Anabel Rodriguez

Experimental study of the application of edible coatings in pumpkin sticks submitted to osmotic dehydration

ABSTRACT Osmotic dehydration (OD) improves the organoleptic characteristics of products; nevertheless, depending on the product, the incorporation of great amounts of solute can be undesirable to consumers. The application of hydrogel coatings would overcome this, acting as a barrier to solute entering the product. An alginate–calcium chloride barrier system (A-CC) was tested with or without pretreatment of Ca2+ as a firmness agent applied to pumpkin sticks. Products were dehydrated in sucrose and glucose solutions (40 and 60% w/w). Different parameters were estimated such as soluble solids uptake, water content, and weight reduction; in addition, texture and microstructural changes were studied. The A-CC configuration complies with the barrier function to solids uptake from the dehydrating solution in case of 60% of sucrose, allowing water exit from the vegetal matrix. The pretreatment with Ca2+ contributed to the retention of product firmness. Integrity, adhesiveness, and thickness of the A-CC system during the OD process were evaluated by environmental scanning electron microscopy (ESEM). In this case, the coating thickness reduction was according to the product weight reduction, indicating that the coated product was dehydrated as an integrated system.

Study and comparison of different drying processes for dehydration of raspberries

ABSTRACT The aim of the present study was to evaluate and compare different drying methods (microwave, hot air + microwave, and osmotic dehydration + microwave) in raspberries (cv. Heritage). A portion of raspberries was pretreated with osmotic dehydration (60°Brix sucrose solution at 20°C for 360 min) and another with hot air drying (HAD) (1.5 m/s air speed at 60°C for 300 min). Pretreated raspberries were then dried by microwave and at three different intensities (3.5, 7.5, and 11 W/g). Physicochemical properties (moisture content, water activity, and drying rate) and quality parameters (optical properties, mechanical properties, antioxidant capacity, and rehydration capacity) of dried raspberries were evaluated. Results showed that the microwave drying (MWD) at 7.5 W/g (50 min and final temperature of 79 ± 5.1°C) allowed a high yield of dried raspberries. The combined processes were not efficient to accelerate the decrease of moisture content, due to the low drying rate of the pretreatments. In terms of quality, none of the drying processes allowed a high retention of the antioxidant capacity. However, they allowed an appropriate rehydration performance. The combination of HAD with MWD allowed obtaining a good appearance and desirable texture on the dried product. Thus, this last option seems to be the best among the drying methods tested, but additional studies are required to improve the efficiency of the process and the effect on the antioxidant capacity during drying.

Comparison of two alternatives of combined drying to process blueberries (O'Neal): Evaluation of the final quality

ABSTRACT In this work, we examined and compared two combined alternatives for the drying of blueberries (O’Neal). Pretreatments of osmotic dehydration (60°Brix sucrose solution at 40°C for 6 h) and hot air drying (HAD) (60°C, 2.5 m/s for 90 min) were performed to reach the same water content. Pretreated blueberries were then dried by microwave at different microwave output power values: 562.5, 622.5, and 750 W. The combined drying processes were also compared with HAD alone (control). The effects of the processes over blueberries were studied in terms of decrease in water content, drying rate (DR), mechanical properties (firmness and stiffness), optical properties (L*, a*, and hue angle (h)), antioxidant capacity, and rehydration capacity. The hot air–microwave drying decreased the process time and presented a high drying rate compared with the osmotic dehydration–microwave processes and the control drying. In terms of quality, the antioxidant and rehydration capacities were the most affected. The results showed that the best drying method to obtain the desired final product was the hot air–microwave drying (750 W).

Generalized Correlations for Characteristic Variables and Thermophysical Properties of Osmotically Dehydrated Fruits

Based on experiments on osmotic dehydration of different fruits, with diverse dehydrating solutions and working conditions, and on an extensive search of the literature on osmotic dehydration, general linear correlations between water content (WC) and soluble solids (SS) content of fruits dehydrated in sugar or alcohol solutions have been developed for each type of fruit that allows characterization of both properties with only one rapid determination of either property. A linear regression for SS vs. WC including all of the experimental data for the seven types of fruits tested (apple, strawberry, pear, kiwi, plum, nectarine, and melon) and the five solutes used in those experiments (sucrose, glucose, xylitol, sorbitol, polyethylene glycol) was obtained with a high regression coefficient. Based on experimental data determined in this work in the range −40 to 40°C, correlations are established between heat capacity and enthalpy with water content and temperature for both fresh strawberries and strawberries that were osmotically dehydrated in different solutions. In addition, general polynomial correlations for the initial freezing temperature of fresh and osmotically dehydrated fruits as a function of water content and type of dehydrating solute are proposed.

Experimental values and correlations of some thermal properties of fresh and osmotically dehydrated stone fruits

Nectarines, plums and peaches were dehydrated with osmotic solutions (glucose at 60°Brix or sorbitol at 60°Brix) at temperature of 35°C for scheduled periods (0.5, 1, 2, 4, 6, 8 h). Thermal properties of fresh and osmotically dehydrated fruits were measured by DSC where samples were heated from −40°C to 40°C at a rate of 2°C min−1. In this work, we obtained experimental data on the variation of water content and soluble solids during the osmotic dehydration of nectarines, peaches and plums in solutions of glucose or sorbitol; experimental data for enthalpy of fresh and osmotically dehydrated nectarines, plums and peaches as a function of temperature and type of osmotic solute; correlations for initial freezing temperature of osmotically dehydrated stone fruits as a function of water content and type of dehydrating solute and correlations between heat capacity or enthalpy and temperature for some fresh and osmotically dehydrated stone fruits.