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An amperometric L-glutamate oxidase (GLOD) electrode based on a multilayer of polymer films was developed for the high selective determination of L-glutamate. The multilayer film consisted of three layers as the following configuration i.e., inner membrane of electropolymerized 1,3-diaminobenzene, middle layer of L-GLOD entrapped in photopolymerized poly(vinylferrocene)–poly(ethylene glycol) hydrogel polymer, and outer dialysis membrane. In this manner, the sensor could eliminate interferences and was able to work at a low potential poised at +0.085 V vs. Ag/AgCl. When used in a flow injection system, the sensor responded to L-glutamate in the range 0.5–8.0 mM, with a sensitivity of 9.48 nA mM−1. The sensor was stable for 5 days of continuous uses (250 assays) and retained 60% activity after 16 days. When used to analyse the L-glutamate contents of a number of different off-the-shelf soy sauces, the sensor gave results in good agreement with the standard colorimetric method.

Spent brewer's yeast was autolysed and the residual solids were further treated with hot alkali to produce a fraction containing 95% (w/w) carbohydrate, with a glycogen content of 42% and an apparent glucan content of ca. 53% (w/w). FTIR and solid state C-NMR spectra basically showed a spectrum characteristic for an impure β-glucan. In vitro, this glucan induced release of prophenoloxidase from intact black tiger shrimp haemocytes. When fed to shrimp at 0.2% (w/w of the feed) for 3 days, significant increases in phenol oxidase, the number of haemocytes and the bacterial killing activity against the pathogen Vibrio harveyi were observed. Furthermore, this feeding resulted in the appearance of an inducible β-(1,3)-glucanase in the liver. Methods to analyse the fractions are discussed and a number of commercial products with potential immune stimulating properties have been included for comparative purposes.

The suitability of using a simple brewer's yeast extract (BYE), prepared by autolysis of complete beer slurry, for growth and sporulation of Bacillus thuringiensis kurstaki was studied in baffled shake flasks. In a standard buffered medium with 2.5% (w/v) glucose and 1% (w/v) brewer's yeast extract, growth of B. t. kurstaki resulted in a low biomass production with considerable byproduct formation, including organic acids and a concomitant low medium pH, incomplete glucose utilization and marginal sporulation, whereas growth in the same medium with a commercial laboratory-grade yeast extract (Difco) resulted in a high biomass concentration, complete glucose utilization, relatively low levels of byproducts and complete sporulation (2.6 × 109 spores/ml). When glucose was left out of the medium, however, growth parameters and sporulation were comparable for BYE and commercial yeast extract, but absolute biomass levels and spore counts were low. Iron was subsequently identified as a limiting factor in BYE. After addition of 3 mg iron sulphate/l, biomass formation in BYE-medium more than doubled, low byproduct formation was observed, and complete sporulation occurred (2.8 × 109spores/ml). These data were slightly lower than those obtained in media with commercial yeast extract (3.6 × 109spores/ml), which also benefited, but to a smaller extent, from addition of iron.

Spent brewer's yeast was autolysed and used as a raw material for the preparation of 5′-GMP-rich yeast extracts. Malt rootlets were used as a source of 5′-phosphodiesterase. The crude enzyme was extracted from malt rootlets and pretreated to inactivate 5′-nucleotidase. The optimum pretreatment conditions were heating at 65 °C for 30 min or 70 °C for 7 min. The effects of autolysis time, phosphodiesterase concentration and incubation period on 5′-GMP content were examined. The suitable autolysis time was 8 h. The preferable enzyme treatment period was in the range of 8–14 h. Longer autolysis and enzyme incubation periods caused a decrease in the 5′-GMP content from 0.7–0.9% (w/w) to 0.2–0.4% (w/w). The 5′-GMP content in extracts from debittered and non-debittered yeast was similar. The highest 5′-GMP content in yeast extract was 0.93% (w/w), obtained with a phosphodiesterase concentration of 1.6unit/ml of yeast extract (5% solids content).

The effect of high pressure homogenization (600 and 1000 bar) prior to autolysis of a commercial pressed baker's yeast was examined. High pressure homogenization released a maximum of 30% of the solids and 34% of the total nitrogen (TN). After autolysis of the whole homogenized slurry, high yields of solids and TN (up to 81 and 85%, respectively) were obtained. Autolysis of non-homogenized controls yielded much lower yield values (30 and 39%, respectively), whereas autolysis in the presence of papain but without prior disruption gave intermediate values (50 and 61%, respectively). The various treatments led to changes in the extract composition: standard autolysates had the highest total nitrogen and true protein weight contents and the lowest carbohydrate content, whereas this trend was reversed when cells were first disrupted before autolysis. In contrast to controls obtained by standard autolysis without or with papain, centrifuged autolysates from pre-homogenized fractions were not clear. Treatment with a combination of a flocculation and a weighting agent clarified the extracts but resulted in a loss of solids (approximately 20%), including nitrogen and carbohydrates.

A flow injection analysis (FIA) system with a modified electrode has been developed and optimized for determination of L-glutamate using L-glutamate oxidase (GLOD) (EC 1.4.3.11). GLOD was immobilized on controlled-pore glass using glutaraldehyde. The optimal potential applied on the working electrode was +700 mV against a platinum (Pt) reference electrode. The optimal pH and flow rate of the carrier buffer were 7.4 and 1.5 ml/min, respectively. A modified electrode was integrated into the FIA system in order to eliminate electroactive interference and it was used to determine L-glutamate in 39 samples of Thai commercial soy sauce products. The results obtained were compared with those obtained from enzymatic assay using glutamate dehydrogenase and those from a chromatographic assay using an amino acid analyser. Good correlations were observed amongst these methods. The results indicated that use of an FIA system with a modified electrode was able to eliminate electroactive interference and was applicable to the determination of L-glutamate in food samples. The modified FIA was faster and simpler than the more common methods of enzymatic and chromatographic analysis.