Effects of puncture combined with ethanol pretreatment on the hot air drying characteristics of carrot slices

Hot air drying is required for the short duration, energy saving, and high quality of food products. In this study, an artificial perforation and alcohol (PAT) pretreatment was proposed to enhance the hot air drying of carrot slices. The carrot was first cut in the transverse and longitudinal sections. Various parameters were then evaluated, including the effective moisture diffusion coefficient, drying rate and time, color, volume shrinkage, rehydration potential, total content of carotenoids and ascorbic acid, antioxidant activity, and flavor. Control samples were selected without pretreatment or subjected to separate perforation (PT) and alcohol maceration (AT) pretreatments. The results demonstrated that the effective moisture diffusion coefficients of pretreated carrot slices ranged from 1.07 to 1.69 times higher in the AT, PT, and PAT pretreatments, compared with the unpretreated samples. The high drying rates enhanced the drying efficacy of carrot slices during hot air drying in the PT, AT, and PAT pretreatments. The drying duration was reduced by 10.61%~50.00%, compared with the unpretreated samples, when the moisture content fell to 0.1 (g water/g dry basis). There was a positive correlation between the drying duration and energy savings. Terefore, at constant drying temperature, air velocigy, and loading, reducing the drying duration implies saving a certain amount of energy. Among them, the PAT pretreated samples exhibited the highest effective moisture diffusion coefficient and drying rate, with the shortest drying duration, followed by the AT, PT, and unpretreated samples. Additionally, a triple enhancement was proposed for the carrot slices drying in the PAT pretreatment: 1) The dehydration load was reduced at the initial dehydration stage in the pretreatment during hot air drying; 2) Capillary water flow was induced by the porous structure that created by artificial perforation; 3) Cell permeability increased to reduce the water flow resistance. Three factors improved the hot air drying of carrot slices. Importantly, the longitudinally-cut carrot slices shared a higher effective moisture diffusion coefficient and drying rate, but a shorter drying time in all pretreatments, compared with the transversely-cut slices. This was attributed to the severe volume shrinkage in the transverse cuts during drying, leading to the less microchannels and impeded moisture diffusion. In physicochemical quality, the hot air drying negatively impacted the quality of fresh carrots. However, the PT, AT, and PAT pretreatments improved the quality of dried carrot slices, in terms of color, volume shrinkage, rehydration, total content of carotenoids and ascorbic acid, antioxidant activity, and flavor. Specifically, PAT pretreatment significantly mitigated the volume shrinkage of longitudinally-cut carrot slices during drying, leading to the least color alteration, the highest saturation and rehydration capacity, as well as the highest content of volatile components. Furthermore, there was the strongest antioxidant activity, due to a relatively high concentration of total carotenoids and ascorbic acid. In conclusion, PAT pretreatment on longitudinal slicing can be an efficient strategy to produce dehydrated carrot slices with superior quality. This research offers practical and effective implications to enhance the hot air drying of fruits or vegetables at an industrial scale.