Recent progress in food processing applications of air impingement technology: A review.

Air impingement method has been widely used in a variety of industrial applications, such as textile and paper drying, turbine cooling, and glass quenching, because it is an efficient technology with high heat and mass transfer rates. This technology has received increasing interest in the field of food processing over the last two decades, such as drying, baking, blanching, freezing, and thawing. In a food processing equipment using air impingement, jets of high-velocity air (with speeds of 10-50 m/s) are directed at a food product. The performance of the system is influenced by several critical elements, including jet velocity, nozzle array diameter and layout, jet distance, and boundary layer characteristics. The use of computational fluid dynamics, an emerging tool, has been shown to be valuable in the analysis of fluid flow and heat and mass transfer in jet impingement systems. The physical properties of impinging jets, such as turbulent mixing in the free jet zone, stagnation, boundary layer formation, recirculation, and their interactions with food products in terms of heat and mass transfer, have been discussed in this article. The benefits and disadvantages of air jet impingement technology in different food processing applications together with potential trends for improving impingement technology performance were identified and discussed. This review not only contributes to a better understanding of the research status of impingement technology on food processing but also triggers new research opportunities in this field in order to provide more healthy and nutritious food in a more sustainable way to the world's growing population.

Yoghurt with candied chestnut: freeze drying, physical, and rheological behaviour.

As a novel product, yoghurt powder was produced by freeze drying and with added candied chestnut puree at ratios of 5, 10, and 20 % by weight. During the freeze drying process, mass loss, water activity, and the moisture content of the samples were determined and the colour (Hunter L, a, b) of the yoghurt powder products was measured. Results showed that increasing the percentage of candied chestnut puree resulted in an increase in water activity, moisture content, and colour change values of the end product. The drying behaviour, drying rate versus free moisture content, was also investigated. It was observed that yoghurt with or without added candied chestnut puree could be satisfactorily freeze-dried. Moreover, the performance of the dried product was observed in a ready-to-use, reconstituted form. For this purpose, the obtained powders were reconstituted to their original moisture contents. Shear stress and apparent viscosity against shear rate in a range of 1-1,000 (1/sec) was then measured by a Haake-Mars rotary viscometer. According to the results, the apparent viscosities of reconstituted products, as plain yoghurt and the one with an added 5 % chestnut puree were lower than that of fresh yoghurt. However, reconstituted yoghurts containing 10 % and 20 % chestnut puree had apparent viscosities higher than fresh yoghurt. Power Law explained well the rheological behaviour of reconstituted yoghurt samples for the applied shear rate range. Based on rheological data and sensory analysis, it was concluded that the freeze dried yoghurt containing 10 % (w/w) candied chestnut puree was an acceptable novel product.