Accurate measurement of water activity (aw) is an important goal for the food industry because aw is a key parameter in microbial growth, biological reaction rates and physical properties. An experimental device was setup using air-product water balance to non-destructively estimate the time-course of mean aw at the food product surface under well-controlled airflow conditions. The device is especially suited for studying the ripening of cheeses and fermented meat products, where water fluxes exchanged between products and air are very low. The validation tests performed with aw-known model products showed that water fluxes of 10(−7) kg s−1 can be estimated with an accuracy better than 2% over very short periods of time, and that surface aw can be estimated with an absolute uncertainty of less than 0.01 aw units. A handful of cheese ripening trials illustrate the potential of the method, highlighting the effects of a low air velocity and high air RH on the water exchanges occurring at a cheese surface, thus demonstrating the strong surface sensitivity to external air conditions.

This paper reports on the development of an approximate empirical-CFD modelling procedure combining experimental correlations for the heat and mass transfer coefficient determination and specific user-defined functions (UDF) implemented in the commercial CFD code Fluent to simulate the interrelationships at play between airflow and transfers of unwrapped food products bathing in air. Transfer modelling required specific calculation to account for water evaporation from the product surface. Mean transfer coefficients at the air-product interface were determined empirically from validated experimental correlations, where the CFD mean air velocity that is calculated in a small user-defined volume surrounding the product was incorporated. Internal water diffusion was taken into account by considering the product as a motionless fluid composed of dry matter and water. The procedure developed allows, in a single CFD calculation using a single mesh, temperature and water concentration fields to be computed and the water loss kinetics of the products to be determined together with the temperature and relative humidity fields of the air flowing around them. Although the established procedure was set up with the aim of assessing heat and water exchanges occurring in large stacks of unwrapped food products subjected to airflow, this last was validated for modelling, first, one-dimensional drying of a meat cylinder and second, three-dimensional drying of five cylindrical plaster casts placed in a row. Its application to the ripening of a row of six cheeses is also discussed.

There has been developed a two-stage method for cooling jars with preserved food after sterilization. The first stage comprises cooling preserves up to t 75-80 deg. C in the air flow, the air velocities being 2.75, 3.7, 4.8, and 5.8 m/sec. The time of cooling reduced as the air velocity increases to be 11, 9, 8.5 and 8 min, respectively. The average speed of cooling gradually increases from 1.82 deg. C/min to 2.5 deg. C/min. The second stage comprises continuing cooling to apply a water film of 60-65 deg. C to the jar surface at 5-10 sec interval. During the process a jar is with a certain frequency is overturned down from the bottom to the cap. The average speed of cooling product is 3.33 deg. C/min at cooling air velocity of 2.75 m/sec and it is gradually increasing to reach 5.4 deg. C/min. Experimentally range of optimum cooling air velocities has been determined to be 4.8-5.8 m/sec, cooling time being decreased by 0.6 min only. A mathematical model has been has been developed describing the two-stage air and water vaporizing cooling time of stewed fruit jars depending on some factors. Relative error in comparing target values with experimental values did not exceed 8%. The developed method is recommended to be used at food canning industry enterprises and for designing continuously operating devices. | Разработан двухэтапный способ охлаждения банок с консервированными продуктами после стерилизации. На первом этапе (до t 75-80 град. С) охлаждение консервов производят в потоке атмосферного воздуха при различных скоростях охлаждающего воздуха – 2,75; 3,7; 4,8 и 5,8 м/с. Продолжительность охлаждения с увеличением скорости охлаждающего воздуха снижается и составляет соответственно 11,0; 9,0; 8,5 и 8,0 мин. При этом средняя скорость охлаждения продукта постепенно увеличивается с 1,82 град. С/м до 2,5 град. С/мин. На втором этапе охлаждение продолжается с нанесением на поверхность банки водяной пленки температурой 60-65 град. С с интервалом 5-10 с, при этом в процессе охлаждения банку с определенной частотой переворачивают с донышка на крышку. Средняя скорость охлаждения продукта составляет 3,33 град. С/мин при скорости охлаждающего воздуха 2,75 м/с и постепенно увеличивается, достигая 5,4 град. С/мин. Экспериментально определен интервал оптимальных скоростей охлаждающего воздуха (4,8-5,8 м/с), при котором продолжительность процесса охлаждения сокращается всего на 0,6 мин. Составлена математическая модель, описывающая продолжительность двухэтапного воздушно-водоиспарительного охлаждения банок с компотами в зависимости от ряда факторов. Относительная погрешность при сопоставлении расчетных значений с опытными не превышала 8%. Разработанный способ рекомендуется для применения на предприятиях консервной промышленности и при проектировании аппаратов непрерывного действия.