James G. Lyng

Ohmic processing: Electrical conductivities of pork cuts

Efficacy of ohmic processing can be influenced by the conductivities of individual components within the food and their behaviour and interactions during the heating process. This study relates to the determination of electrical conductivities of a selection of pork meat cuts used in meat processing. Conductivity measurements of pork cuts indicated that lean is highly conductive compared to fat and addition of fat to lean reduced the overall conductivity but the addition of fat over the range (i.e. 0-100%) was non-linear. Light microscopy suggested that differences in the conductivities of leg and shoulder lean (entire) (0.76 vs. 0.64 Sm(-1), respectively) could be due to the denser muscle fibre structure and/or higher intra-muscular fat in shoulder vs. leg. This could be of significance for ohmic processing of full muscle products.

Combined effect of temperature and pulsed electric fields on apple juice peroxidase and polyphenoloxidase inactivation.

Pulsed electric fields (PEF) were applied to freshly prepared apple juice using a laboratory scale continuous PEF system to study the feasibility of inactivating peroxidase (POD) and polyphenoloxidase (PPO). Square wave PEF using different combinations of electric field strength, pre-treatment temperature and treatment time were evaluated in this study and compared to conventional pasteurisation (72°C; 26s). Inactivation curves for the enzyme were plotted for each parameter and inactivation kinetics were calculated. Results showed the highest level of decrease in the enzymatic activity of 71% and 68%, for PPO and POD, respectively, were obtained by using a combination of preheating to 50°C, and a PEF treatment time of 100μs at 40kV/cm. This level of inactivation was significantly higher (P<0.05) than that recorded in juice processed by conventional mild pasteurisation where the activity of PPO and POD decreased by 46% and 48%, respectively. The kinetic data for the inactivation of both enzymes could be described using a 1st-order model (P<0.001).

A survey of the dielectric properties of meats and ingredients used in meat product manufacture

The objective of the present study was to improve understanding of interactions between microwave (MW) and radio frequency (RF) radiation and meat/meat products. Dielectric properties at 27.12, 915 and 2450 MHz of lean, fat, aqueous solutions/suspensions and meat blends of typical ingredients used in meat product manufacture were measured. In addition temperature rises of ingredient/meat blends were compared following RF or MW heating. Frequency affected dielectric properties as did composition, with fat having lower dielectric activity than lean. Dielectric properties at MW frequencies appeared more sensitive to composition changes. Ingredients could be subdivided into groups having either lower or higher dielectric activity than lean, with concentration influencing which group an ingredient fell into. When low and high dielectric activity ingredients were incorporated into lean meats dielectric properties did not correlate well with temperature rises indicating the importance of other factors in addition to dielectric properties in determining temperature rise.

Ohmic cooking of whole beef muscle - optimisation of meat preparation.

Uniform ohmic heating of solid foods primarily depends on the uniformity of electrolyte distribution within the product. Different preparation techniques were tested in an attempt to ensure an even salt dispersion within a full beef muscle (biceps femoris). Meat pieces were soaked, injected and tumbled using a range of procedures before ohmic cooking at pasteurization temperatures. A final preparation method (multi-injection (five points) with a 3% salt solution followed by 16h tumbling) was validated. Selected quality parameters of the ohmically cooked products were compared to steam cooked products. Ohmically heated meat had a significantly (P<0.05) uniform lighter and less red colour. Cook loss was significantly lower (P<0.05) in ohmic samples and in relation to tenderness ohmic heated samples were tougher (P<0.05) though the difference was only 5.08N. Comparable cook values were attained in the ohmic and conventionally cooked products.

Effect of radio frequency cooking on the texture, colour and sensory properties of a large diameter comminuted meat product

Radio frequency (RF) cooking is a form of dielectric heating similar to microwave heating. In this study an optimised cooking protocol was developed for pasteurising 1 kg cased meat emulsion samples, which were immersed in 80 °C circulating water during cooking. Subsequently, selected quality attributes of RF pasteurised samples were compared to steam pasteurised samples, by instrumental and sensory methods. Instrumental assessments show that RF heated meat batters had a greater ability to hold water, were significantly harder, chewier and gummier (P<0.001), while having less cook colour development than their steam cooked counterparts. Differences were also detected by sensory methods. In conclusion, while differences were detected, it is possible that these could be eliminated by adjusting the cooking protocol to produce similar cook values in RF samples to those in products cooked by steam.

Effectiveness of High Intensity Light Pulses (HILP) treatments for the control of Escherichia coli and Listeria innocua in apple juice, orange juice and milk.

High Intensity Light Pulses (HILP) represent an emerging processing technology which uses short (100-400 μs) light pulses (200-1100 nm) for product decontamination. In this study, model and real foods of differing transparencies (maximum recovery diluent (MRD), apple and orange juices and milk) were exposed to HILP in a batch system for 0, 2, 4 or 8 s at a frequency of 3 Hz. After treatment, inactivation of Escherichia coli or Listeria innocua was evaluated in pre-inoculated samples. Sensory and other quality attributes (colour, pH, Brix, titratable acidity, non-enzymatic browning, total phenols and antioxidant capacity (TEAC)) were assessed in apple juice. Microbial kill decreased with decreasing transparency of the medium. In apple juice (the most transparent beverage) E. coli decreased by 2.65 and 4.5 after exposure times of 2 or 4 s, respectively. No cell recovery was observed after 48 h storage at 4°C. No significant differences were observed in quality parameters, excepting TEAC and flavour score, where 8 s exposure caused a significant decrease (p<0.05). Based on these results, HILP with short exposure times could represent a potential alternative to thermal processing to eliminate undesirable microorganisms, while maintaining product quality, in transparent fruit juices.

The effect of salt and fibre direction on water dynamics, distribution and mobility in pork muscle: A low field NMR study

The effect of salt concentration and fibre orientation on water within the meat matrix was investigated by low-field nuclear magnetic resonance (LF-NMR), water-binding capacity (WBC), diffusion studies and histological analysis. Pork M. longissimus thoracis et lumborum samples were cured with 5.7, 15.3 or 26.3% w/w NaCl at a parallel or perpendicular fibre direction. NMR transverse (T2) relaxation identified three water components (T2b, T21 and T22) which all exhibited characteristics correlated to WBC. Results indicated that T2b increases with increasing NaCl concentration. Increasing intra-myofibrillar water and decreasing extra-myofibrillar water resulted in the highest WBC. Water diffused more quickly into the extra-myofibrillar space in samples cured at a parallel fibre direction. This water remained loosely bound in samples cured with the saturated solution (26.3% w/w NaCl) leading to decreased WBC. This study provides further information on water binding within the meat matrix by applying the results of LF-NMR to traditional water-binding theories.

PEF based hurdle strategy to control Pichia fermentans, Listeria innocua and Escherichia coli k12 in orange juice.

The combination of pulsed electric fields (PEF) and bacteriocins in a hurdle approach has been reported to enhance microbial inactivation. This study investigates the preservation of orange juice using PEF in combination with nisin (2.5 ppm), natamycin (10 ppm), benzoic acid (BA; 100 ppm), or lactic acid, (LA; 500 ppm). Pichia fermentans, a spoilage yeast frequently isolated from orange juice, Escherichia coli k12 or Listeria innocua were inoculated into sterile orange juice (OJ) with, and without, added preservatives. The antimicrobial activity over time was evaluated relative to an untreated control. The effect of PEF treatment (40 kV/cm, 100 micros; max temperature 56 degrees C) was assessed on its own, and in combination with each antimicrobial. The acidic environment of OJ inactivated E. coli k12 (1.5log reduction) and L. innocua (0.7log reduction) slightly but had no effect on P. fermentans. PEF caused a significant decrease (P<0.05) in the viability of P. fermentans, L. innocua and E. coli k12 achieving reductions of 4.8, 3.7 and 6.3log respectively. Nisin combined with PEF inactivated L. innocua and E. coli k12 in a synergistic manner resulting in a total reduction to 5.6 and 7.9log respectively. A similar synergy was shown between LA and PEF in the inactivation of L. innocua and P. fermentans (6.1 and 7.8log reduction), but not E. coli k12. The BA-PEF combination caused an additive inactivation of P. fermentans, whereas the natamycin-PEF combination against P. fermentans was not significantly different to the effect caused by PEF alone. This study shows that combining PEF with the chosen preservatives, at levels lower than those in current use, can provide greater than 5log reductions of E. coli k12, L. innocua and P. fermentans in OJ. These PEF-bio-preservative combination hurdles could provide the beverage industry with effective non-thermal alternatives to prevent microbial spoilage, and improve the safety of fruit juice.

Inactivation of Escherichia coli in a Tropical Fruit Smoothie by a Combination of Heat and Pulsed Electric Fields

Moderate heat in combination with pulsed electric fields (PEF) was investigated as a potential alternative to thermal pasteurization of a tropical fruit smoothie based on pineapple, banana, and coconut milk, inoculated with Escherichia coli K12. The smoothie was heated from 25 degrees C to either 45 or 55 degrees C over 60 s and subsequently cooled to 10 degrees C. PEF was applied at electric field strengths of 24 and 34 kV/cm with specific energy inputs of 350, 500, and 650 kJ/L. Both processing technologies were combined using heat (45 or 55 degrees C) and the most effective set of PEF conditions. Bacterial inactivation was estimated on standard and NaCl-supplemented tryptone soy agar (TSA) to enumerate sublethally injured cells. By increasing the temperature from 45 to 55 degrees C, a higher reduction in E. coli numbers (1 compared with 1.7 log(10) colony forming units {CFU} per milliliter, P < 0.05) was achieved. Similarly, as the field strength was increased during stand-alone PEF treatment from 24 to 34 kV/cm, a greater number of E. coli cells were inactivated (2.8 compared with 4.2 log(10) CFU/mL, P < 0.05). An increase in heating temperature from 45 to 55 degrees C during a combined heat/PEF hurdle approach induced a higher inactivation (5.1 compared with 6.9 log(10) CFU/mL, respectively [P < 0.05]) with the latter value comparable to the bacterial reduction of 6.3 log(10) CFU/mL (P> or = 0.05) achieved by thermal pasteurization (72 degrees C, 15 s). A reversed hurdle processing sequence did not affect bacterial inactivation (P> or = 0.05). No differences were observed (P> or = 0.05) between the bacterial counts estimated on nonselective and selective TSA, suggesting that sublethal cell injury did not occur during single PEF treatments or combined heat/PEF treatments.

Antimicrobial effect and shelf‐life extension by combined thermal and pulsed electric field treatment of milk

Aims:  The impact of a combined hurdle treatment of heat and pulsed electric fields (PEF) was studied on native microbiota used for the inoculation of low‐fat ultra‐high temperature (UHT) milk and whole raw milk. Microbiological shelf‐life of the latter following hurdle treatment or thermal pasteurization was also investigated.