Many low-molecular weight molecules, including amino acids, saccharides, polyols, peptides, and synthetic compounds, are well known to elicit the sensation of sweetness, whereas most proteins are tasteless and flavorless. However, some proteins do elicit a sweet taste response on the human palate. Sweet-tasting proteins have potential as low-calorie sweeteners and as substitutes for sucrose for industrial applications, and could be useful in clarifying the mechanisms by which we perceive a sweet taste. However, despite a number of investigations assessing the relationship between sweetness and the structures of sweet-tasting proteins, no common feature has been identified in either their tertiary structures or amino acid sequences. However, most sweet-tasting proteins are basic and have high isoelectric points. Here, we first review site-directed mutagenesis and chemical modification studies on the charged residues of thaumatin and lysozyme. Efforts to increase the production yield of recombinant lysozyme and thaumatin from the yeast Pichia pastoris are then described. We conclude by introducing our recent investigations into the atomic-resolution structural analysis of thaumatin, and a cell-based assay using sweet taste receptors.

This study aimed to investigate the effect of young barley leaf extract powder (YBL) on the fecal composition of gut microbiota and cecal levels of short-chain fatty acids (SCFA) in rats. Rats were fed either a control diet, 2% (w/w) YBL diet, or 10% (w/w) YBL diet for 4 weeks. Compared to the control group, the 10% YBL diet significantly increased the cecal volume and the cecal SCFA levels. Notably, the bacterial community, as determined by terminal restriction fragment length polymorphism (T-RFLP) patterns for fecal 16 S ribosomal RNA gene, showed a significant dose-dependent increase in Prevotella sp. with YBL dietary supplementation. In conclusion, YBL affects the growth of certain genus of the gut microbiota in rats and is associated with the cecal levels of SCFA.

Miso products, such as instant miso paste, are seasonings made from fermented soybeans, and must be subjected to the inactivation of microbial spores to ensure a long shelf life. To inactivate spores in miso by conventional heating, it is necessary to heat the miso at a high temperature for a long time. However, heating can degrade the color and flavor of miso. In this study, continuous joule heating under pressure was employed to inactivate Bacillus subtilis spores in miso. A 2.7 log reduction in B. subtilis spores was achieved by applying joule heating, in which the temperature was raised to 98°C in 4 s, and then maintained for 4 s at 0.75MPa. No change in color was observed after joule heating, in contrast to the darkened color after conventional heating.

The behavior of Alicyclobacillus acidocaldarius and related species, isolated from food manufacturing environments and food ingredients, in acidic beverages was investigated. These bacteria are frequently detected in the raw materials of beverages. A. acidocaldarius and related species grew at temperatures between 40 and 55°C in beverages with an osmotic pressure from 250 to 300mOsm, so-called isotonic drinks. Ascorbic acid at 350mg/L inhibited growth, likely due to ascorbic acid’s consumption of dissolved oxygen in the isotonic drinks. In contrast, bacterial growth was observed in the absence of ascorbic acid. No bacterial growth was detected in fruit drinks with an osmotic pressure of 700mOsm in the absence of 350 mg/L ascorbic acid. Bacterial growth resulted in an off-odor and flavor deterioration of the drinks. Bacterial growth was promoted by the addition of 0.1% fruit juice in a glucose solution at 700mOsm. Moreover, the bacteria grew well even in the presence of 500mg/L ascorbic acid in a hypotonic drink with an osmotic pressure of 200mOsm. Our results suggest that A. acidocaldarius and related species are potential spoilage bacteria of acidic beverages, especially so-called hypotonic drinks. Therefore, bacterial decontamination using a membrane filter during the manufacturing process of acidic beverages is necessary. Further, detailed analysis of the beverage ingredients is important to control bacterial growth in acidic beverages.

We have developed a method for preventing histamine accumulation during fish paste processing. In this study, we investigated the suppressive effect of citric acid addition on histamine accumulation in fish paste. Furthermore, we tried to determine the optimal time for citric acid addition with respect to both suppression of histamine accumulation and the effects on the taste of the fish paste produced. We found that on adding citric acid during the preparation of fish paste (addition of citric acid anhydrous to obtain a 0.6% [w/w] final concentration), both the growth of histamine-producing bacteria and the accumulation of histamine were inhibited. The earlier citric acid was added during processing, the greater was its effectiveness in suppressing histamine accumulation. Furthermore, early addition of citric acid during the fermentation process suppressed the accumulation of various metabolic products of microorganisms, such as lactic acid and citric acid. Therefore, early addition of citric acid contributed to a reduction in sourness, which could be attributed to lower subsequent production of acidic metabolic products by microorganisms, without influencing the “umami” taste of the fish paste. We conclude that the ideal strategy for preventing histamine production without altering the taste of fish paste is to add citric acid to obtain a final concentration of 0.6% (w/w), as early as possible during processing.

Postprandial hyperglycaemia control is important to prevent or treat diabetes, and also to lower the risk of complications. The glycemic index (GI) is one of the indices for controlling postprandial blood glucose levels. Both blood glucose control in diabetic patients and GI determinations require frequent blood samplings by self-monitoring of blood glucose, which is a major burden for patients and subjects. Thus, a new non-invasive method for measuring blood glucose is highly desirable. We examined the GI for five kinds of combination meals such as rice with vegetables (cabbage, komatsuna, tomato, soybean, and nagaimo) using white rice as a reference food. In addition, the non-invasive blood glucose determination using near-infrared (NIR) spectroscopy was studied as part of the GI determination. Spectra in the short-wavelength NIR region from 700 to 1050nm were measured on the palm of the hand. For each 2-h GI test, the blood glucose level was estimated from the spectral intensity by using simple linear regression analysis. Our results indicate that the GI for all combination meals was below 100 as determined by the reference food, and the blood glucose value was accurately estimated using the NIR technique during each GI test.

The hyaluronidase-inhibitory activity of rhamnan sulfate, a sulfated polysaccharide obtained from cultivated Monostroma nitidum, was investigated in a hyaluronidase inhibition study using different concentrations (0.1 to 0.2mg/mL) of rhamnan sulfate. Consequently, rhamnan sulfate showed a concentration-dependent enhancement of the inhibition rate. In another hyaluronidase inhibition study, sulfated polysaccharides (fucoidan, porphyran, κ-carrageenan, λ-carrageenan, and rhamnan sulfate) were used to examine the relationship between the inhibition rate and the sulfate group content. Results showed that the activity tended to intensify with increases in the sulfate group content of sulfated polysaccharides, suggesting a strong association between the activity and the sulfate group.