Innovative application of lemon by-products as a drying aid in the freeze-drying of lemon: an analysis of drying kinetics and powder quality

Abstract The production of a non-sticky powder requires suitable drying aids or carriers, which are one of the key elements in the drying of fruit. High-molecular-weight substances are used as drying aids in the industry owing to their high glass transition temperature (T g ). Fiber-rich by-products from the processing of fruit juices could potentially provide high molecular weight components suitable as drying aids. Consequently, the aim of this study was to assess a new drying aid obtained from lemon juice by-products in the freeze-drying of lemons and to determine the freeze-drying kinetics and physicochemical properties of lemon powder. For this purpose, lemon juice by-products were used directly as a drying aid and mixed with lemon juice to produce a puree. The prepared puree was dried in a freeze dryer, the resulting powder product was subsequently ground and sieved through two sieves of 40 and 80 mesh, and the effect of the particle size on the quality characteristics of the powder was evaluated. Nine different thin-layer drying models were applied to determine the freeze drying kinetics. It was found that the Wang and Singh model best explained the drying kinetics of the lemon samples, and the average value of the effective diffusivity was determined to be 7.12 × 10 − 10 m 2 /s. The water activity (0.306 < 0.320) and porosity (65.86 < 66.67%) of the lemon powder with smaller particle size were lower those of the lemon powder sample with larger particle size, while the glass transition temperature (53.07 > 49.07 ℃), the total phenolic content (6.712 > 6.041 mg GAE/g dw), vitamin C (91.03 > 87.44 mg/100 g), the compressed bulk density (535. 2 > 510.9 kg/m 3 ), particle density (1568 > 1533 kg/m 3 ), flowability (Carr index value of 24.82 > 21.44%) and wettability (178.4 > 57.06 s) were higher, indicating that the particle size influences the physical properties and thus the chemical properties of the powder product. The dried samples showed higher antioxidant activity than the fresh sample. The use of lemon by-products as a drying aid preserved the original color of the fruit.