Novel energy-saving cryogenic equipment for walnut kernel peeling

In the food industry, walnut kernels are used in baking, pastry and confectionery as well as for flavouring drinks and ice creams.  The peeling of the skin coating the walnut kernel is a value-added first step before any further processing. This process reduces the tannin content, thus improving the taste of the final walnut product. However, current methods of walnut kernel peeling are limited, either by high energy consumption or by low peeling efficiency. 

 

A recent study examined cryogenic treatment methods for walnut kernel peeling. The advantage of cryogenic treatment over high-temperature treatment is that it can inhibit metabolism and achieve long-term preservation without damaging the texture and composition of the products. For walnuts, previous studies have shown that walnut kernels do not spoil in a cryogenic environment. 

 

Liquid nitrogen immersion for walnut kernel peeling 

 

Walnut kernels separate from their skin when cooled directly in a liquid nitrogen (LN₂) bath at a temperature of −196 °C. It is the difference in the coefficient of thermal expansion between the walnut kernel and its skin that leads to the separation of the two. The main disadvantage of this method is the high LN₂ consumption. Indeed, during the process, the N₂ vapour is discharged right after the liquid-to-gas phase transition of the cooling medium. In other words, only the latent heat of LN₂ is used, while a significant part of the cooling power of low-temperature N₂ gas is wasted. 

 

The authors of the study verified the effectiveness of LN₂ immersion for walnut kernel peeling. They found that the liquid nitrogen immersion method could indeed achieve complete peeling, but its energy consumption was very high (14.61 kg LN₂ for 200 kg walnut kernels).

 

Novel cryogenic equipment for walnut kernel peeling 

 

The authors designed an industrial-scale cryogenic pre-treatment equipment capable of recovering cold energy and reducing LN₂ consumption.

   

Walnut kernels at room temperature were transported into the equipment where they were cooled to −160 °C by cold gas/LN₂. After low temperature treatment, the walnut kernel skins started to shrink and crack. They were then gradually removed from the system by introducing air. The prototype equipment could peel and discharge 200 kg of walnut kernels every 20 min.  

 

Room temperature air was used to transfer heat within the system. Its temperature gradually lowered to −160 °C as a result of the heat exchange occurring with the low-temperature N₂ and the cold walnut kernels. Afterwards, the temperature of air gradually raised to room temperature as it passed through the walnut kernels at a higher temperature. Finally, the air was discharged through the exhaust vent. Since both the walnut kernels and the working air were at room temperature when entering/leaving the equipment, the authors hypothesised that their system could theoretically achieve complete recovery of the cooling energy. 

 

Their results showed that the prototype equipment could provide similar peeling performance to that achieved by immersion in LN₂ while saving up to 92.81% of the cooling power. Furthermore, the greater the mass of walnut kernels being processed, the greater the energy-saving advantage for the prototype equipment. 

 

 

Source 

Zhao, Y., Chen, L., Ji, W., Guo, J., & Wang, J. (2021). Study on a novel energy-saving cryogenic pre-treatment equipment for walnut kernel peeling. Food Control, 121. https://doi.org/10.1016/j.foodcont.2020.107650