PHYSICAL-PROPERTIES OF ALGINATE-COATED ONION (ALLIUM-CEPA) SKIN

Onion is an economically important vegetable often used as a spice. Fr esh onion undergoes weight loss at ambient temperatures. The skin's la yers drop off with time and a hydrocolloid coating preventing this wou ld be very advantageous in terms of shelf-life extension, export and d omestic salability. Successful coating relies on a study of the skin's structure and chemical properties. Scanning electron micrographs of a ir-dried coated onion-skin specimens were analyzed in parallel with cr yo-electron microscope of 'fresh' coated skin. The transparent alginat e coating adhered to the onion's outer skin and adopted its shape. The coated onion exhibited extended shelf life, reduced water loss and im proved gloss. A study of the skin surface helps tailor the film (and t he coating composition) to the vegetable. The surface of the onion ski n was therefore characterized by two methods. The first was designed t o study its roughness using a Surftest and this yielded roughness valu es of similar to 5-6 mu m Pa. Onion skin was found to be smoother than that of garlic or pepper. Fine structural analysis of the onion skin, using specimen areas no larger than similar to 250 mu m2 (excluding v asculature), was performed by atomic force microscopy. Skin surface pr otrusions of no more than similar to 78 nm were found. To estimate min eral diffusion from the coating solution to the skin, the mineral cont ent of the skin before and after coating and peeling was checked by in ductively coupled plasma-atomic emission spectroscopy. The residual am ount of mineral found in the skin layers suggests possible penetration of the coating ingredients or cross-linking solution into the skin. ( C) 1998 Published by Elsevier Science Ltd. All rights reserved.