Pilot-plant test of mushroom instant soup by vacuum freeze drying based on energy consumption analysis

Mushroom instant soup is one of the most popular snack products in the diet of modern consumers. The typical consumption pattern is dramatically ever increasing for ready-to-eat, convenient, and cheap snacks, especially in most developing countries. The high-class mushroom can also provide delicious taste and abundant nutritive value. However, it has not yet been fully investigated how to develop instant soup with rich flavor and nutritional value.Compared with other preparation methods, freeze drying is commonly recognized to allow obtaining a higher quality product which can be easily rehydrated and retain organoleptic properties. In general, freeze-drying allows the dehydration of materials by sublimation at low temperature and sub-atmospheric pressure, thereby retaining higher nutritional value, flavor, color, and creating a porous structure in the dried products, all of which are desired attributes of dried products. However, freeze-drying is a typical high-energy-consuming operation, due mainly to the necessity of freezing products, heating the frozen samples to induce sublimation and lowering the total pressure of the dehydration chamber. As a result, freeze-drying in the food industry is restricted only to be applied in high-value-added products. Therefore, many attempts have been made to modify and optimize the whole drying process in order to extend the application fields of freeze-drying. A variety of products has been investigated on freeze drying using various theoretical models. Currently, the specific freeze- drying process can be simulated in many different approaches. An appropriate drying model needs to define the coordinate system, space dimensions, sublimation interface type, meshing technique, parameters variations over time and of time discretization. As such, various process variables can be used to evaluate the efficiency of the freeze-drying process, in terms of productivity and product quality (obtaining the highest quality in the shortest cycle time), especially in large scale production. Additionally, it is also highly required that the optimization should be implemented in-line pilot scale. In this study, a pilot plant test was conducted to investigate the energy-saving preparation of mushroom instant soup with an online regulation integrated freeze-drying. The operation parameters were analyzed to obtain optimal operation conditions, including the freezing stage (freezing temperature, thickness of loaded soup), sublimation stage (drying chamber pressure, heating temperature), and desorption stage (moisture conversion point, heating process). Especially, step-by-step heating was proposed and the freeze-drying curve was fitted in the desorption stage. The results showed that an optimal combination of parameters was achieved, where the pre-freezing temperature was set at -35 ℃, the thickness of loaded soup was 15 mm, the drying chamber pressure was 25 Pa, and the water content of the soup was 83.1%; the temperature of the heating plate in the sublimation stage was -10 ℃, the starting point in the desorption stage was 1 148 min (at this time, the moisture content of mushroom instant soup was 9.2%), and the heating (-10 ℃ → 2.5 ℃ (70 min) → 15 ℃ (70 min) → 27.5 ℃ (70 min) → 40 ℃ (144 min)) in the desorption stage. In this case, the the yield was 99.2%, overall score for sensory quality indicator was 8.37±0.52, the energy consumption was 1.75 kW•h/kg, and the water content of soup was 4.3%. Additionally, the fitting results of freeze drying curve during desorption stage showed that the Boltzmann model was well fit with the freeze-drying process in the desorption stage. The finding can provide a sound reference and technical support to the parameters optimization and energy consumption on the freeze-drying, particularly for the application of freeze-drying in the production of modern convenient food. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.