Effect of water hardness on antibacterial activity of green tea extract was studied using tap water and mineral water. The agar medium containing green tea made with six samples of tap water or two samples of mineral water, were tested for Escherichia coli culture, and the colony forming number was compared with that on the agar medium containing each water. Increasing water hardness resulted in increasing antibacterial activity of green tea. Using the high hardness (1468 mg/L) mineral water, the ratio of colony forming number on the green tea medium to one on the water medium was 0.016, which is 43 times lower than that on the low hardness (18 mg/L) tap water. The principal component analysis based on the concentrations of mineral elements in water samples, suggested that the high concentrations of Ca and Mg in water, and low concentration of Si increased antibacterial activity of green tea. The water hardness increased the amount of (-)-epicatechin gallate, not the total amount of catechins in green tea extract. The results are discussed considering the formation of complexs with catechin with Ca and Mg and the change in antibacterial activity of green tea.
To keep the safety of oyster intended for raw eating, monitoring of norovirus (NoV) contamination by rapid and sensitive method is important. We had developed a rapid method using bead homogenization to recover NoV particles from oyster tissues followed by RNA detection using reverse-transcription, real-time PCR. But there were still remains a need to improve the sensitivity. Therefore we re-evaluated bead homogenization conditions and studied the usefulness of enzymatic digestion after the bead homogenization. In the comparison of homogenization condition (speed and time), the mildest condition (3,500 rpm for 15 sec) showed the highest positive rate (60%) of NoV. On the contrary, when proteinase K digestion was added after bead homogenization, high intensity homogenization at 4,500 rpm for 15 sec showed high positive rate compared with the low intensity homogenization. In the comparison of digestion with two different enzymes, proteinase K and α-amylase, that digest organic compounds in digestive tissues after the bead homogenization, α-amylase digestion showed higher NoV GII RNA copy number (p<0.01) in average. At least 60-min digestion time by α-amylase was required. These results show that the bead homogenization method coupled with α-amylase digestion is useful to recover NoV particles in oyster digestive tissues.