BIOCHEMICAL PHYSIOLOGY OF OBESUMBACTERIUM PROTEUS,A COMMON BREWERY CONTAMINANT

In defined media, arginine plus either glutamate or aspartate supports growth of Obesumbaeterium proteus but additional amino acids enhance yields. Glucose, fructose, mannose and galactose are rapidly utilized; maltose and starch give slower growth. Fermentation products include acetic, formic, 2-oxoglutaric, pyruvic and succinic acids. With brewer's wort as growth medium, the concentrations of arginine, lysine, NH₃, aspartate, serine, tyrosine and leucine decrease whereas several other amino acids increase in concentration. Fermentation products include acetoin, ethanol, lactic acid, fusel alcohols, some volatile acids and dimethylsulphide. Measurements of oxygen uptake by washed suspensions in the presence of a variety of substrates suggest synthesis of (i) a terminal electron transfer chain that is repressed anaerobically and (ii) a glucose oxidase complex. Radio-respirometry indicates that the EMP is not the sole pathway for glucose metabolism. Cytochrome b₁ is detected in the bacterial cells but not cytochromes a and c. Glucose represses the activities of several enzymes of the EMP and HMP, but not lactate dehydrogenase and 3 enzymes of the citric acid cycle.     

EFFECT OF OXYGEN CONCENTRATION ON THE BIOCHEMICAL AND CHEMICAL CHANGES OF STORED LONGAN FRUIT

ABSTRACT

Longan fruits were stored for 6 days in atmosphere of 5, 21 (air) or 60% O₂ (balance N₂) at 28C and 90–95% relative humidity to examine effects of low and high O₂ concentration on enzymatic browning and quality attributes of the fruit. Changes in pericarp browning, pulp breakdown, disease development, total phenol content, activities of phenol metabolism‐associated enzymes, relative leakage rate, α,α‐diphenyl‐β‐picrylhydrazy (DPPH) radical scavenging activity, and contents of total soluble solids, titratable acidity and ascorbic acid were evaluated. Storage of fruit in a 5% O₂ atmosphere markedly delayed pericarp browning in association with maintenance of high total phenolic content and reduced activities of polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonia lyase. Moreover, the fruit stored in a 5% O₂ atmosphere exhibited a lower relative leakage rate and higher DPPH radical scavenging activity than fruit stored in air. This presumably was beneficial in maintaining compartmentation of enzymes and substrates, and thus, reducing pericarp browning. Pulp breakdown and disease development were also reduced by exposure to a 5% oxygenatmosphere. On the contrary, exposure of longan fruit to a 60% O₂ atmosphere accelerated pericarp browning, pulp breakdown and decay development. PPO and POD activities and relative leakage rate were similar for control and 60% O₂‐treated fruit after 4 and 6 days of storage. Furthermore, treatment with 60% O₂ significantly decreased the phenolic content and DPPH scavenging activity of fruit. In addition, exposure to 5 or 60% O₂ resulted in a higher level of total soluble solids, but a lower level of ascorbic acid of longan fruit flesh. In conclusion, exposure to a 5% O₂ atmosphere showed great potential to reduce pericarp browning and extend shelf life of longan fruit.

PRACTICAL APPLICATIONS

Pericarp browning and pulp breakdown are the major causes of deterioration in postharvest longan. Conventional controlled atmosphere with low O₂ and high CO₂ is effective in maintaining quality and extending shelf life of fruits and vegetables, including inhibition of tissue browning. In this study, 5%‐controlled atmosphere reduced significantly pericarp browning, pulp breakdown and rot development. It could potentially be useful as a postharvest technology of longan fruit for reducing or replacing the use of chemicals such as SO₂ and fungicides, but it requires further investigation.     

RECENT ADVANCES IN THE RAPID DETECTION OF BREWERY MICRO-ORGANISMS AND DEVELOPMENT OF A MICROCOLONY METHOD

Recent developments in the rapid detection of low concentrations of micro-organisms are reviewed. A modification of the microcolony method, involving uptake of optical brighteners by developing yeast colonies, is described. The method includes the use of incident light microscopy and appears to offer advantages over other detection methods.     

DETECTION OF WILD YEASTS IN THE BREWERY

The potential of an established culture medium, Wallerstein Laboratories' nutrient agar, for the detection of wild yeasts has been evaluated and recommended for microbiological quality control in brewery laboratories. Wild yeast strains, including Saccharomyces species, can be differentiated from strains of Saccharomyces cerevisiae by the colour, form and rate of growth of their colonies on this medium within 2–3 days. The sensitivity of the method is such that one wild yeast can be detected in a Saccharomyces cerevisiae population of the order of 10° cells.     

THE RAPID DETECTION OF BREWERY SPOILAGE MICRO-ORGANISMS

Recent developments in the rapid detection of low concentrations of micro-organisms are discussed and their relevance to improved brewery quality control methods is assessed. A method based on the change in pH of a general purpose growth medium appears to offer the advantages of both time and sensitivity over methods currently in use.     

新型O2指示剂用于气调包装的研究

利用皂石的反射紫外线作用,在表面活性剂的作用下将亚甲蓝、抗坏血酸引入皂石的层间结构中,从而制成一种新型的氧气指示剂并解决了亚甲蓝易于被光降解的问题,克服了其它变色型氧气指示剂需要避光保存的问题。该氧气指示剂在氧气含量小于0.1%时为红色,当氧气含量上升到0.5%时迅速变为蓝色,具有较灵敏的检测氧气的功能。并且探讨了表面活性剂在氧气指示剂中的作用。     

NEW METHOD FOR THE DETERMINATION OF HOT-WATER EXTRACT OF MALT

A new method for the measurement of hot-water extract of malt is described. In this method malt grist is continuously extracted in a Soxhlet apparatus under reduced pressure, the pressure being chosen such that the extraction is carried out at 65·5° C. In a comparison of malt extracts by the new method and by the Standard Institute of Brewing method the former gives lower extract values. The new method provides certain advantages: it requires a smaller malt sample, is rather quicker to carry out, requires less manipulation and avoids the problem of accounting for spent grain volumes.     

NEW METHOD FOR THE DETERMINATION OF YIELD STRESS1

A method is proposed for determining the yield stress of liquid and semi-liquid foods. The described technique makes use of a modified Fisher Autotensiomat apparatus. A yield stress range of from 20 dynes/cm² to about 900 dynes/cm² was covered by products such as condensed milk and tomato paste, respectively. The data were compared with the results obtained with a Ferranti-Shirley cone and plate viscometer, a Chainomatic Beckers balance and a Thomas-Stormer viscosimeter. With the exception of the data from the latter instrument, the results were in good agreement.     

THE INCIDENCE AND SIGNIFICANCE OF WILD SACCHAROMYCES CONTAMINANTS IN THE BREWERY

The persistence of low levels of contamination by non-brewing Saccharomyces through several batch fermentations establishes the immuno-fluorescent method as a very sensitive procedure for estimating the microbiological purity of pitching yeasts. Trade return figures for draught beers show that in this brewery the principal cause for high rejection rates has, on several occasions, been contamination of pitching yeasts with “wild” Saccharomyces. The recommendation is made that pitching yeasts should be discarded when the level of infection achieves 100 cells of wild Saccharomyces per million cells of brewing yeast.     

DETECTION OF WILD YEASTS IN THE BREWERY EFFICIENCY OF DIFFERENTIAL MEDIA

The efficiency of five differential media in the detection of wild yeasts was compared. On the basis of the types of wild yeast they are able to detect, these differential media were classified into two groups. Group I consists of actidione medium and lysine medium, and is suitable for the detection of non-Saccharomyces wild yeasts. In this group, the lysine medium detected more species and a higher percentage of wild yeasts than the actidione medium. Group II consists of crystal violet medium, SDM, and Lin's medium, and is suitable for the detection of Saccharomyces wild yeasts. In this group, Lin's medium is superior to crystal violet medium and SDM for wild yeast detection. It is suggested that lysine medium and Lin's medium be employed together for the detection of wild yeasts in the brewery.     

FACTORS AFFECTING THE SURVIVAL OF LYOPHILIZED BREWERY YEAST STRAINS

Some of the factors that contribute to the loss of viability of brewery yeast strains during lyophilization (freeze-drying) have been investigated. A lyophilization technique for the maintenance of brewery yeast strains with higher viabilities than those previously reported has been developed. Three lyophilized strains of ale yeast still had a survival rate of 60% after periods of storage of up to three years, while a lager yeast strain maintained a viability of approximately 50% during storage for eighteen months.     

APPLICATION OF THE POLYMERASE CHAIN REACTION TO THE RAPID ANALYSIS OF BREWERY YEAST STRAINS

The rapid discrimination of closely-related Saccharomyces cerevisiae strains can pose a significant problem to breweries, in particular where closely related strains are being used simultaneously to manufacture different products. In this study, two PCR approaches have been examined to assess their usefulness for the discrimination of brewery ale and lager yeast strains. PCR using arbitrary primers (RAPD PCR) was found unsuitable for such an application since the DNA profiles generated from brewery strains were generally found to be identical, due presumably to the close genetic relatedness of these yeasts. In contrast, PCR using δ sequence primers could rapidly differentiate between many ale and lager strains and characteristic profiles for these were generated. This method could also be applied directly to yeasts isolated from brewery worts or from active dried yeast preparations. Results of such analyses were available within the working day.     

THE VALUE OF LONG-CHAIN FATTY ACID COMPOSITION IN THE IDENTIFICATION OF SOME BREWERY YEASTS

The cellular long-chain fatty acids of 72 strains representing 29 species of brewery yeasts were extracted by saponification and analysed as methyl esters by gas chromatography. With this method, it was possible to distinguish between Saccharomyces cerevisiae and five other groups from the brewery industry, only four hours after they were obtained from 48 h old cultures under standard conditions. This compares favourably with the 7 to 10 days for conventional methods.     

PREDICTING THE ELECTRORHEOLOGICAL BEHAVIOR OF MILK CHOCOLATE

Electrorheology is the study of the effects of electric fields on the flow properties of fluids. These materials generally exhibit an increase in apparent viscosity and a greater yield stress over an unelectrified sample. The phenomenon requires an electric field and polar particles suspended in an insulating oil. A standard concentric cylinder viscometer, fitted with custom made electrical attachments, was converted into an electrorheometer. This system allowed control of DC voltage (0–450 Volts mm^?1), fluid temperature (35C–40.5C), and shear rate (0.022 s^?1– 8.744 s^?1) while observing the resulting effects on the shear stress. Dimensional analysis was used to study the rheological response of milk chocolate when subjected to electric fields. Dimensionless groups were identified to explain the phenomena, and multiple regression analysis was used to predict the electrorheological flow behavior of milk chocolate. Over the ranges of input variables, the electric field-induced forces dominated the rheological response over thermal forces, and a prediction equation for apparent viscosity was developed as a function of electric field strength and shear rate. This mathematical expression allows for prediction of milk chocolate rheology in the presence of an electric field.     

THE MAJOR BIOCHEMICAL CONSTITUENTS OF AN ACID EXTRACT OF BARLEY FLOUR AS USED IN VISCOSITY DETERMINATIONS BY THE FALLING BALL METHOD

The viscosity of an acid extract of barley flour, measured as its ‘falling time’, has been used as a guide to malting potential.²² One such extract, made from a sample of Maris Mink, was examined in detail to identify the principle biochemical constituents and to investigate their contribution to viscosity. The extract contained a high concentration of carbohydrate (3·30 mg/ml) divided into two discrete fractions of high and low molecular weight (in the weight ratio of 39:61) by gel filtration. The high molecular weight component was polydisperse and consisted principally of β-glucan with some starch and pentosan also present. The low molecular weight fraction contained mostly fructose, glucose and some pentose. Protein present in the extract (1·07 mg/ml) was derived almost exclusively from the albumin and globulin (salt soluble) fraction of barley protein. Studies on the viscosities of reconstituted preparations of these components indicated that β-glucan made the major contribution to the viscosity of the extract. This conclusion is supported by observed changes in viscosity after the extract was treated with β-glucanase, α-amylase and protease.