Occurrence of aflatoxins, ochratoxin A, and fumonisins in retail foods in Japan

We conducted a survey of aflatoxin B1, B2, G1, and G2, ochratoxin A, and fumonisin B1, B2, and B3 contamination in various foods on the retail market in Japan in 2004 and 2005. The mycotoxins were analyzed by high-performance liquid chromatography, liquid chromatography-mass spectrometry, or high-performance thin-layer chromatography. Aflatoxins were detected in 10 of 21 peanut butter samples; the highest concentration of aflatoxin B1 was 2.59 microg/kg. Aflatoxin contamination was not found in corn products, corn, peanuts, buckwheat flour, dried buckwheat noodles, rice, or sesame oil. Ochratoxin A was detected in oatmeal, wheat flour, rye, buckwheat flour, green coffee beans, roasted coffee beans, raisins, beer, and wine but not in rice or corn products. Ochratoxin A concentrations in contaminated samples were below 0.8 microg/kg. Fumonisins were detected in popcorn, frozen corn, corn flakes, and corn grits. The highest concentrations of fumonisins B1, B2, and B3 in these samples were 354.0, 94.0, and 64.0 microg/kg, respectively.     

Enumeration of fungi in fruits by the most probable number method

In this study, enumeration methods for fungi in foods were evaluated using fruits that are often contaminated by fungi in the field and rot because of fungal contaminants. As the test methods, we used the standard most probable number (MPN) method with liquid medium in test tubes, which is traditionally used as the enumeration method for bacteria, and the plate-MPN method with agar plate media, in addition to the surface plating method as the traditional enumeration method for fungi. We tested 27 samples of 9 commercial domestic fruits using their surface skin. The results indicated that the standard MPN method showed slow recovery of fungi in test tubes and lower counts than the surface plating method and the plate-MPN method in almost all samples. The fungal count on the 4th d of incubation was approximately the same as on the 10th d by the surface plating method or the plate-MPN method, indicating no significant differences between the fungal counts by these 2 methods. This result indicated that the plate-MPN method had a number agreement with the traditional enumeration method. Moreover, the plate-MPN method has a little laborious without counting colonies, because fungal counts are estimated based on the number of plates with growing colonies. These advantages demonstrated that the plate-MPN method is a comparatively superior and rapid method for enumeration of fungi.     

Modeling and predicting the simultaneous growth of Listeria monocytogenes and natural flora in minced tuna

The growth kinetics of Listeria monocytogenes and natural flora (NF) in minced tuna from 2 to 30 °C were examined, and a simultaneous growth model was developed. The inhibiting effect of the NF on the growth of L. monocytogenes was examined by inoculating different levels of NF isolated from the minced tuna. The kinetic data were fitted to the Baranyi model and estimated the growth parameters such as specific growth rate (μ(max)), maximum population density (N(max)), and lag time. The temperature and inoculated NF dependency on the μ(max) of L. monocytogenes and NF were described by modified Ratkowsky's square-root model. As the initial NF level increased, the slopes of the square-root models were decreased for both L. monocytogenes and NF. The N(max) of L. monocytogenes was described as a function of temperature and inoculated NF level. Simultaneous growth prediction of L. monocytogenes and NF under constant temperature conditions was examined by using the differential equations based on the Baranyi model with the effect of interspecies competition substituted into the developed μ(max) and N(max) models. The root mean square errors between the model prediction and the observation for L. monocytogenes and NF were 0.42 and 0.34, respectively. Predictive simulation under fluctuating temperature conditions also demonstrated a high accuracy of simultaneous prediction for both L. monocytogenes and NF, representing the root mean square errors of 0.19 and 0.34, respectively. These results illustrate that the developed model permits accurate estimation of the behavior of L. monocytogenes in minced tuna under real temperature history until consumption.     

Occurrence of beauvericin and enniatins in wheat flour and corn grits on the Japanese market,and their co-contamination with type B trichothecene mycotoxins

The contamination levels of beauvericin and four enniatins, A, A₁, B and B₁, in 207 samples of wheat flour and corn grits on the Japanese market were determined by an analytical method based on LC-MS/MS. The toxins were extracted from samples with acetonitrile–water (85:15, v/v) and then purified with C18 cartridges. The method was validated in a single laboratory using spiked samples at two levels; the recovery of the five toxins ranged from 91.1% to 113.8%. Enniatin B was frequently detected in imported wheat flour (81.8%) and domestic wheat flour (85.6%), and the highest concentration of enniatin B was present in a domestic wheat sample (633 μg kg^?1). In corn grits, beauvericin was found in 34% of the samples, but enniatins were not detected at all. The maximum concentration of beauvericin in corn grits was 26.1 μg kg^?1. Deoxynivalenol and nivalenol in the same samples were determined by a method using an immunoaffinity column. Co-contamination of deoxynivalenol and enniatins was observed in 61% of the imported wheat samples and in 58% of the domestic wheat samples. These results suggest the need for a risk assessment for cyclic depsipeptide mycotoxins in Japan and a study on the synergistic effect of deoxynivalenol and enniatins.     

Crystal Structure of an Archaeal Tyrosyl-tRNA Synthetase Bound to Photocaged L-Tyrosine and Its Potential Application to Time-Resolved X-ray Crystallography

Genetically encoded caged amino acids can be used to control the dynamics of protein activities and cellular localization in response to external cues. In the present study, we revealed the structural basis for the recognition of O-(2-nitrobenzyl)-L-tyrosine (oNBTyr) by its specific variant of Methanocaldococcus jannaschii tyrosyl-tRNA synthetase (oNBTyrRS), and then demonstrated its potential availability for time-resolved X-ray crystallography. The substrate-bound crystal structure of oNBTyrRS at a 2.79 Å resolution indicated that the replacement of tyrosine and leucine at positions 32 and 65 by glycine (Tyr32Gly and Leu65Gly, respectively) and Asp158Ser created sufficient space for entry of the bulky substitute into the amino acid binding pocket, while Glu in place of Leu162 formed a hydrogen bond with the nitro moiety of oNBTyr. We also produced an oNBTyr-containing lysozyme through a cell-free protein synthesis system derived from the Escherichia coli B95. ΔA strain with the UAG codon reassigned to the nonnatural amino acid. Another crystallographic study of the caged protein showed that the site-specifically incorporated oNBTyr was degraded to tyrosine by light irradiation of the crystals. Thus, cell-free protein synthesis of caged proteins with oNBTyr could facilitate time-resolved structural analysis of proteins, including medically important membrane proteins.     

Effect of Retrograded Waxy Corn Starch on Bread Staling

Waxy corn starch pastes (10%) were stored at 5 °C for up to 35 days, and the powder specimens of retrograded starches thus obtained were added to wheat flour for bread baking at a level of 5%. The effect of retrograded starch on the staling of bread was determined. The loaf which contained retrograded waxy corn starch, which was prepared by storing the 10% paste at 5 °C for 7 days, showed an increase in specific volume and the results of the sensory evaluation showed that it was very acceptable. During the storage of bread, the increase in firmness and decrease in degree of gelatinization were suppressed by adding retrograded waxy corn starch. The moisture content of bread crumbs did not relate to firmness. Added retrograded waxy corn starch decreased the final viscosity of flour. The crystalline region of retrograded waxy corn starch used for bread baking included longer chains from amylopectin which in raw starch occurred in the amorphous region.     

Pasting Properties of Various Retrograded Starches Isolated with Ethanol

The pasting properties of various retrograded starches were studied by means of RVA. Most retrograded starch pastes showed a low‐temperature peak which did not appear in the viscograms of any raw starch sample: for wheat and rice starches, both a high‐temperature peak shown in the viscogram of their raw samples and a low‐temperature peak appeared, while for other cereal starches and potato starch, only a low‐temperature peak was detected in the retrograded paste. The low‐temperature peak began to appear, when the degree of gelatinization decreased below 60—70%. The viscosity of both peaks tended to increase to a maximum value and then decrease during storage.