Freeze-drying is an important processing unit operation in food powder production. It offers dehydrated products with extended shelf life and high quality. Unfortunately, food quality attributes and grinding characteristics are affected significantly during the drying process due to the glass transition temperature (during drying operation) and stress generated (during grinding operation) in the food structure. However, it has been successfully applied to several biological materials ranging from animal products to plants products owning to its specific advantages. Recently, the market demands for freeze-dried and ground food products such as spices, vegetables, and fruits are on the increase. In this study, the effect of the freeze-drying process on quality attributes, such as structural changes, the influence of glass transition during grinding, together with the effect on grinding efficiency in terms of energy requirement, grinding yield, and morphological changes in the powder as a result of temperature, drying time were discussed. An overview of models for drying kinetics for freeze-dried food sample, and grinding characteristics developed to optimize the drying processes, and a prediction of the grinding characteristics are also provided. Some limitations of the drying process during grinding are also discussed together with innovative methods to improve the drying and grinding processes.

Grinding is a staple size-reduction process to produce food powders in which the powdered form is chemically and microbiologically stable and convenient to use as end products or intermediate products. The moisture content of food materials before grinding is a particularly important factor, since it determines the materials’ physical properties and the powder properties, such as flowability after grinding. Generally, the moisture content of food materials is closely related to its energy requirement for grinding, because the energy expenditure required to create new surfaces varies. Grinding models used to analyze and predict the grinding characteristics, including energy, have been developed in many studies. The moisture content also influences powder flow properties. The inter-particle liquid bridges among the particles are due to the moisture in powders; therefore, the flowability of powders is interrupted because of the increase of the cohesiveness of the powder. Understanding the grinding characteristics related to various moisture contents is, theoretically and experimentally, an important cornerstone in optimizing the grinding processes used in food industries. In this review, comprehensive research of the effect of moisture content on the grinding process and powder properties is presented.

Computational fluid dynamics (CFD) is a powerful tool to model fluid flow motions for momentum, mass and energy transfer. CFD has been widely used to simulate the flow pattern and temperature distribution during the thermal processing of foods. This paper discusses the background of the thermal processing of food, and the fundamentals in developing CFD models. The constitution of simulation models is provided to enable the design of effective and efficient CFD modeling. An overview of the current CFD modeling studies of thermal processing in solid, liquid, and liquid-solid mixtures is also provided. Some limitations and unrealistic assumptions faced by CFD modelers are also discussed.

Sea cucumber (Stichopus japonicus Selenka, SC) was dried using far infrared radiation drying (FIRD) and air drying (AD). The effect of the drying temperature of FIRD and AD on the drying rate of SC was investigated. Temperatures varied at 60, 70 and 80C, with a constant air velocity (13.2 m/s). Thin-layer drying models were applied to describe the drying process of SC with FIRD and AD. The Midilli and Kucuk equation showed the best fit (R² > 0.99). The effective moisture diffusivity (D_eff) was estimated by the diffusion model based on the Fick's second law. The D_eff values of FIRD and AD treated samples varied in the range of 3.39 × 10⁻¹⁰ to 5.16 × 10⁻¹⁰ m²/s and 2.81 × 10⁻¹⁰ to 3.81 × 10⁻¹⁰ m²/s, respectively. The activation energy (E_a) based on the drying temperature showed that the thermal sensitivity of FIRD was higher than that of AD. The scanning electron microscope images showed more damages on the surface of AD treated sample than that of FIRD. Practical Applications Dried sea cucumbers are highly value-added products. Currently, solar radiation is mainly used to produce the dried sea cucumber, but solar radiation drying requires more than 5 days under suitable environment. Such a long drying period causes sanitation problems as well as more cost to produce the dried sea cucumber. Our study showed that the far infrared radiation drying (FIRD) dramatically shortens the drying time. The quality of dried sea cucumber using FIRD was better than that of the air drying, because the surface hardening of the sea cucumber was minimized by the radiation heating mechanism. FIRD might be a good technology to replace the solar drying as well as the air drying to produce the dried sea cucumber.

The progress of food 3D printing (3DP) applications demands a full understanding of the printing behavior of food materials. Computational fluid dynamics (CFD) simulation can help determine the optimum processing conditions for food 3DP such as layer height, deposit thickness, volume flow rate, and nozzle shape and diameter under varied material properties. This paper mainly discusses the application of CFD simulation for three core processes associated with 3DP: (1) flow fields in the nozzle during the extrusion process; (2) die swelling of materials at the die (the exit part of the nozzle); and (3) the residual stress of printed products. The major achievements of CFD simulation in food 3DP with varied food materials are discussed in detail. In addition, the problems and potential solutions that modelers encountered when utilizing CFD in food 3DP were explored.

Yun Ju Lee · Won Byong Yoon. JABC 2020;63:1-8. https://doi.org/10.3839/jabc.2020.001