Pasteurization of Pacific Oysters by Radiation: Post-Mortem Changes in Nucleotides During Storage at 0-2°C

SUMMARY: The concentration of nucleotides was lower in the adductor muscle of the oyster (1.64 μmoles/g) than in the remaining dark tissues of the oyster (2.75 μmoles/g). The concentration was less in the whole oyster meats (2.87 μmoles/g) than is usually found in fish muscle, or other marine invertebrates.
In addition to the adenine nucleotides and inosine monophosphate, uridine triphosphate, guanosine triphosphate, guanosine diphosphate, guanosine monophosphate and guanosine diphosphate-mannose were found in the fresh oysters. Samples collected in summer had greater concentrations of nucleotides than similar winter samples. lnosine monophosphate formed rapidly from adenosine triphosphate during storage at 0°–2°C, while the turnover rate of inosine monophosphate was slow and reflected low 5'-nucleotidase activity. Hypoxanthine, inosine, guanosine, guanine and uracil were formed during ice storage. The nucleotide breakdown in oysters was not changed by 2 mrads of radiation dose. Total nucleosides and free bases increased during storage of both unirradiated and irradiated samples. During the latter part of the storage period the concentrations of nucleosides and free bases were considerably greater in the irradiated samples. This difference probably is due to the utilization of these compounds by bacteria in the un-irradiated samples. After 15 days of storage bacteria had increased to more than 10'organisms per g, while the counts for irradiated samples were very low (less than 103 per g).     

Effect of Chlorinated Wash Water on Vibrio cholerae in Oyster Meats

Live oysters exposed to seawater artificially contaminated with Vibrio cholerue, El Tor, Inaba, in a closed system for 18 hr were harvested, shucked, sampled and placed into blower tanks containing 25 and 50 ppm chlorine for 5- and 10-min intervals in each tank. Regardless of chlorine concentrations encountered during the blowing process, V. cholerae MPNs of oyster meats sampled after the prescribed contact period were essentially equivalent to those of oysters sampled at the end of the 18-hr exposure period. Chlorination of the water used in blower tanks did not eliminate V. cholerae, El Tor, Inaba, organisms from oyster meats.     

Analysis of the Costs and Economic Feasibility of Requiring Postharvest Processing for Raw Oysters

Because of concerns about Vibrio vulnificus, the U.S. Food and Drug Administration is considering requirements for postharvest processing (PHP) of oysters harvested from the Gulf of Mexico during warm‐weather months and intended for raw consumption. As described in the paper, feasible PHP methods for warm‐weather‐harvested oysters include cool pasteurization, high hydrostatic pressure, and low‐dose gamma‐irradiation. We estimate that the costs of applying PHP are approximately 5 to 6 cents per half‐shell oyster intended for raw consumption. However, most oyster processors have insufficient volumes to cost‐effectively install PHP equipment. To assist these smaller operations, central PHP facilities operated by a 3rd party would be needed. A geographic information system analysis that minimized volume‐weighted travel distances from each Gulf oyster operation identified 6 optimal PHP facility locations in the Gulf region. Even with the establishment of central PHP facilities, some oyster operations will become unprofitable and be at risk for closure.     

北部湾海区三种常见牡蛎的蛋白质及氨基酸营养分析与评价

为探究北部湾海区常见的熊本牡蛎、香港牡蛎和近江牡蛎三种牡蛎的蛋白质和氨基酸含量、蛋白质营养价值及基于氨基酸含量的综合品质评价差异.采用国家标准测定牡蛎中蛋白质含量,利用高效液相色谱法测定牡蛎中氨基酸组成及含量,分别进行蛋白质营养价值评价,并以牡蛎中的氨基酸组成和含量为指标进行主成分分析(Principal compon...     

Low temperature pasteurization to reduce the risk of vibrio infections from raw shell-stock oysters

Vibrio vulnificus and V. parahaemolyticus are natural inhabitants of estuarine environments and may be transmitted to humans by ingestion of raw oysters. This study focused on the use of low temperature pasteurization, to reduce these Vibrio spp. to nondetectable levels, thus reducing the risk of infection associated with raw oyster consumption. Artificially inoculated V. vulnificus and V. parahaemolyticus and naturally-contaminated V. vulnificus in live oysters were pasteurized at 50%deg;C for up to 15min. Samples of processed and unprocessed oysters were enumerated for V. vulnificus, V. parahaemolyticus, and aerobic spoilage bacteria for 0-14 days. Low temperature pasteurization was effective in reducing these pathogens from > 100000 to non-detectable levels in less than 10min of processing. Spoilage bacteria were reduced by 2-3 logs, thus increasing the shelf-life for up to 7 days beyond live unprocessed oysters. Vibrio vulnificus in control oysters was reduced by 102 during ice storage alone. Following pasteurization and during a temperature storage abuse study (24h at 22°C), V. vulnificus was not recovered. During this storage period spoilage bacteria exceeded 1 million/g oyster meat.     

Prevalence and characterization of Salmonella serovars isolated from oysters served raw in restaurants

To determine if Salmonella-contaminated oysters are reaching consumer tables, a survey of raw oysters served in eight Tucson restaurants was performed from October 2007 to September 2008. Salmonella spp. were isolated during 7 of the 8 months surveyed and were present in 1.2% of 2,281 oysters tested. This observed prevalence is lower than that seen in a previous study in which U.S. market oysters were purchased from producers at bays where oysters are harvested. To test whether the process of refrigerating oysters in restaurants for several days reduces Salmonella levels, oysters were artificially infected with Salmonella and kept at 4°C for up to 13 days. Direct plate counts of oyster homogenate showed that Salmonella levels within oysters did not decrease during refrigeration. Six different serovars of Salmonella enterica were found in the restaurant oysters, indicating multiple incidences of Salmonella contamination of U.S. oyster stocks. Of the 28 contaminated oysters, 12 (43%) contained a strain of S. enterica serovar Newport that matched by pulsed-field gel electrophoresis a serovar Newport strain seen predominantly in the study of bay oysters performed in 2002. The repeated occurrence of this strain in oyster surveys is concerning, since the strain was resistant to seven antimicrobials tested and thus presents a possible health risk to consumers of raw oysters.     

The effect of a novel photodynamic activation method mediated by curcumin on oyster shelf life and quality

In this paper, the effect of photodynamic method mediated by curcumin (PDT) on the shelf life and quality of pacific oysters during storage at 5 ± 1 °C were analyzed. In our previous study we investigated the optimal treatment conditions of photodynamic method mediated by curcumin to sterilization were 10 uM photosensitizer concentration and 5.4 J/cm² light energy density. Under these conditions, the effect of a novel photodynamic activation method mediated by curcumin on oyster shelf life and quality was researched. The total bacterial counts, TVB-N content and sensory analysis were used to evaluate the effects on oyster shelf life. The oyster shelf life was prolonged from 8 days to 12 days after photodynamic treatment and the oysters in the treatment group displayed notable odor retention, produced fewer odor corrupting substances when the control group oysters reached the end of their shelf life (day 8). Texture, free amino acid contents and fatty acid levels were applied to estimate the quality of the treated oysters. The texture had no significant change after treated with PDT. At the end of oyster shelf life, compared PDT group (PDT) with control group (control), total free amino acid contents (control: 234.30 mg/100 g, PDT: 813.02 mg/100 g) was higher and free fatty acid levels (control: 0.071 mEq/L, PDT: 0.0455 mEq/L) displayed lower in PDT group. This indicated that the treated oysters oxidized minimally, decayed slowly, decomposed fewer nutrients and had lower metabolic levels of spoilage microorganisms. PDT has a positive effect on prolonging oyster shelf life and its quality.     

Effectiveness of icing as a postharvest treatment for control of Vibrio vulnificus and Vibrio parahaemolyticus in the eastern oyster (Crassostrea virginica)

The focus of this research was to investigate the efficacy of icing as a postharvest treatment for reduction of the levels of Vibrio vulnificus and Vibrio parahaemolyticus in commercial quantities of shellstock oysters. The experiments were conducted in June and August of 2006 and consisted of the following treatments: (i) on-board icing immediately after harvest; (ii) dockside icing approximately 1 to 2 h prior to shipment; and (iii) no icing (control). Changes in the levels of pathogenic Vibrio spp. during wholesale and retail handling for 2 weeks postharvest were also monitored. On-board icing achieved temperature reductions in all sacks in accordance with the National Shellfish Sanitation Program standard, but dockside icing did not meet this standard. Based on one-way analysis of variance, the only statistically significant relationship between Vibrio levels and treatment occurred for samples harvested in August; in this case, the levels of V. vulnificus in the noniced oysters were significantly higher (P < 0.05) than were the levels in the samples iced on-board. When analyzing counts over the 14-day storage period, using factorial analysis, there were statistically significant differences in V. vulnificus and V. parahaemolyticus levels by sample date and/or treatment (P < 0.05), but these relationships were not consistent. Treated (iced) oysters had significantly higher gaping (approximately 20%) after 1 week in cold storage than did noniced oysters (approximately 10%) and gaping increased significantly by day 14 of commercial storage. On-board and dockside icing did not predictably reduce the levels of V. vulnificus or V. parahaemolyticus in oysters, and icing negatively impacted oyster survival during subsequent cold storage.     

Effects of high-pressure and heat treatments on physical and biochemical characteristics of oysters (Crassostrea gigas)

Physical and biochemical changes in oysters following high-pressure (HP) treatment at 260 MPa for 3 min or heat treatment (cool pasteurisation (CP) at 50 °C for 10 min or traditional pasteurisation (TP) at 75 °C for 8 min) were investigated and compared to changes in untreated oysters. HP or TP oysters had higher (P < 0.05) pH values (6.49–6.58) than untreated or CP oysters (6.45–6.46). HP and heat treatment both modified the gross composition of oyster tissue. The protein content of HP-treated oysters (6.9%) was significantly (P < 0.05) lower compared to control or heat-treated oysters (7.9–9.1%). The moisture content of HP-treated whole oyster tissue (86.5%) was higher than that of heat-treated or untreated oysters (83.5–84.7%), but HP or CP treatments did not affect the salt content or water activity of oysters. However, all treatments increased Hunter L- (66.3–68.9) while decreasing a- (− 1.6 to − 2.4) and b- (15.8–14.5) values of oyster tissue; overall, HP treatment had less negative effects on tissue colour of oysters than thermal treatments. HP-treated, CP and TP oysters had higher shucking yields (15.5%, 12.5% and 2.6%, respectively) than untreated oysters. One significant advantage of HP treatment over heat treatment of oysters was that the former process opened the oyster and separated the muscle of the oyster from the shell.     

Study on the volatile profile characteristics of oyster Crassostrea gigas during storage by a combination sampling method coupled with GC/MS

In this work, a combination sampling method, including headspace solid-phase microextraction (HSSPME) and steam distillation (SD), was used to study the oyster volatiles during storage followed by GC–MS detection. Twenty and twenty seven volatile compounds of fresh and deteriorated oysters were identified respectively by HSSPME, and 16 oyster volatiles were isolated by SD. HSSPME and SD were suitable for low-boiling-point and high-boiling-point volatiles, respectively. Therefore, the combination of HSSPME and SD could obtain more species of oyster volatiles during storage than any single method. Different volatile profile characteristics during oyster storage obtained by HSSPME were specified by principal component analysis. The top ten volatiles contributing most to the difference of oyster volatile profile characteristics during storage were distilled by common model analysis. The results tentatively suggested that the difference of entire volatile profile characteristics during oyster storage would provide more precise alarming information for oyster deterioration during storage than individual volatiles.     

Effects of a commercial heat-shock process on Vibrio vulnificus in the American oyster, Crassostrea virginica, harvested from the Gulf Coast

Oysters (Crassostrea virginica) harvested from the Gulf Coast, containing 10(2) to 10(4) most probable number (MPN) per gram of Vibrio vulnificus, were subjected to a commercial heat-shock process. After 1 to 4 min at internal oyster meat temperatures exceeding 50 degrees C, shellstock oysters were shucked, chilled, washed, and packed. V. vulnificus and total bacterial levels in Gulf Coast oysters were significantly reduced from 1 to 4 logs in the finished product. Similar reductions were not observed in shellstock oysters that were subject to conventional processing. Under the National Shellfish Sanitation Program, heat shocking is an acceptable process to use to assist in the shucking of shellstock. This research revealed that the heat-shock process may also serve to significantly reduce V. vulnificus in summer Gulf Coast oysters.     

Elimination of fecal coliforms and F-specific RNA coliphage from oysters (Crassostrea virginica) relaid in floating containers

Declining oyster (Crassostrea virginica) production in the Chesapeake Bay has stimulated aquaculture based on floats for off-bottom culture. While advantages of off-bottom culture are significant, the increased use of floating containers raises public health and microbiological concerns, because oysters in floats may be more susceptible to fecal contamination from storm runoff compared to those cultured on-bottom. We conducted four commercial-scale studies with market-size oysters naturally contaminated with fecal coliforms (FC) and a candidate viral indicator, F-specific RNA (FRNA) coliphage. To facilitate sampling and to test for location effects, 12 replicate subsamples, each consisting of 15 to 20 randomly selected oysters in plastic mesh bags, were placed at four characteristic locations within a 0.6- by 3.0-m "Taylor" float, and the remaining oysters were added to a depth not exceeding 15.2 cm. The float containing approximately 3,000 oysters was relaid in the York River, Virginia, for 14 days. During relay, increases in shellfish FC densities followed rain events such that final mean levels exceeded initial levels or did not meet an arbitrary product end point of 50 FC/100 ml. FRNA coliphage densities decreased to undetectable levels within 14 days (16 to 28 degrees C) in all but the last experiment, when temperatures fell between 12 and 16 degrees C. Friedman (nonparametric analysis of variance) tests performed on FC/Escherichia coli and FRNA densities indicated no differences in counts as a function of location within the float. The public health consequences of these observations are discussed, and future research and educational needs are identified.     

IMPACT OF MODIFIED ATMOSPHERE PACKAGING ON THE OSMODEHYDRATED PAPAYA STABILITY

Papaya pieces were osmotically dehydrated in a 50°Brix sucrose solution containing calcium lactate (0.05 M) and lactic acid (0.02 M) as additives for 1 h at 25C, packed in polyethylene terephthalate (PET) containers and stored at 5C for 15 days. Fresh fruit pieces packed in PET containers and under atmosphere condition were used as control samples. Sensory acceptance, microbiological count, CO₂ and O₂ content inside the packages, color, mechanical properties and weight loss of the product were evaluated. PET containers prevented weight loss during storage. The utilization of calcium lactate as additive was effective in maintaining fruit hardness during refrigerated storage. The use of passive modified atmosphere associated with refrigeration was adequate to preserve the osmotically dehydrated fruit for 15 days, showing a good sensory acceptance by the consumers during the storage time.     

Endogenous Factors Affecting Oxidative Stability of Beef Loin, Pork Loin, and Chicken Breast and Thigh Meats

ABSTRACT:  The susceptibility of meats from different animal species (chicken breast [CB] and thigh [CT], pork [PL], and beef [BL]) to lipid oxidation was studied. The amounts of TBARS in raw PL, CB, and CT did not change during a 7-d storage period. TBARS values of raw BL, however, significantly increased during 7-d storage because of high heme iron content, high lipoxygenase-like activities, and low 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities. Ferric ion reducing capacities (FRC) were detected in all raw meats, but their characteristics were different: storage-unstable in CB and CT and storage-stable in PL and BL. Ferric ion reducing capacities in raw CB and CT was higher than those of PL and BL, and could be related to their high oxidative stability. The TBARS values of cooked meat increased significantly with storage. The rates of TBARS increase in cooked CT and BL were significantly higher than those of cooked CB and PL after a 7-d storage. Nonheme iron content in cooked BL was higher than other meats and increased significantly after 7 d. Cooked BL had a higher amount of heat-stable FRC, which acted as a prooxidant in the presence of high free ionic irons, than other meats. Therefore, high heat-stable FRC and increased nonheme iron content in cooked BL were responsible for its high susceptibility to lipid oxidation. Despite relatively low nonheme iron and heat-stable FRC levels, cooked CT showed similar levels of TBARS to cooked BL after a 7-d storage because of its high PUFA content.     

Oyster-to-oyster variability in levels of Vibrio parahaemolyticus

This study examined the variability in the levels of total and pathogenic Vibrio parahaemolyticus in individual oysters. Twenty oysters were collected on three occasions (in June, July, and September 2001) from a site near Mobile Bay, Ala. Ten of these oysters were tested immediately, and 10 were tested after 24 h of storage at 26 degrees C. Levels of total and pathogenic V. parahaemolyticus were determined by alkaline phosphatase-labeled DNA probe procedures targeting the thermolabile hemolysin and thermostable direct hemolysin genes, respectively. Similar V. parahaemolyticus levels (200 to 2,000 CFU/g) were found in nearly 90% of the oysters (for all sampling occasions) prior to storage. The log-transformed densities (means +/- standard deviations) of V. parahaemolyticus in oysters immediately after harvest were 2.90 +/- 0.91, 2.88 +/- 0.36, and 2.47 +/- 0.26 log10 CFU/g for June, July, and September, respectively. After storage for 24 h at 26 degrees C, the mean V. parahaemolyticus densities increased approximately 13- to 26-fold. Before storage, pathogenic V. parahaemolyticus was detected in 40% (10 to 20 CFU/g) of the oysters collected in June and July but was not detected in any oysters collected in September. After storage, pathogenic V. parahaemolyticus was detected in some oysters at levels of > 100 CFU/g. These data should aid in the development of sampling protocols for oyster monitoring programs and in the determination of exposure distributions associated with raw oyster consumption.     

Accumulation of paralytic shellfish poison (PSP) and biotransformation of its components in oysters, Crassostrea gigas, fed with the toxic dinoflagellate Alexandrium tamarense

As a part of our studies on the mechanism of uptake of paralytic shellfish poison (PSP) and the kinetics of its accumulation in bivalves, oysters Crassostrea gigas were experimentally contaminated with PSP by being fed with the toxic dinoflagellate Alexandrium tamarense for 2, 4, 6, 8 and 10 days. Temporal variations in the PSP contents and their profiles in oysters during the feeding experiment were monitored by high-performance liquid chromatography (HPLC) and the toxin profile of the oysters was compared with that of A. tamarense. Toxins excreted from the infested oysters into the seawater for 2 and 10 days were recovered and analyzed by HPLC. PSP toxicity rapidly appeared in the tissues of oysters and their toxicity levels reached 0.6 (0.3), 2.2 (1.1), 1.0 (0.5), 3.4 (1.6) and 1.1 (0.5) MU/g (nmol/g) shucked meat at 2, 4, 6, 8 and 10 days, respectively. The accumulation rates of toxin, calculated from the total amount (nmol) of toxins expressed by the total cell number fed during the exposure period and the toxicity of the oysters, were 14.1, 18.7, 5.1, 14.9 and 3.2% for 2, 4, 6, 8 and 10 days. During feeding experiments, the toxin profile of oysters changed substantially, showing marked differences from the proportions found in the toxigenic dinoflagellate used as food. The toxin components in this strain existed almost exclusively as beta-epimers, which accounted for 66.3 mol% of the total. This contrasts with the case of the oysters, where the beta-epimers represented 24.8, 29.8, 25.1, 27.3 and 25.2 mol% of the total at 2, 4, 6, 8 and 10 days, respectively. The amount of gonyautoxin-1 (GTX1) accumulated in oysters increased linearly and slowly for 8 days and the maximum content of GTX1 reached 51.3 mol%. The composition of GTX group compounds recovered from the seawater in which the oysters had been reared was a little different from that within the oyster tissues.     

The Effects of Freezing on the Survival of Salmonella and E. coli in Pacific Oysters

SUMMARY: A mixed inoculum of Salmonella derby or S. typhimurium and Escherichia coli I was injected into the intestinal region of Pacific oysters (Crassostrea gigas) which were then frozen by four methods. Frozen oysters were stored at O°F, and survival of the inoculated bacteria was determined over a period of two weeks. In separate experiments, inoculated oysters were homogenized and then stored, unfrozen, at 32°F and −30°F (frozen). Routinely, bacterial counts and pH readings were taken of all samples during the course of experiments.
Both species of Salmonella proved to be highly sensitive to freezing, regardless of the freezing method, and showed a survival of 1% or less after 48 hr. E. coli proved less sensitive, showing a wide and capricious variability of survival during the first week of storage, with survival ranging from 10 to 30%. Generally, however, most samples showed a decline comparable to that of salmonellae after two weeks'storage. Because of the fluctuation in E. coli counts after freezing, it is difficult to correlate the numbers of E. colt in frozen shellfish with the count in unfrozen shellfish. Therefore, it would be inappropriate to apply coliform standards for fresh oysters to the frozen product.
In separate studies using inoculated oyster homogenates held at 32° and −30°F for 168 hr, a higher survival rate of E. coli and salmonellae was noted in samples held at −30°F. However, since results obtained were based solely on bacterial counts, it is not possible to say with certainty that these results indicate a protective effect by oyster homogenates against the adverse effects of freezing. Significantly, the results of these experiments did not agree with results obtained with whole oysters, thus indicating the inadvisability of attempting to apply results of homogenate studies to the whole oyster.     

不同前处理方式对牡蛎肽营养成分的影响

目的探讨新鲜牡蛎肉不同前处理方式对所得牡蛎肽营养成分的影响。方法选择2种前处理方式,对所得牡蛎肽的基本成分、氨基酸、牛磺酸、锌、硒、多肽的含量以及相对分子质量的分布进行对比分析。结果新鲜牡蛎肉直接酶解得到的牡蛎肽A牛磺酸的含量为96.79 g/kg,而蒸煮弃去汤液后酶解所得的牡蛎肽B的含量为49.50 g/kg;牡蛎肽B锌和硒的含量明显低于牡蛎肽A中的含量(P0.05);牡蛎肽A中游离氨基酸总量是牡蛎肽B的3.13倍,但牡蛎肽B中多肽的含量是牡蛎肽A中的1.55倍;对于分子量180~1000 u组分,牡蛎肽A和牡蛎肽B中的含量分别占39.00%和69.39%。结论新鲜牡蛎肉经过微沸蒸煮弃去汤液后再制备牡蛎肽,将损失大量的游离氨基酸、牛磺酸和锌、硒等可溶性营养成分。