡处理等辅助工艺使米粒结构疏松化 ,以使β -淀粉酶更加均匀地分布与作用于米粒的各个部位。结果表明 ,文章建立的二次蒸煮二次浸泡工艺框架效果良好 ,以此为基础 ,可望最终获食味良好 ,回生抑制效果显著的米饭制品。     

Amylolytic Hydrolysis of Native Starch Granules Affected by Granule Surface Area

ABSTRACT: Initial stage of hydrolysis of native starch granules with various amylolytic enzymes, α‐amylase from Bacillus subtilis, glucoamylase I (GA‐I) and II (GA‐II) from Aspergillus niger, and β‐amylase from sweet potato showed that the reaction was apparently affected by a specific surface area of the starch granules. The ratios of the reciprocal of initial velocity of each amylolytic hydrolysis for native potato and maize starch to that for rice with the amylolytic enzymes were nearly equivalent to the ratio of surface area per mass of the 2 starch granules to that of rice, that is, 6.94 and 2.25, respectively. Thus, the reciprocal of initial velocity of each enzymatic hydrolysis as expressed in a Lineweaver–Burk plot was a linear function of the reciprocal of surface area for each starch granule. As a result, it is concluded that amylolytic hydrolysis of native starch granules is governed by the specific surface area, not by the mass concentration, of each granule.     

Starch determination,amylose content and susceptibility to in vitro amylolysis in flours from the roots of 25 cassava varieties

BACKGROUND: Cassava cultivars are classified following different criteria, such as cyanogenic glucoside content or starch content. Here, flours from the roots of 25 cassava varieties cultivated simultaneously in a single plantation, were characterized in terms of starch content (SC), amylose content (AC), α‐amylolysis index (AI) and gel formation ability. Resistant starch content (RS) was measured in 10 of the samples. RESULTS: Cassava flours exhibited high SC, low AC and low AI values, with differences among varieties. Cluster analysis based on these parameters divided the cultivars in four groups differing mainly in SC and AC. AI and AC were inversely correlated (r = ? 0.59, P < 0.05) in 18 of the cultivars, suggesting AC as an important factor governing the susceptibility to enzymatic hydrolysis of starch in raw cassava. Differences in susceptibility to amylolysis, assessed by RS, were also recorded in the sample subset analyzed. Most flours yielded pastes or gels upon heating and cooling. Gels differed in their subjective grade of firmness, but none exhibited syneresis, confirming the low retrogradation proclivity of cassava starch. CONCLUSION: Some differences were found among cassava samples, which may be ascribed to inter‐cultivar variation. This information may have application in further agronomic studies or for developing industrial uses for this crop. Copyright © 2011 Society of Chemical Industry     

Selected functional properties of waxy corn and potato starches after illumination with linearly polarised visible light

Aqueous suspensions (30%) of waxy corn and potato starches were illuminated for 5–50 h with linearly polarised visible light (λ > 500 nm). Molecular weights (M?_w) and radii of gyration (R?_g) of the amylopectin and amylose fractions of illuminated waxy corn starch, and the amylopectin, intermediate, and amylose fractions of illuminated potato starch were measured by high‐performance size exclusion chromatography coupled with multiangle laser light scattering and refractive index detection. The weight‐average molecular weight (M?_w) and radius of gyration (R?_g) of the amylopectin fraction of native waxy corn starch were 14.45 × 10⁷ and 161.1 nm respectively. After 15 h of illumination a decrease in M?_w (5.80 × 10⁷) and R?_g (117.6 nm) was observed. Illumination for 25 h, led to an increase in M?_w (7.60 × 10⁷) and R?_g (134.0 nm). Further illumination, up to 50 h resulted in a slight decrease in M?_w (6.74 × 10⁷). The molecular weight and radius of gyration of the amylopectin fraction of native potato starch were 21.30 × 10⁷ and 207 nm respectively. Illumination for 15 h led to a decrease in M?_w (14.87 × 10⁷) and R?_g (141.5 nm), followed by an increase in both values after 25 h (18.97 × 10⁷, 146.6 nm) and 50 h (19.69 × 10⁷, 207.1 nm) of illumination. Illumination influenced the swelling power, solubility, susceptibility to α‐amylolysis and X‐ray diffractogram of the starches. A varying increase in the solubility passed through a minimum after 25 h of illumination. The X‐ray diffraction pattern and susceptibility to enzymatic hydrolysis of waxy corn starch did not change, but in potato starch a gradual, illumination time‐dependent increase in the amylolysis rate took place. This effect could result from the reduction in crystallinity of the starch as indicated by the X‐ray diffraction pattern. Copyright © 2003 Society of Chemical Industry     

COMPUTER SIMULATION OF THE β-AMYLOLYSIS OF STARCH COMPONENTS*

In an attempt to model quantitatively the product distributions arising from the degradation of starch by the conjoint action of α- and β-amylase, such as occurs in the commercial mashing of malted cereal, a computer program has been written to simulate the amylolysis of amylose or amylopectin or a mixture of these components. Using Monte-Carlo techniques the program simulates the action of either α-amylase or β-amylase or both enzymes acting conjointly. The program was tested for pure β-amylolysis. Parameters were estimated by a non-linear regression technique and predicted product concentrations were compared with relevant experimental data for amylose and amylopectin substrates and for mixtures of these starch components. The simulation model accurately predicted the results of amylolysis except at high product concentrations where the deviations between predicted and experimental values ranged from 1% to 4%.     

用基因重组技术构建可降解淀粉和产生酒精的酵母工程菌

将酵母Ty转座子的δ序列 ,黑曲霉糖化酶cDNA及G418抗性基因neo重组进酵母整合型质粒YIplac12 8获得含LEU2 ,neo双标记基因 ,糖化酶cDNA的高整合型表达载体YIp12 8D 17N ,转化GRF18后获得整合型酵母转化子GRF18(YIp12 8D 17N)糖化酶基因在该菌株获得高效表达 ,产物分泌到胞外。Southern印迹分析证明 ,糖化酶基因已整合进工程菌染色体。GRF18(YIp12 8D 17N)在含 2 0 %可溶性淀粉的培养基中培养 2 4h ,淀粉水解率在98%以上 ,酒精浓度达到 9 5 % (v/v)在 2 5 %淀粉中酒精浓度达到 10 2 %。     

Effect of shear stress extrusion intensity on plasticized corn flour structure: Proteins role and distribution

Plasticized corn flour‐based materials were prepared by extrusion and injection molding. Extrusion of corn flour blends (75% wet basis (wb)—glycerol (5 or 10% wb)—water) was performed in a twin‐screw extruder with either one or three shearing zones. Native corn flour is mainly composed of corn starch granules surrounded by proteins layers. Therefore, the destructuration of corn flour by thermomechanical treatments was analyzed (i) by techniques essentially allowing to monitor corn starch amorphization (differential scanning calorimetry, X‐ray diffractometry, determination of water sorption isotherms, susceptibility to hydrolysis by amylolytic enzymes) (ii) and via proteins layers role and distribution observed by confocal scanning laser microscopy and comparing the susceptibility of corn starch to hydrolysis by amylolytic enzymes in the presence or not of a protease. Both corn starch granules amorphization and proteins dispersion and aggregation were more pronounced for materials extruded in a screw profile with three shearing zones. For materials extruded in a screw profile with one shearing zone, the amorphization of starch was higher in materials made with 5% wb glycerol, whereas the proteins dispersion and aggregation was more pronounced in materials made with 10% wb glycerol. A barrier role of proteins to hydrolysis of corn starch by amylolytic enzymes was demonstrated and discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012