Effects of vacuum freezing pre-drying on the textural properties of explosion puffing dried fruit and vegetable crisps

Explosion puffing drying is well known as one of the most novel processing of food products. The raw materials are also required to be pre-dried at the given moisture content. A preferred combination of vacuum freeze-drying and explosion puffing drying can be utilized to produce the crispy products of fruit and vegetable with better color and higher brittleness. Among them, vacuum freeze-drying can be used to maintain a better shape, and then form a more loose porous structure in the dried samples. This study aims to explore the effect of pre-drying treatment on the structural and textural properties of different fruit and vegetable crisps after vacuum freeze-drying combined with explosion puffing drying. Three moisture conversion points were also selected (60 %, 45 %, and 30 % moisture content in wet base). The moisture status, cell structure, shrinkage, porosity, stress-relaxation properties, and texture profile analysis (TPA) were analyzed for the pre-dried samples. The drying time, energy consumption, and texture properties were determined for the dried crisp samples. The results showed that the moisture content gradually decreased, and the free water gradually disappeared with the pre-drying, while the immobilized water was the main component of water in the samples. The decreased degree of free water content presented a significant difference among different fruit and vegetable slices, when the vacuum freeze-drying to 60 % moisture content. The free water contents of apples, peaches, pears, and potatoes decreased by 94.94%, 89.76%, 91.53%, and 99.49%, respectively. A large amount of free water was removed from the pre-dried samples during the early drying stage. The shrinkage rate of dried crisps decreased gradually, as the water conversion point continued to decrease, whereas, there was an increase in the porosity, hardness, chewiness, and elastic modulus. The higher shrinkage ratios were found in the crisps of apple, peach, and pear, compared with the rest. The higher hardness was found in the crisps of potato and yam. Green radish crisps shared the more severe fractures of the cell wall and the highest porosity. There was a significant difference in the texture properties of fruit and vegetable crisps with the different moisture conversion points (P<0.05). Once the water conversion point was 60%, the hardness and brittleness of the fruit and vegetable crisps were relatively higher, with the potato and yam crisps having the highest hardness, whereas, the peach and pear crisps had the highest brittleness. The dried crisps presented a too-soft texture and poor taste at the lower water conversion point. The reason was that the fruits and vegetables with the higher moisture easily formed the dry layer on the surface of the samples during expansion, resulting in a higher hardness of the chips. Once the moisture content was lower, the skeleton structure in the samples were basically formed after vacuum freeze-drying, while the subsequent explosion puffing drying then resulted in insufficient expansion force and minimal impact on the texture of the samples. The porosity of green radish crisps was significantly higher than that of the rest when the water conversion point was 30%. The skeleton in the samples was basically formed at the later stage of drying. The lower shrinkage ratio resulted in a larger internal porosity. The higher the water conversion point was, the shorter the combined drying time was, particularly for the fruits and vegetables, except for peach and pear, and the lower the energy consumption for all fruits and vegetables was. The statistical analysis revealed that the moisture conversion point and porosity played a crucial role in the texture properties of the fruit and vegetable crisps during vacuum freeze-drying. The finding can provide a significant implication on the drying time and energy consumption.