Physical properties of alginate-coated onion (Allium cepa) skin

Onion is an economically important vegetable often used as a spice. Fresh onion undergoes weight loss at ambient temperatures. The skin's layers drop off with time and a hydrocolloid coating preventing this would be very advantageous in terms of shelf-life extension, export and domestic salability. Successful coating relies on a study of the skin's structure and chemical properties. Scanning electron micrographs of air-dried coated onion-skin specimens were analyzed in parallel with cryo-electron microscope of 'fresh' coated skin. The transparent alginate coating adhered to the onion's outer skin and adopted its shape. The coated onion exhibited extended shelf life, reduced water loss and improved gloss. A study of the skin surface helps tailor the film (and the coating composition) to the vegetable. The surface of the onion skin was therefore characterized by two methods. The first was designed to study its roughness using a Surftest and this yielded roughness values of ∼5-6 μm Ra. 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 ∼250 μm² (excluding vasculature), was performed by atomic force microscopy. Skin surface protrusions of no more than ∼78 nm were found. To estimate mineral diffusion from the coating solution to the skin, the mineral content of the skin before and after coating and peeling was checked by inductively coupled plasma-atomic emission spectroscopy. The residual amount of mineral found in the skin layers suggests possible penetration of the coating ingredients or cross-linking solution into the skin.