Thermal behavior of food materials during high pressure processing

Knowledge about temperature and pressure history is important for evaluating process uniformity during high pressure processing (HPP). Process uniformity is further influenced by the food’s thermal properties. This study was conducted to estimate thermal conductivity and heat of compression of selected materials under pressure to evaluate thermal behavior of foods under pressure.

Thermal conductivity (k) of selected foods was determined using a line heat source probe. The probe was calibrated using distilled water. Probe specific calibration factors were developed by comparing experimental data against National Institute of Standards and Technology (NIST) data for water. Thermal conductivity of selected liquid (apple juice, canola oil, clarified butter, honey and high fructose corn syrup) and solid foods (carrot, cheddar cheese, guacamole, chicken breast and chicken fat) were determined at pressures between 0.1 and 700 MPa. The process temperatures used were 25° for liquids and 25, 50 and 75° for solid foods. Thermal conductivity increased linearly for all foods with increasing pressures. Among the liquids tested, water and apple juice had the highest k (0.82 W/m°C), while fatty foods had the lowest (0.4 W/m°) at 700 MPa. In solid foods, k increased with increasing moisture content and process temperature and decreased with increasing fat content. Carrot had the highest k (0.90 W/m°), while chicken fat had the lowest k (0.43 W/m°C) at 700 MPa and 75° The combined uncertainty in the measured k values ranged from 0.6% (canola oil) to 4.2% (chicken fat). Effect of polarity and molecular structure on heat of compression (δ) was analyzed in a separate study. While polar liquids showed a linear trend with the δ, non-polar liquids exhibited a nonlinear relationship. Heat of compression decreased with increasing polarity index (8.8° per100 MPa for chloroform to 3° per100 MPa for water). Change in carbon chain length (C₂ to C₄) and degree of saturation (C_18:1 to C_18:3) of selected fatty acids had effect on δ only at elevated pressures. Empirical relationships were developed for predicting k and the maximum rise in temperature under pressure.