Sterilization of Scrambled Eggs in Military Polymeric Trays by Radio Frequency Energy

ABSTRACT: There is a current need for fresh-cooked-like yet shelf-stable egg products for U.S. military combat rations. Novel thermal processes based on radio frequency (RF) energy can shorten the heating time and reduce overheating. This technology was explored to produce shelf-stable egg products for combat ration development. Clostridium sporogenes (PA 3679) spores were used as a surrogate to validate the RF sterilization process to control Clostridium botulinum . Decimal reduction time (D value) of PA3679 in scrambled eggs was determined using aluminum thermal-death-time (TDT) tubes. The thermal inactivation kinetic information was then used in inoculated pack studies to validate a novel thermal process based on 27.12 MHz radio frequency (RF) pilot scale unit. Trays of scrambled eggs inoculated with PA 3679 were subjected to 3 processing levels: target process (F₀ about 5.3 min), under-target process (F₀ about 3.0 min), and over-target process (F₀ about 9.1 min). The results of the microbial challenge study showed that microbial destruction from the RF process agreed with sterilization values calculated from time-temperature data measured at the cold spot in the treated trays. A comparison of RF- and retort-treated scrambled eggs showed significant differences in the degree of lightness ( L *) and redness ( a *). RF-processed egg was less brown compared with conventional retorted eggs. Retort treatment of fully cooked scrambled eggs had higher hardness, springiness, and smaller cohesiveness than RF -treated samples. This study suggests that RF thermal processes can produce safe, shelf-stable sterilized scrambled eggs for both military and civilian uses.     

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.     

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.     

Chlorophyll, colour and pH changes in H.T.S.T. processed green pea puree

Summary. Pea puree of pH 6.95 and pH 8.45 was heat processed in thermal death-time tubes at temperatures between 115.6°C and 148.9°C to a process value of F₀= 6.0. Chlorophyll pigments, Hunterlab colour indices and pH were determined before and after high temperature-short time (H.T.S.T.) processing and during storage for 18 months at 20°C, 2.8°C, and −23.3°C.
Highly significant correlations were found between per cent conversion of chlorophylls to pheophytins and objective colour indices derived from tristimulus measurements on stored pea puree. the degree of chlorophyll conversion, and hence puree colour, were both markedly affected by process temperature, product pH, storage time and storage temperature, but long-term storage stability was achieved only by combinations of two or more variables.
Changes in pH of processed and stored puree were directly related to changes in pigments and colour, and it appears that pH control during processing and storage offers the most likely means of colour and pigment retention in heat processed chlorophyll-containing foods.     

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.     

Kinetics of Clostridium sporogenes PA3679 Spore Destruction Using Computer-Controlled Thermoresistometer

A modified version of a computer-controlled thermoresistometer was used, with and without micropurge, to study the inactivation kinetics of Clostridium sporogenes PA 3679 spore destruction between 121–143°C in phosphate buffer (pH 7) and in mushroom extract acidified with citric acid. A shorter temperature come up time was observed with micropurge. The thermal death time (TDT) curve for spores in phosphate buffer with micropurge followed a straight line (z = 9.5°C). Without micropurge the curve could be described by two lines with z = 10.0 °C for temperatures up to 132.5 °C and z = 18.3 °C for higher temperatures. The spore heat resistance in mushroom extract was lower than in phosphate buffer. D_T. values decreased exponentially as temperature increased, but acidification did not reduce thermal resistance at high temperatures.     

BEHAVIOR OF STREPTOCOCCUS PYOGENES IN MASHED POTATOES

Streptococcus pyogenes is widely recognized as a human pathogen. Whereas person-to-person transmission is the most common transmission mechanism for this pathogen, some outbreaks of S. pyogenes disease have been reported to occur in association with consumption of contaminated foods such as shrimp or potato salads. In this study, the behavior of S. pyogenes was studied in mashed potatoes as a function of storage temperature, types and amount of background biota and type of ingredients. Combined mashed potatoes (potatoes, butter, milk, egg and table salt) or plain mashed potatoes (potatoes only) were inoculated with a 5-strain cocktail of S. pyogenes and stored at 7, 25, 35 or 37C. At intervals during storage, samples were collected for counting S. pyogenes in blood agar plates or blood agar added with sodium azide, polymyxin and crystal violet. Mashed potatoes obtained from fast-food restaurants were used to determine the fate of S. pyogenes as affected by changes in aerobic mesophiles, coliform and lactic acid bacteria counts. S. pyogenes was able to survive in mashed potatoes stored at 7C and to grow in mashed potatoes stored at 25 or 37C with lag phase lengths of 3 and 2 h and generation times of 26.0 and 25.3 min, respectively. The generation time of S. pyogenes in plain mashed potatoes was 30.7 min at 35 C. Presence of active background biota at 2–3 log₁₀ CFU/g concentrations did not prevent growth of S. pyogenes when stored at 35C. These results contribute to a better understanding of the potential for S. pyogenes to cause foodborne outbreaks.     

Thermal treatment of aseptically processed tomato paste

The heat resistance characteristics of Bacillus coagulans (NRRL B-1103) spores suspended in buffer (pH 7.0, 4.5), tomato serum and tomato paste were studied. It was found that the heat resistance of spores was reduced significantly when buffer of pH 4.5, tomato serum or tomato paste was used as suspension medium instead of buffer pH 7.0. This effect was more apparent at higher temperatures. It was concluded that a thermal process of F_105°C = 3 min is capable of causing at least 3D destruction of spores of the most heat resistant strains of B. coagulans     

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.     

The heat resistance of spores of Clostridium botulinum 213B over the temperature range 120 to 140°C

The heat resistance of spores of Clostridium botulinum strain 213B heated in phosphate buffer (pH 7.0) was determined over the temperature range 120 to 140°C using a computer controlled thermoresistometer. The predicted D _121°C value of approximately 0.13 min and z value of 11.0°C (obtained by linear regression) was close to the classical D_121°C of 0.2 min and z value of 10°C used in process calculations. These data indicated that, according to the heat resistance of this culture, the z value of C. botulinum can be considered constant over the whole range 120 to 140°C. This also provided support for the recommended classical process calculations, using a z value of 10.0°C, being adequate if extrapolated to 140°C.     

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.     

Dielectric Properties of Potatoes and Potato Chips

SUMMARY— Dielectric properties (dielectric constant, ɛ'_r; loss tangent, tans; and dielectric loss factor, ɛ"_r) were determined for raw potatoes and for potato chips, using the precision slotted line technique. Measurements on raw potatoes were made at 77°F (25°C) and at frequencies of 300, 1,000, and 3,000 MHz (megahertz). Measurements on potato chips were made at three moisture levels; at 77°F (25°C), 125°F (51.6°C), and 180°F (82.2°0; and at frequencies of 300, 1,000, and 3,000 MHz. Moisture, total lipids, and nitrogen contents are reported for both the raw potatoes and the potato chips.
As would be expected from their high moisture content and the presence of various dissolved salts, the raw potatoes possess extremely high dielectric values.
Potato chips show a rapidly decreasing dielectric loss as moisture content is reduced; the loss values of the chips approach those of the oil used for frying them after moisture has been reduced to approximately 3% and the oil content has been increased accordingly.
For finish drying of potato chips, a frequency of 3,000 MHz will result in 3–4.5 times greater power production in the chips than will the use of a frequency of 1,000 MHz (in the moisture and temperature range at which the data were obtained). However, this difference in power production is due almost entirely to the difference in frequency since the difference in dielectric loss values at the two frequencies is quite small.     

THERMAL INACTIVATION KINETICS of TRYPSIN AT ASEPTIC PROCESSING TEMPERATURES

Kinetics of thermal inactivation of trypsin (bovine pancreas) were evaluated in the temperature range, 90–130C, common to aseptic processing of high/low acid foods. Aliquots of trypsin in buffer, at three pH values, 3.8, 5.1 and 6.0, were subjected to selected heat treatments at various temperatures. Kinetic parameters were evaluated from the residual enzyme activity. Reference k and D values (at 121.1C) ranged from 0.0719 to 0.349 min⁻¹ and 32.0 to 6.6 min, and E_a and z values ranged from 84.9 to 69.9 kJ/mole and 33.1 to 39.9C, respectively, in the pH range 3.8–6.0. the thermal inactivation resistance of trypsin in the acid and low acid pH range makes it a potential bioindicator for high temperature thermal processes.     

Dielectric Properties of Mashed Potatoes Relevant to Microwave and Radio-frequency Pasteurization and Sterilization Processes

ABSTRACT: Dielectric properties of mashed potatoes relevant to microwave and radio-frequency (RF) pasteurization and sterilization processes were measured over 1 to 1800 MHz and 20 °C to 120 °C. Effects of moisture content (81.6% to 87.8%, wb) and salt content (0.8% to 2.8%, wb) were investigated. Dielectric loss factors and constants decreased with frequency. Dielectric loss factors increased with temperature and salt content, but dielectric constants were not significantly affected. Ionic conductivity played a dominant role at low frequencies. Power penetration depth increased with moisture content and decreased with temperature, frequency, and salt content. Regression equations were developed to relate the dielectric properties to temperature, moisture, and salt contents.     

HIGH PRESSURE–HIGH TEMPERATURE STERILIZATION: FROM KINETIC ANALYSIS TO PROCESS VERIFICATION†

The inactivation of Clostridium sporogenes PA 3679 spores by high pressure at high temperatures (HP–HT) in phosphate buffer was investigated in a lab‐scale temperature‐controlled HP system (QFP‐6) with an internal heater to maintain the sample temperature. Some inactivation of spores occurred during the pressurization come‐up time (CUT) and depressurization time. The inactivation of PA 3679 was found to be exponential during the adiabatic holding period of the HP cycle at constant pressures and temperatures. The inactivation rate increased with both pressure and temperature. The kinetic parameters – such as D‐values at tested temperatures and pressures that are necessary for the design of process parameters of HP sterilization process – were determined. Within the pressure range of 600–800 MPa, the calculated D‐values ranged from 270.3 to 357.4 and 49.0 to 67.6 s at 91 and 108C, respectively. These studies provided basic data on the effects of pressure and temperature on the inactivation of PA 3679 spores under conditions applicable to the development of preservation specifications for commercial HP–HT processing of low acid foods. The spore strips of C. sporogenes were used as indicators for microbiological verification of delivered lethality of HP–HT sterilization process at different processing conditions in a pilot scale HP vessel.     

CHARACTERIZATION OF DYE-SENSITIZED PHOTOOXIDATION OF MUSHROOM TYROSINASE

Dye-sensitized photooxidation was investigated as a nonthermal means to inactivate food quality-related enzymes, using mushroom tyrosinase as a model. Illumination of tyrosinase in the presence of either rose bengal or riboflavin resulted in an apparent first-order destruction of enzyme activity. Both dye and light were required, and photoinactivation was favored by increasing levels of dissolved oxygen. The rose bengal-sensitized photooxidation was generally more rapid than that caused by riboflavin (at 0.01% dye), with k_i values ranging from 0.74 to 1.66 h⁻¹ and 0.25 to 1.23 h⁻¹, respectively. First-order rate constants for photoinactivation decreased with decreasing temperatures (E_a was 8.4 to 12.2 kcal mol⁻¹ between 20° and 50°C), increasing protein concentration (0.175 to 1.40 mg⁻¹ ml), increasing sodium phosphate buffer concentration (10 to 200 m M) and increasing ionic strength (0.02 to 0.20). The dependence of enzyme photoinactivation rates on pH (between 6.0 and 9.0) resembled a titration curve with a pK of 7.5, and maximal rates were observed at pH 8.0–9.0.     

TEXTURE-STRUCTURE RELATIONSHIPS IN HEAT-INDUCED SOY PROTEIN GELS

Structural factors responsible for textural properties of heat-induced soy protein gels were investigated using microscopic and mechanical testing techniques. The gels were prepared by heating 20% soy protein pastes for 30 min at temperatures ranging from 25 to 130°C . Gel hardness increased linearly with the heating temperature up to 80°C , and decreased when the gels were heated at over 90°C , especially over 120°C . The equilibrium modulus estimated by tensile stress relaxation experiments was of the order of 10⁴ - 10⁵ dyne/cm², suggesting the presence of crosslinks, and there was a good correlation between the equilibrium modulus and the hardness of the gels. Solubility in phosphate buffer containing 2 -mercaptoethanol and/or urea suggested that the gel network was formed through crosslinking of the disulfide-, hydrogen- and hydrophobic-bonding types, and that the textural properties were governed by the degree of the network formation controlled by the heating temperature. SEM images of the 80°C -induced hard gels revealed a porous structure having membranous walls of thin compact film. With the 40°C -induced soft gels, the formation of the porous structure was not yet adequate, while a partial collapse of this structure was observed in the 120°C -induced fragile gels.     

Thermal destruction of bacterial spores immobilized in food/alginate particles

Spores of Clostridum sporogenes PA 3679 (NCIB 8053) and Bacillus stearothermophilus (NCIB 8919) were immobilized in large (0·8–2·4 cm) cubes consisting of alginate mixed with pureed potatoes, pea or meat. These were heated in experimental heating chambers at temperatures between 120 and 150°C. Theoretical predictions of the efficiency of the sterilization process were then compared with experimental data obtained from spore survival levels. The results indicate that this method may be used for the experimental substantiation of thermal processes received by large particulate foods.     

Temperature Mapping of Particles During Aseptic Processing with Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) was used to obtain 2-dimensional temperature maps in potatoes undergoing aseptic processing. The change in precession frequency of protons served as the temperature indicator. The larger particles (6.9 and 3.84 cm3) showed a δT (T_surface - T_center) of up to 22±0.4°C 45s after exiting the heat exchanger with the δ (T_outlet - T_inlet) of the carrier fluid in the heat exchanger at 30 to 45°C. No AT was measured between the center and the surface of particles <2.05 cm³ pumped at <22.7 L/min. The average fluid to particle convective heat transfer coefficient (h_fp) for the heat exchanger and holding tube was calculated using a finite element method. The hfp was from 600 to 2500 and >3000 W/m²°C for the large (6.9 cm³ cubes) and smaller particles respectively.