ORIGINAL PAPER Drying Technology: Trends and Applications in Postharvest Processing Arun S. Mujumdar & Chung Lim Law Received: 17 November 2009 / Accepted: 26 March 2010 / Published online: 23 April 2010 # Springer Science+Business Media, LLC 2010 Abstract Thermal drying technologies have attracted significant R&D efforts owing to the rising demand for improved product quality and reduced operating cost as well as diminished environmental impact. Drying materials may appear in the form of wet solid, liquid, suspension, or paste, which require drying to extend the period of storage, ease of transportation, and for downstream processing to produce value added products. Most of these materials are heat-sensitive and require careful drying; conventional hot air drying can be detrimental to the retention of bioactive ingredients. High temperature tends to damage and denature the product, destroy active ingredients, cause case hardening and discoloration, etc. This article briefly summarizes some of the emerging drying methods and selected recent developments applicable to postharvest processing. These include: heat pump-assisted drying with multimode and time-varying heat input, low and atmospheric pressure superheated steam drying, modified atmosphere drying, intermittent batch drying, osmotic pretreatments, microwave-vacuum drying, etc. Keywords Dehydration . Bioactive ingredients . Preservation . Energy savings . Quality Introduction Drying is one of the most energy-intensive unit operations in postharvest processing. This unit operation is applied to reduce the water content of products such as various fruits, vegetables, agricultural and herbal products, etc. after harvest. The purpose of reducing the water content is to prolong the shelf-life of the products of bio-origin by reducing the water activity to a level low enough where growth of microorganisms, enzymatic reactions, and other deteriorative reactions are inhibited. Some bio-origin products such as herbs have to be dried before the active ingredients can be extracted. Furthermore, the products in the dry form weigh less and thus reduce transportation costs. The harvested bio-origin products are diverse in physical, chemical, and biochemical properties. A large assortment of dryers has been developed to dehydrate and preserve these products to meet different quality and cost requirements. Over 500 dryer types have been reported in the technical literature, and about 100 types are commercially available. Differences in dryer design are due to different physical attributes of the product, different modes of heat input, different operating tem- perature and pressure, different quality specifications on the dried product, etc. Most conventional dryers use hot air as the drying medium, convection as the single mode of heat transfer, and are operated at atmospheric pressure under steady drying conditions. For smaller capacities and long drying times (e.g., solar dryers), batch operation is preferred for obvious reasons. Conventional dryers have several limitations, e.g., non- uniform product quality due to over-drying/under-drying caused by long or inadequate or non-uniform exposure to the drying medium; long drying times due to low contacting efficiency between the drying medium and solids being A. S. Mujumdar Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore e-mail: mpeasm@nus.edu.sg C. L. Law (*) Department of Chemical and Environmental Engineering, Faculty of Engineering, The University of Nottingham, Malaysia Campus, Broga Road, Semenyih, Selangor 43500, Malaysia e-mail: chung-lim.law@nottingham.edu.my Food Bioprocess Technol (2010) 3:843–852 DOI 10.1007/s11947-010-0353-1