Infrared and Microwave Assisted Hot Air Drying of Lemon (Citrus limon L.)

Abstract

The Lemon (Citrus limon L.) has many important natural chemical components, including citric acid, ascorbic acid, minerals and flavonoids. Storage of lemons is difficult due to enzymatic activity and production of fermentative metabolites. Pectinesterase and peroxidase are the active enzymes present in the peel and juice sacs of lemon, respectively. In the present study different pretreatments like osmotic pre-concentration, chemical dipping (ascorbic acid and calcium chloride), ultra-sonication and blanching (water and steam) were given to lemon to inactivate pectinesterase and peroxidase enzymes. Osmotic dehydration was found to be a good pretreatment to attain higher moisture loss and solid gain making lemon more suitable for further drying. The optimal condition of osmotic dehydration was found to be 20% salt concentration and 30 °C osmotic solution for 180 min to attain high moisture loss, less solid loss and required salt uptake within allowable limits. Also, steam blanching was found to be most suitable pretreatment among other pretreatments to inactivate enzymes. Therefore, osmotically pretreated lemons followed by one minute steam blanching were used for infrared hot air drying. Infrared hot air drying of osmotically pretreated and steam blanched lemon slices was effective in partially drying (reduce moisture content less than 30% w.b.) during one hour without entering in drastically falling rate period. Microwave hot air finish drying was found to be energy and drying time saving with minimum deterioration of the quality of product. Also, microwaves helped in maintaining the higher drying rates in final stage where removal of moisture was difficult. The hybridization of these two drying methods overcame the limitations and combined advantages of infrared and microwaves at appropriate moisture levels of the product