Starch digestibility of various Japanese commercial noodles made from different starch sources.

To examine the effect of starch variety and structure-related attributes on noodle starch digestibility, cooked Japanese commercial noodles, such as Harusame (mung bean starch), Malony (potato starch) and Udon (wheat flour), were kinetically analyzed during simulated in vitro digestion. The kinetic constant of the homogenized noodle slurry (10.8 × 10-2-22.9 × 10-2) was 10-20 times higher than that of the intact one (0.9 × 10-2-1.5 × 10-2), whereas the equilibrium starch hydrolysis (%) was equivalent. Therefore, the noodle microstructure, which is influenced by the present of binding agent and enhancer, should relate to the starch digestibility. Also, the starch digestibility at the early small intestine phase was associated with the cohesiveness and water retention ability of the cooked noodles. These results indicate that the internal microstructures of cooked noodles and the binding agent properties play an important role in the starch digestibility of noodles.

Studies of the texture, functional components and in vitro starch digestibility of rolled barley.

This study aimed to describe the effects of the cooking condition, including water ratio (WR) and soaking time (ST), on the texture of rolled barley, and to evaluate its ß-glucan content, total phenol content and in vitro starch digestibility. Rolled barley (75 g) was soaked in 1.5×, 2.0× and 2.5× weight of aliquots of Milli-Q water at room temperature, for 30 and 90 min, respectively, and then cooked using a rice cooker. Barley and rice were also pearled (polished), cooked and examined. The WR and ST did not affect the adhesiveness of rolled barley but were negatively related to its firmness. Rolled barley and barley had higher ß-glucan and polyphenol contents and less starch hydrolysis during in vitro digestion than rice. The starch granules in rolled barley are enclosed by thick cell walls, which may be responsible for its firmness and low starch digestibility.

The importance of an oral digestion step in evaluating simulated in vitro digestibility of starch from cooked rice grain.

To examine the effect of oral digestion step in a simulated in vitro starch digestion model, the digestibility of intact, homogenized and actual chewed cooked rice grains was investigated and analyzed. The kinetics of starch digestibility were calculated from changes in the hydrolysis percent of starch that were achieved during simulated small intestinal digestion stage. Morphological and histological microscopic tissue structures were also examined. Compared with the trend of starch hydrolysis changes of the actual chewed grain, 1.3U/ml of salivary α-amylase concentration treated for 60min was regarded as a mimicked condition to the simulate in vitro oral digestion step in this study. The results showed that the equilibrium percent of starch hydrolysis for all of the samples ranged from 84.2% to 95.9% with no significant differences observed regardless of whether the oral digestion step was included (p>0.05). In contrast, the kinetic constant, which is one of the measure of starch digestion rate during small intestinal stage, significantly increased with the degree of grain homogenization increased: 120s>actual chewed ≥1s>intact, for both the gastrointestinal and oral plus gastrointestinal processes. These results indicated that the kinetic constant was influenced by the change of cooked rice grain structure in oral digestion step that would be related to increase in enzyme accessibility to rice starch. Thus, rice grain digestibility was affected by grain-scale structural changes, including grain tissue damages which were normally observed during the oral digestion step.

Impact of structural characteristics on starch digestibility of cooked rice.

To examine the impact of structural characteristics of cooked rice grains on their starch digestibility, a simulated in vitro gastro-small intestinal digestion technique was applied to intact and homogenised cooked rice samples. The starch hydrolysis percentage increased during simulated small intestinal digestion, in which approximately 65% and 24% of the starch was hydrolysed within the first 5min, for homogenised and intact cooked rice, respectively. The kinetic constant of homogenised cooked rice, which was regarded as an estimated digestion rate, was ∼8 times higher than the intact cooked rice. The homogenised and intact samples were also examined for any microstructural changes occurring during the in vitro digestion process using fluorescent and scanning electron microscopy. In the intact samples, the aleurone layers of the endosperm remained as thin-film like layers during in vitro digestion and thus may be regarded as less digestible materials that influence cooked rice digestibility.

Impact of the degree of cooking on starch digestibility of rice - An in vitro study.

The impact of cooking degree on the starch digestibility of a non-waxy, cooked rice was examined through a simulated gastro-small intestinal in vitro digestion model. The starch hydrolysis of both non-homogenised and homogenised cooked rice samples during simulated digestion was also compared in order to examine the impact of grain structure on starch digestibility. Polished rice grains were cooked in boiling water for 10min (partially cooked) and 20min (fully cooked) to obtain samples of different cooking degree. No significant differences in the equilibrium starch hydrolysis (%) were observed among the partially and fully cooked grains, although significant differences were observed among the uncooked and cooked samples. The equilibrium starch hydrolysis (%) of homogenised rice was higher than its non-homogenised counterpart. These results showed that rice starch digestibility should not be affected by the cooking degrees related to starch gelatinisation, but was influenced by the modification/destruction of the grain structure during mechanical processing.

Microbial conversion of glucose to a novel chemical building block, 2-pyrone-4,6-dicarboxylic acid.

2-Pyrone-4,6-dicarboxylic acid (PDC) is a catabolic intermediate in Sphingobium sp. SYK-6 (previously characterized as Sphingomonas paucimobilis SYK-6), which is a degrader of lignin-derived aromatic compounds. Recently, PDC has been also characterized as a novel starting material for several potentially useful synthetic polymers. In a previous study, we constructed a biosynthetic system in which PDC was generated efficiently from a chemically synthesized compound, protocatechuate. In order to develop an alternative system for production of PDC, we tried to generate it from glucose, which is a low-cost sugar that can be obtained from abundant cellulosic wastes and biomass crops. We designed a metabolic bypass to PDC from the shikimate pathway in recombinant Escherichia coli cells. PDC accumulated in the medium of recombinant E. coli cells that had been transformed with genes isolated from Emericella niger, E. coli, Pseudomonas putida, and Sphingobium sp. SYK-6. The yield of PDC depended on the combination of genes that we introduced into the cells and on the specific of host strain. Under optimal conditions, the yield and titer of PDC were, respectively, 17.3% and 0.35 mg/l when the concentration of glucose was 2 g/l and the culture volume was 50 ml. Our results open up the possibility of novel utilization of biomass as the source of a useful chemical building block.