Reducing Sugar Production in Sweet Potatoes Heated by Electromagnetic Radiation

The apparent reaction rate constant needed to generate reducing sugar was determined by heating a thin slice of sweet potato using thermal conductive heating. This value was used to predict reducing sugar production in sweet potatoes cooked by electromagnetic irradiation. The generation of reducing sugar in the thin slice was not observed at temperatures <65°C or >85°C, but it increased linearly during the early stage of heating. The Arrhenius plot had a peak of approximately 83°C, allowing determination of the values for activation energy and frequency factor. Then, using the values obtained for apparent reaction rate constant, the yields of reducing sugar in sweet potatoes cooked by infrared (IR) and microwave (MW) heating were calculated and compared with experimental data. Although the calculated values exceeded the experimental values in the early stages of electromagnetic irradiative heating, the calculated amounts of reducing sugar generally agreed with the experimental values. Moreover, when the time needed to heat the sweet potato from 65°C to 85 °C was longer than approximately 8 min, the yield of reducing sugar was maximized for both MW and IR heating. These results indicated that the yield of reducing sugar did not depend on the heat transfer mechanism and that the amount of reducing sugar produced in heat-treated sweet potatoes could be predicted.

Quantitative evaluation of antifungal activity of metallic oxide powders (MgO, CaO and ZnO) by an indirect conductimetric assay

AIMS

To evaluate antifungal activities of MgO, CaO and ZnO powders quantitatively by indirect conductimetric assay.

METHODS AND RESULTS

Candida albicans NBRC1060, Saccharomyces cerevisiae NBRC1950, Aspergillus niger NBRC4067 and Rhizopus stolonifer NBRC4781 were used as test micro-organisms. The indirect conductimetric assay, in which the change in electrical conductivity of an alkaline solution (NaOH) is produced by absorption of CO2 from microbial metabolism, could offer a simple and rapid evaluation of the antifungal activity within 24-48 h. The conductivity curves obtained for MgO, CaO and ZnO were analysed using the growth inhibition kinetic model proposed by Takahashi for calorimetric evaluation, and the kinetic parameters and minimum inhibitory concentration ([I]100) could be determined. MgO and CaO powders exhibited the antimicrobial activities against all fungi used in this study and showed little differences between types of fungi. However, although ZnO powder inhibited fungal growth, the values of [I]100 were over 100 mg ml-1.

CONCLUSIONS

Although a common method for evaluating antifungal activity requires over 5-7 days, the indirect assay could provide a rapid and quantitative evaluation of antifungal activity within approx. 2 days, and MgO and CaO were found to have antifungal activities.

SIGNIFICANCE AND IMPACT OF THE STUDY

The indirect assay can be applicable for simple and rapid evaluation of the antimicrobial activity of insoluble or slightly soluble materials with high turbidity such as antibacterial ceramic powders. Moreover, these materials can be useful for controlling fungi in food processing and the environment.

Measurement of fungi by an indirect conductimetric assay

AIMS

To investigate the growth of fungi using an indirect conductimetric assay and derive, experimentally and theoretically, the relationship between microbial concentration and electrical conductivity change.

METHODS AND RESULTS

The indirect assay, in which change in electrical conductivity of an alkaline solution (NaOH) is produced by absorption of CO2 from microbial metabolism, was conducted with the Bactometer (bioMerieux, Marcy-l'Etoile, France) for the enumeration of fungi. A linear relationship was obtained between detection time and logarithmic initial microbial concentration. This indirect assay used growth media, which could not be used in the direct conductimetric assay, to monitor fungal growth.

CONCLUSIONS

The indirect assay does not depend on the growth media and the turbidity of sample and could offer a simple and rapid assay for the measurement of fungal growth under various conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

The indirect assay is applicable for rapid detection of fungi, estimation of the growth rate and evaluation of antifungal activity.

Quantitative evaluation of antibacterial activities of metallic oxide powders (ZnO, MgO and CaO) by conductimetric assay

Antibacterial activities of metallic oxide (ZnO, MgO and CaO) powders against Staphylococcus aureus and Escherichia coli were quantitatively evaluated by measuring the change in electrical conductivity of the growth medium caused by bacterial metabolism (conductimetric assay). The obtained conductivity curves were analyzed using the growth inhibition kinetic model proposed by Takahashi for calorimetric evaluation, and the metallic oxides were determined for the antibacterial efficacy and kinetic parameters. The parameters provide some useful indicators for antimicrobial agents, such as the dependence of antibacterial activity on agent concentration, and the affinity between the agent and the bacterial cells. CaO was the most effective, followed by MgO and ZnO, against E. coli. On the other hand, ZnO was the most effective for S. aureus and was suggested to have a strong affinity to the cells of S. aureus.

Indirect conductimetric assay of antibacterial activities

The applicability of indirect conductimetric assays for evaluation of antibacterial activity was examined. The minimal inhibitory concentration (MIC) obtained by the indirect method was consistent with that by the direct conductimetric assay and the turbidity method. The indirect assay allows use of growth media, which cannot be used in the direct conductimetric assay, making it possible to evaluate the antibacterial activity of insoluble or slightly soluble materials with high turbidity, such as antibacterial ceramic powders.

Photoreceptor cell differentiation in retinoblastoma demonstrated by a new immunohistochemical marker mucin-like glycoprotein associated with photoreceptor cells (MLGAPC)

AIMS

For further understanding of specific differentiation in retinoblastoma, we studied the expression of newly detected mucin-like glycoprotein associated with photoreceptor cells (MLGAPC), which is specific for photoreceptor cells of retina and analogous to interphotoreceptor matrix proteoglycan-1 (IMPG1).

METHODS AND RESULTS

Surgically enucleated retinoblastomas (n=21; undifferentiated type, n=15, differentiated type, n=6) were immunohistochemically studied with a polyclonal antibody against MLGAPC, and 17/21 cases (81%) showed positive staining of tumour cells. We classified various staining patterns and structures into four groups: type 1 showing a granular intracellular scattered staining pattern with round small cells; type 2 showing a reticular staining pattern between spindle-shaped tumour cells; type 3 showing radiating staining from the centre of Homer-Wright rosettes; type 4 showing ring-shaped, radiating and granular staining associated with Flexner-Wintersteiner rosettes. Eleven of 15 undifferentiated retinoblastomas (73%) showed type 1 or 2, and all the six differentiated cases showed type 3 or 4. Image analysis of immunostaining revealed an increase in MLGAPC-positive area from 0.48% in undifferentiated cases to 1.60% in differentiated cases, and a negative correlation was shown between mitotic frequency and MLGAPC-positive area.

CONCLUSIONS

This study proved MLGAPC as a valuable marker of retinoblastoma, and that photoreceptor differentiation takes place even in 'undifferentiated' retinoblastoma.

Kinetic analysis of the bactericidal action of heated scallop-shell powder

Shell powder of scallop (Patinopecten yessoensis) was exposed to heat treatment at between 200 and 1000 degrees C, and the bactericidal action of the powder slurry was investigated. Shell powder heated at 700 degrees C or higher exhibited bactericidal action against Escherichia coli, Salmonella typhimurium, Staphylococcus aureus and Bacillus subtilis (vegetative cells). The death of bacteria in the shell powder slurry followed first-order reaction kinetics, and the apparent death rate constant (k) was determined. An increase in exposure temperature enhanced the bactericidal action. The bactericidal action is due to calcium oxide that is converted from calcium carbonate, which is the main component of the shell powder, by heat treatment. The slurry temperature is found to significantly affect the bactericidal action of the shell powder. The slope of the Arrhenius plot of k for E. coli and S. aureus that were grown at 37 degrees C exhibited a discontinuous point at approximately 22 degrees C, at which the values of activation energy for the death of bacteria in the powder slurry changed. This temperature corresponds to that of the phase transition of cell membrane lipids. The bactericidal action of the shell powder is greater than that of a NaOH solution of identical pH. Although the pH of the shell powder slurry is high, the slurry was considered to possess other antibacterial mechanisms in addition to that of alkalinity.

Heated scallop-shell powder slurry treatment of shredded cabbage

The main component of scallop-shell powder is calcium carbonate (CaCO3). Through heat treatment, CaCO3 in the shell is converted to CaO, which exhibits antibacterial activity. The disinfecting effect of heated scallop-shell powder on shredded cabbage was investigated for various powder concentrations (0.1 to 1.0 g dm(-3)) and treatment temperatures (10 to 40 degrees C). Scallop-shell powder treatment was found to reduce the aerobic bacteria count in cabbage, with increasing effectiveness at higher powder concentrations and treatment temperatures. Coliforms were completely eliminated within 5 min with as little as 0.1 g dm(-3) powder treatment. During storage at 4 degrees C, aerobic bacterial counts did not increase after powder treatment, whereas counts increased with water-washing or sodium hypochlorite treatment at 200 microg dm(-3). The inactivation pattern of bacterial cells in shredded cabbage involved an accelerated decline followed by an extended tail at powder concentrations of 0.1 and 0.5 g dm(-3). We postulate that a fraction of bacterial cells in the initial population becomes tolerant to the shell powder. A proposed model accurately predicts the reducing bacterial counts on shredded cabbage by scallop-shell powder treatment. The decrease in the L-ascorbic acid content of shredded cabbage was approximately 20 to 30% for scallop-shell powder treatment at 0.1 and 0.5 g dm(-3) (20 degrees C), which is almost identical to that by sodium hypochlorite treatment at 200 micorg dm(-3).