0960–3085/04/$30.00+0.00 # 2004 Institution of Chemical Engineers www.ingentaselect.com=titles=09603085.htm Trans IChemE, Part C, June 2004 Food and Bioproducts Processing, 82(C2): 133–142 REVIEW PAPER YOGHURT POWDER—A REVIEW OF PROCESS TECHNOLOGY, STORAGE AND UTILIZATION P. KUMAR and H. N. MISHRA* Post Harvest Technology Centre, Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur, India Y oghurt powders possess various nutritional and therapeutic values. Antitumour activity is associated with the cell wall of starter bacteria and so the activity remains even after drying. The primary objective of drying yoghurt is to preserve it in a shelf-stable powdered form of high quality without a need for refrigeration. Such powder can be prepared by various methods, such as freeze-drying, spray-drying, microwave-drying and vacuum- drying. Before drying, it is beneficial to concentrate yoghurt by methods including cloth bag method, mechanical, centrifugation, ultrafiltration and vacuum concentration. S. thermophilus shows less sensitivity in comparison to L. bulgaricus, during freeze- as well as spray-drying of yoghurt. Cultured products sold with any claims of health benefits should meet the criteria of suggested minimum number of more than 10 6 cfu g 1 at the expiry date. Freeze-dried yoghurt can be stored for up to 1–2 years at 4  C. After 1 year of storage, the powder typically contains total bacterial counts of 10 6 cfu g 1 . Active cultures are guaranteed for 1 year under cool, dry conditions for spray-dried yoghurt; it can be used in the bakery and confectionary industries. Keywords: yoghurt, L. bulgaricus; S. thermophilus; freeze-drying; spray-drying; microwave- drying. INTRODUCTION Yoghurt is a highly nutritious protein-rich product obtained by fermentation of milk with S. thermophilus and L. bulgaricus. The product is highly acceptable to consu- mers because of its flavour and aroma, mainly attributed to acetaldehyde, and its texture. The shelf life of yoghurt is short, i.e., 1 day under ambient condition (25–30  C) and around 5 days at 7  C (Salji et al., 1987), which hinders its commercialization. Yoghurt is maintained at 2–4  C through- out the distribution chain, which not only avoids risk of spoilage from yeasts and moulds but also prevents further activity by starter culture. This, however, adds to the cost of the product. Improvement of the shelf life of yoghurt can be obtained by lowering its water content by draining of whey. Another method is drying, e.g. freeze-, spray- or microwave-drying, the primary objective of which is to preserve the product in a shelf-stable powder form of high quality without the need for refrigeration. Dried yoghurt requires less packaging and storage costs because of the reduction of bulk, and no refrigeration is required. Balanced and substantial retention of the original properties of yoghurt in the dehydrated product is desirable. YOGHURT: CONSTITUENTS AND PROPERTIES Yoghurt is a widely consumed semisolid fermented milk product. Viability and activity of yoghurt bacteria are important commercial considerations for survival through- out shelf life and transit through acidic conditions in the stomach, as well as effects of enzymes and bile salts in the small intestine. Experts contend that putting the term ‘yoghurt’ on the label of a product with only trace amounts of starter cultures, or one with yoghurt flavours or powders and no live bacteria, is deceptive to consumers. Soon, legislation may require for all products which contain yoghurt to have minimum levels of active cultures of Streptococcus thermophilus and Lactobacillus bulgaricus (Anonymous, 1983b). In addition to its high nutritional value, yoghurt possesses antagonistic and therapeutic values. The valuable sensory characteristics of yoghurt are due to its content of carbonyls, mainly acetaldehyde, acetone, acetoin, diacetyl and ethanol, produced by yoghurt bacteria. The low pH of yoghurt creates an undesirable environment for the growth of most spoilage micro-organisms other than yeasts and moulds (Gilliland, 1991). 133 *Correspondence to: Dr. H.N. Mishra, Post Harvest Technology Centre, Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur-721 302, India. E-mail: hnm@agfe.iitkgp.ernet.in