Microbial Safety in Radio-frequency Processing of Packaged Foods

ABSTRACT: Thermal resistance of Clostridium sporogenes (PA 3679) was determined at 115.6 °C, 118.3 °C, and 121.1 °C (240 °F, 245 °F, and 250 °F, respectively) in phosphate buffer (pH 7.0) and mashed potatoes (pH 6.3) using aluminum thermal-death-time (TDT) tubes developed at Washington State Univ. D-values were 1.8, 1.1, and 0.62 min in phosphate buffer and 2.2, 1.1, and 0.61 min in mashed potatoes at 115.6 °C, 118.3 °C, and 121.1 °C, respectively. Z-values were 12 °C and 10 °C in phosphate buffer and mashed potatoes, respectively. The thermal inactivation kinetic results were then used to validate a novel thermal process based on 27.12 MHz radio frequency (RF) energy. Trays of mashed potatoes inoculated with PA 3679 were subjected to 3 processing levels: target process (F₀∼4.3), under-target process (F₀∼2.4), and over-target process (F₀∼7.3). The microbial challenge test data showed that microbial destruction from the RF process agreed with the calculated sterilization values. This study suggests that thermal processes based on RF energy can produce safe and shelf-stable packaged foods.     

Determination of a process time for a new product: canned low acid artichoke hearts

In order to obtain a process time for canned low acid artichoke hearts, the heat resistance of Clostridium sporogenes PA 3679 in phosphate buffer (pH 7) and in artichoke puree (pH 5.2) was determined in the range of 100–118°C. D _T values in artichoke puree were determined by the most probable number and plate count methods. D _121°C values were deduced by extrapolation of the curves. An F 8.3/121 (see nomenclature section) target value of 1.8 min, equivalent to an F ₀= 2.5 min, was established considering a D _121°C value of 0.36 min for artichoke puree. The cold point for canned artichoke hearts in 1/2 kg (71.5x117 mm) cans was determined, a set of heat penetration and cooling curves were obtained and the values of the parameters ' j ' and ' f ' were found. A process time was deduced, 21.5 and 23 min of heating at 121°C, following the methods of Patashnik (1954) and Hayakawa (1970, 1974) respectively, in order to reach the F ₀ target of 2.5 min for this new canned food. This process was adequate, as the inoculated experimental pack test showed no altered cans out of 50 inoculated after 21.5 min of heating at 121°C.     

Thermal Resistance of Spores of Five Strains of Clostridium botulinum Type E in Ground Whitefish Chubs

SUMMARY– Thermal death-time determinations in the temperature range of 165°F (73.9°C) to 185°F (85°C) were made of spores of five strains of Clostridium botulinum type E in tubed fish paste prepared from whole Great Lakes whitefish chubs. Estimated F₁₇₆ (80°C) values for the destruction of approximately 1×10⁶ spores per gram of fish paste were as follows: for strain Alaska, 34.2 min; Beluga, 17.0 min; 8E, 14.0 min; Iwanai, 12.5 min; and Tenno, 13.2 min. Estimated 95% confidence limits about these F₁₇₆ values were for strain Alaska, 23.4-49.5 min; Beluga, 14.2-20.2 min; 8E, 8.2-17.7 min; Iwanai, 10.2–15.4 min; and Tenno, 9.3–18.5 min. D₁₇₆ values calculated from our F₁₇₆ values by Schmidt's (1957) equation were for strain Alaska, 4.3 min; Beluga, 2.1 min; 8E, 1.8 min; Iwanai, 1.6 min; and Tenno, 1.6 min. Z_y values obtained from thermal death-time determinations were for strain Alaska, 13.2; Beluga, 13.3; 8E, 10.3; Iwanai, 13.6; and Tenno, 13.1°F.     

Clostridium sporogenes PA 3679 and its uses in the derivation of thermal processing schedules for low-acid shelf-stable foods and as a research model for proteolytic Clostridium botulinum

The putrefactive anaerobe Clostridium sporogenes PA 3679 has been widely used as a nontoxigenic surrogate for proteolytic Clostridium botulinum in the validation of thermal processes for low-acid shelf-stable foods, as a target organism in the derivation of thermal processes that reduce the risk of spoilage of such foods to an acceptable level, and as a research model for proteolytic strains of C. botulinum. Despite the importance of this organism, our knowledge of it has remained fragmented. In this article we draw together the literature associated with PA 3679 and discuss the identity of this organism, the phylogenetic relationships that exist between PA 3679 and various strains of C. sporogenes and proteolytic C. botulinum, the heat resistance characteristics of PA 3679, the advantages and limitations associated with its use in the derivation of thermal processing schedules, and the knowledge gaps and opportunities that exist with regard to its use as a research model for proteolytic C. botulinum. Phylogenetic analysis reviewed here suggests that PA 3679 is more closely related to various strains of proteolytic C. botulinum than to selected strains, including the type strain, of C. sporogenes. Even though PA 3679 is demonstrably nontoxigenic, the genetic basis of this nontoxigenic status remains to be elucidated, and the genetic sequence of this microorganism appears to be the key knowledge gap remaining to be filled. Our comprehensive review of comparative heat resistance data gathered for PA 3679 and proteolytic strains of C. botulinum over the past 100 years supports the practice of using PA 3679 as a (typically fail-safe) thermal processing surrogate for proteolytic C. botulinum.     

Effect of Heat Processing on the Texture Profile of Canned and Retort Pouch Packed Oil Sardine (Sardinella longiceps) in Oil Medium

ABSTRACT: Sardine in oil was canned at 3 different Lethality values (F₀ 5,7, and 9) in retort pouches and aluminum cans in stationary retort. Process time calculations were done mathematically to find out the actual process time. Sardine in oil processed in retort pouches had a lower process time compared with aluminum cans for each F₀ value. Instrumental texture analysis was done using a food texture analyzer to study the effect of thermal processing on the texture of thermally processed sardine in oil in aluminum cans and retort pouches. As the F₀ value and cook value increased in both the retort pouches and aluminum cans, the textural properties showed a decreasing trend. Hardness of the product canned in both aluminum cans and retort pouches decreased with increase in F₀ values.     

Microbiological Validation of Microwave-Circulated Water Combination Heating Technology by Inoculated Pack Studies

A 915‐MHz Microwave‐Circulated Water Combination (MCWC) heating technology was validated for a macaroni and cheese product using inoculated pack studies. Before the tests, heat resistances of a Clostridium sporogenes (PA 3679) spore crop were determined in neutral phosphate buffer and macaroni and cheese product. Trays of macaroni and cheese products were subjected to 3 processing levels: target process (F₀= 2.4), under target process (F₀= 1.2), and over target process (F₀= 4.8). The inoculated packs were evaluated by count‐reduction method and end‐point method. The microbial results showed that microbial destruction resulting from MCWC heating technology matched the calculated degree of sterilization (F₀ value). This study suggests that the MCWC heating technology has potential in sterilizing packaged foods.     

Numerical method for prediction of final microbial load in inoculated cylindrical cans of tomato concentrate heated in a pilot plant

A finite difference numerical method to predict temperatures and destruction of a thermally labile species in cylindrical cans of conduction heating food was applied to compute final microbial load in an inoculated 18°Brix canned tomato concentrate at pH = 4.5 processed under conditions simulating industrial practice. Predictions of product temperature and final microbial load in three can sizes compared well with experimental heat penetration data and estimation of surviving species of Bacillus coagulans ATC 8038 spores inoculated into the cans. The application of the model as an aid to establishing process schedules is illustrated for a can with a required IS¹⁴₁₀₀= 11.44 min. The model predicts that a 29.5 min heating time at 105.6°C would provide an IS¹⁴₁₀₀= 11.77 min.     

Establishing processing conditions for canning cowpea seeds in tomato sauce

The thermal process time and sensory attributes of Ife-BPC cowpeas canned in tomato sauce were studied. Heat resistance of Bacillus stearothermophilus 1518 was determined by the plate count method at 114.5 and 120.6°C. Surviving spores were cultured and TDT curves were plotted from these data to obtain D values of 2.2 ± 0.2 min at 120.6°C and 7.5 ± 0.5 min at 114.5°C. Heat penetration studies were conducted on two can sizes and at two operating temperatures. Thermal process parameters were determined using both the general and formula methods. There was no significant difference ( P < 0.05) in the process times determined by the two methods although slightly higher values were obtained with the formula method. The medium can size required a longer process time to approach the retort temperature than the small can size. The lethal effect of sterilizing at 120.6°C resulted in a lower D-value, higher sterilizing value (F₀), and higher safety ratio (a/b) than at 114.5°C. The final F₀ at the end of processing was greater than the targeted F₀ (11.0 min) from the 5D value, suggesting that the processing time is adequate for safe food production. Taste panel assessments were conducted to identify optimal processing conditions. The sensory attributes of dry and semi-dry Ife-BPC cowpeas canned in tomato sauce were compared with those of imported baked beans. Semi-dry processed cowpeas had high consumer preference over the dry seeds, and acceptable canning qualities similar to those of the imported baked beans.     

Performance evaluation of aluminum test cell designed for determining the heat resistance of bacterial spores in foods

Thermal inactivation kinetic studies are necessary to determine heat resistances of spores in the development of new thermal processes for low-acid shelf-stable products. Most currently available sample holders used for solid and semi solid samples in the kinetic studies take long time to reach the target sample temperature, hence fail to provide isothermal condition. In this research, novel aluminum test cells were developed to facilitate easy loading and unloading samples in a hermetically sealed 1 ml cavity to evaluate the heat resistance of bacterial spores when heated at temperatures above 100 °C. Design of the test cell was governed by minimum come-up time. A finite element model based on the commercial software ‘FEMLAB’ was used to simulate transient heat transfer and finalize the test cell dimensions. Performance of the new test cell was evaluated against capillary and aluminum thermal death time tube methods in characterizing the heat resistance of Clostridium sporogenes PA 3679 spores in a phosphate buffer and mashed potato at 121 °C. D₁₂₁ values of PA 3679 spores in both the phosphate buffer and mashed potato using the new test cells were not significantly different (P>0.05) from those by the capillary tube method. The results indicated that the new test cell is appropriate for studying the inactivation kinetics of bacterial spores in microbial validation of conventional and novel thermal processes for low-acid shelf-stable foods.     

Efficacy of Sulfuric Acid Scarification and Disinfectant Treatments in Eliminating Escherichia coli O157: H7 from Alfalfa Seeds Prior to Sprouting

ABSTRACT: E. coli O157:H7 reduction on inoculated alfalfa seeds was investigated using acid scarification treatments with or without subsequent application of sanitizers. Scarification with 0.1 to 2N H₂SO₄ for 2.5 to 45 min did not affect (p ≤ 0.05) seed viability. E. coli O157:H7 was reduced by 2.1 to 5.0 logs after treating with 0.1 to 2N H₂SO₄ for 5 to 20 min. Combined scarification (0.5N H₂SO₄) and H₂O₂ or CH₃COOH treatments enhanced microbial destruction by less than 1 log compared to sanitizer alone. Chlorine, Na₂CO₃, or Na₃PO₄ treatments preceded by scarification did not significantly increase microbial destruction compared to sanitizer alone. Appreciable reductions in seed germination were only observed with chlorine treatments.     

Sterilization of Foodstuffs Using Radio Frequency Heating

ABSTRACT: A study was conducted with a pilot-scale sterilization system based on 27-MHz radio frequency (RF) energy to investigate the effectiveness in shortening process time and in improving quality for foods sealed in 6-pound military-ration polymeric trays. Chemical marker M-1 was used to evaluate heating uniformity in 20% whey protein gels as a model food, and macaroni and cheese was processed to assess the influence of RF process on product quality. With the RF system, a lethality ( F ₀= 10 min) was achieved in both model food and macaroni and cheese within 30 min with relative uniform heating, compared to a 90 min conventional retort process that delivered a similar lethality.     

INACTIVATION OF BACTERIAL SPORES BY COMBINED ACTION OF HYDROSTATIC PRESSURE AND BACTERIOCINS IN ROAST BEEF

Foodborne bacterial spores are normally resistant to high hydrostatic pressure; however, at moderate pressure, they can be induced to germinate and outgrow. At this stage, they can be killed by bacteriocin-based biopreservatives (BP-containing pediocin and nisin at 3:7 ratio; BP_X, BP + 100 μg/mL lysozyme; BP_Y, BP_X+ 500 μg/mL Na-EDTA). Based on this principle, spores of the meat spoilage organism, Clostridium laramie (1–2 × 10² spores/bag) alone or a mixture of four clostridial spores (5 × 10³ spores/bag), Clostridium sporogenes, Clostridium perfringens, Clostridium tertium, and Clostridium laramie, were inoculated in roast beef in the presence of 5000 AU/g of bacteriocin-based biopreservatives. The roast beef samples were subjected to hydrostatic pressure (HP) at 345 MPa for 5 min at 60C and stored at 4 or 12C for 84 days or at 25C for 7 days. The HP treatment of roast beef samples inoculated with a mixture of clostridial spores could be stored for 42 days at 4C. The HP in combination with either BP_X or BP_Y extended the shelf-life of roast beef up to 7 days at 25C. The combined treatment of HP and BP controlled the growth of C. laramie spores and extended the shelf-life of roast beef for 84 days when stored at 4C.     

DIFFERENTIAL ACCUMULATION OF THE MOLECULAR FORMS OF POLYGALACTURONASE IN TOMATO MUTANTS

Endopolygalacturonase [poly (1–4-α-D galacturonide) glycanhydrolase, EC 3.2.1.15] activity was measured in samples of cv. Rutgers, Nr (never ripe) and F₁ hybrids of rin (ripening inhibitor, + rin, ++), nor (nonripening, ++, + nor) and rin × nor (+ rin + nor) fruits taken at three day intervals during ripening. Day 1 of ripening was taken as the day on which the first persistent increase in ethylene production was recorded. The enzyme was first detected on day 3 in Rutgers, F₁ rin and F₁ nor, and on day 6 in Nr and F₁ rin × nor. The Rutgers and F₁ rin samples softened more rapidly than the other samples and they also contained more enzyme activity. Enzyme activity developed less rapidly in Nr than in any other samples, but the Nr and the nor hybrid, samples softened at comparable rates. In Rutgers and F₁ rin, a high molecular weight form of the enzyme (PG1, Mr 100 000) predominated to day 6, and a lower molecular weight form (PG2, Mr 43 000 - 46 000) progressively increased after day 6. In all F₁ nor, F₁ rin × nor and Nr samples the enzyme was predominately or entirely as PG1. The interrelationship of PG1 and PG2 and their roles in ripening are discussed.     

Edible Film Coating to Minimize Eggshell Breakage and Reduce Post-Wash Bacterial Contamination Measured by Dye Penetration in Eggs

ABSTRACT: The mechanical and bacterial barrier properties of washed (tap water, sodium carbonate, Na₂CO₃, sodium hypochlorite, NaOCl) noncoated eggs and eggs coated with soy protein isolate (SPI), whey protein isolate (WPI), carboxylmethyl cellulose (CMC), or wheat gluten (WG) were evaluated. All coated eggshells showed greater puncture strength than those of noncoated eggs. The film-coated eggs reduced post-wash bacterial penetration as measured by dye penetration method. WPI coating completely inhibited blue dye penetration on average. These results suggest that WPI coatings can be used to reduce breakage of eggshell and egg microbial contamination.     

Microbial Inactivation Kinetics in Soymilk during Continuous Flow High-Pressure Throttling

ABSTRACT:  The thermal resistance of  Clostridium sporogenes  PA 3679 ATCC 7955 was determined in soymilk (pH 7) and 0.1% peptone water (pH 7) by the capillary tube method. In the continuous flow high-pressure throttling, the temperature of soymilk increased due to instantaneous pressure release and the additional heat was supplied by a heat exchanger to achieve the set temperature. The soymilk was immediately cooled after a short preset hold time to below 40 °C. A significant increase in the heat resistance was observed in  C. sporogenes  spores when heated in soymilk in comparison to 0.1% peptone water. The  D ₁₂₁-value for spores in soymilk was approximately 3-folds higher than peptone water. The  z- value was also much higher in soymilk as compared to that in 0.1% peptone water. Continuous flow high-pressure throttling (HPT) from 207 or 276 MPa to atmospheric pressure reduced the microbial populations in inoculated soymilk up to 6 log cycles when the holding times were 10.4, 15.6, and 20.8 s and the process temperatures were 85, 121, 133, and 145 °C, respectively. The sporicidal effect increased as the operating pressure, time, and temperature were increased. More injured spores were found at 207 MPa than at 276 MPa, indicating that lower pressure caused cell injury whereas high pressure caused cell death.     

Modified Spore Inoculation Method for Thermal-Process Verification of Pinto Beans and Green Beans Canned in Two Large Reusable Containers

As an alternative to standard metal cans, 2 large, reusable containers with volumetric capacities 3.6 to 3.8 times larger than a standard #10 can were constructed of nickel‐plated steel. Process times and cold point determinations were previously determined using mathematical approximations of heat‐penetration data. It was necessary to verify the adequacy of these process times using inoculated containers. Because it was impractical to use traditional inoculated can procedures, a modification was developed using spores of Clostridium sporogenes PA3679 immobilized in alginate beads contained in brass tubes. Adequacy of the calculated thermal processes was demonstrated; no viable anaerobic spores survived in the inoculated packs.     

Feasibility of utilizing bioindicators for testing microbial inactivation in sweetpotato purees processed with a continuous-flow microwave system

ABSTRACT:  Continuous-flow microwave heating has potential in aseptic processing of various food products, including purees from sweetpotatoes and other vegetables. Establishing the feasibility of a new processing technology for achieving commercial sterility requires evaluating microbial inactivation. This study aimed to assess the feasibility of using commercially available plastic pouches of bioindicators containing spores of Geobacillius stearothermophilus ATCC 7953 and Bacillus subtilis ATCC 35021 for evaluating the degree of microbial inactivation achieved in vegetable purees processed in a continuous-flow microwave heating unit. Sweetpotato puree seeded with the bioindicators was subjected to 3 levels of processing based on the fastest particles: undertarget process ( F ₀ approximately 0.65), target process ( F ₀ approximately 2.8), and overtarget process ( F ₀ approximately 10.10). After initial experiments, we found it was necessary to engineer a setup with 2 removable tubes connected to the continuous-flow microwave system to facilitate the injection of indicators into the unit without interrupting the puree flow. Using this approach, 60% of the indicators injected into the system could be recovered postprocess. Spore survival after processing, as evaluated by use of growth indicator dyes and standard plating methods, verified inactivation of the spores in sweetpotato puree. The log reduction results for B. subtilis were equivalent to the predesigned degrees of sterilization ( F ₀). This study presents the first report suggesting that bioindicators such as the flexible, food-grade plastic pouches can be used for microbial validation of commercial sterilization in aseptic processing of foods using a continuous-flow microwave system.     

Shelf-life of chicken liver/egg paté in modified atmosphere packages

Shelf-life of paté (salad in Israel) made by blending fried chicken liver and onions with hard-boiled eggs was extended, from 6 days in air to 14 days by Modified Atmosphere Packaging (MAP), using high CO₂/low O₂ mixtures. Aerobic microbial growth, mainly B. subtilis , and lipid oxidation (as TBA values) were largely inhibited, during 14 days, and odour up to 8 days (not tested beyond). the microbial growth inhibition was due to high CO₂ levels. Oxidation was prevented due to microbial inhibition and not due to reduced O₂.     

Changes in rheological behaviour and functional properties of hen's egg yolk induced by processing and fermentation with phospholipases

Functional changes of hen's egg yolk rheological behaviour as a result of processing (pasteurisation, freeze-drying) and modifying its phospholipids using phospholipase A₁ (PLA₁) and phospholipase A₂ (PLA₂) were studied. It could be shown how fermentation with phospholipases affects the different steps of heat-induced gelation of egg yolk. As a consequence of fermentation of native yolk with 0.1% PLA₁, the first viscosity maximum was shifted from 79 to 97 °C. In fermented pasteurised yolk, the homogenous gel-like consistency from the first viscosity maximum was even stabilised up to 100 °C. The second viscosity ascent, which indicates solid gel structures (caused by aggregation of denatured proteins), was prevented entirely. In contrast to pancreatic PLA₂, fermentation of egg yolk with microbial PLA₁ resulted in a hydrolysis of approximately 78% of phosphatidylethanolamine and a significant change in flow behaviour (of yolk solutions).