Advances in radio frequency and ohmic heating of meats

Abstract

Interest in both radio frequency (RF) and ohmic heating of foods has increased in recent years. In particular, their application to meat products has been investigated. A critical factor is the dielectric constants of the products being heated. These have been measured for meats. In particular, the dielectric constant ε′, dielectric loss factor ε″, thermal heat capacity c p , thermal conductivity k and thermal diffusivity α of two local comminuted meat products of differing diameters, both pork based, (pork luncheon roll PLR and white pudding WP) were measured between 5 and 85 °C. Radio frequency (RF) and microwave (MW) ε″ values varied across 5–85 °C ( P < 0.05). Microwave ε′ and ε″ values for WP tended to peak at 45 °C and decrease thereafter, whereas for PLR, ε′ and ε″ peaked at 65 °C which appeared to match potato starch gelatinisation within this product. WP and PLR had significantly higher c p values at 25 °C, which corresponded to the melting point of pork fat. At 85 °C, k values were higher ( P < 0.05) than at 5, 25 and 45 °C but were not higher than values at 65 °C. Thermal diffusivity α values increased with temperature ( P < 0.05). For ohmic heating, the electrical conductivity becomes the controlling variable. 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. Conductivity measurements on 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 S m −1, respectively) could be due to the denser muscle fibre structure and/or higher intra-muscular fat in the shoulder vs. the leg meat. This could be of significance for ohmic processing of full muscle products. Of course, for both forms of heating, the quality of the heated product becomes the critical factor. The effect of radio frequency cooking, on the quality (assessed by cook yield, water holding capacity texture profile analysis, penetration test, Warner–Bratzler shear, colour and sensory evaluation) and cooking time of two types of pork products (leg ham and shoulder ham) were compared to steam cooked samples. RF cooking of the hams resulted in a shorter cooking time. Instrumental measurements indicated that RF heated samples had a higher cook yield ( P < 0.05), but a lower water holding capacity ( P < 0.05). Texture profile analysis indicated that RF cooked samples were harder ( P < 0.05), particularly for leg hams. A sensory panel also indicated that panellists could distinguish between radio frequency and steam cooked samples ( P < 0.05).