On stabilization of acidified milk drinks – a review Mohammad Shirkhani, Asghar Khosrowshahi Asl Department of Food Technology Faculty of Agriculture, Urmia University Urmia, Iran Rebin_shirkhany@yahoo.com Ashkan Madadlou Department of Food Technology Institute of Chemical Technologies, Iranian Research Organization for Science & Technology (IROST) Tehran, Iran Ashkan.madadlou@gmail.com Abstract— Acidified milk drinks are popular drinks diversely consumed in many countries. Lowering of pH to less than 4.2 can be performed by direct addition of acids or activity of lactic acid bacteria. Separation of serum, due to aggregation of milk proteins, is the main industrial defect in these types of beverages. Hydrocolloids are very commonly used to prevent serum separation in acidified milk drinks by increasing the viscosity, electrostatic or/and steric repulsion. Viscosity increase can also be achieved by employing of exopolysaccharides-producing starter cultures. Casein-whey proteins ratio alteration, up to a certain level, is capable of stabilizing the system. However, homogenization pressure increase has no significant influence on the prevention of serum separation in acidified milk drinks. Keywords-acidified milk drinks; stability; milk proteins; hydrocolloids; exopolysaccharides I. INTRODUCTION Acidified milk drinks (AMD) are popular in many countries. Acidified products can be obtained by microbial fermentation or direct addition of acid using fruit juice or acids such as citric, malic or glucono-δ- lactone. Most of the research has been conducted on drinkable yogurts which are made from milk fermented with lactic acid bacteria. Fermented milk products are classified into the following categories on the basis of physical characteristics: (a) viscose products; (b) diluted or beverage products; (c) carbonated products [1]. From the microbial point of view, fermented drinking products may contain live starter culture bacteria as a fresh type or be extended shelf-life with no live micro-organisms by heating the final product [2]. Drinking type fermented milk products are known as Laban drink in most Arab countries, Lassi and Dahi in India, Viili, Tafil, filmjolk in Scandinavian countries, Ayran in Turkey and Doogh in Iran [1]. On the whole, they differ from each other in the amount of added water, fat and salt/sugar content, rheological properties and taste [3]. In Europe, for instance, consistency of the product is higher when compared with the low viscose drinking fermented milk produced in Asia and Middle East [1]. Low fat content in some drinking yoghurt products is appealing to diet- conscious consumers. In current commercial productions of AMDs, the reduction of fat content is normally achieved using skim milk to prepare the yoghurt, and usually the final product is flavored with essential oils, such as those extracted from mint, oregano, thyme, etc. containing salt (in Ayran and Doogh) or fruit juice containing sugar [4]. Serum separation is the main textural defect in AMDs during storage, which is industrially called “Wheying off”. It is the separation of product into a casein-rich lower layer and a clear upper layer of serum [4]. Stabilizers are commonly added during the manufacturing of these products in order to avoid sedimentation of milk solids and whey separations in the package [2]. In this study, we endeavored to review a wide range of articles reported in literature and interpret the given information on materials which are used to stabilization of AMDs products and their mechanisms. The aim was to generate some new interest in this field and to elaborate on some possible new direction for research. II. WHEY SEPARATION MECHANISM The caseins form the largest protein component in most milks of industrial significance representing 80% of the total milk protein. The major caseins are  S2 -,  S1 -, β- and -, and they are in a ratio of 1: 4: 4: 1, respectively and presenting predominantly in the form of hydrated association colloids called casein micelles [5]. The α and β-caseins are clustered in the center of the micelle; the κ- casein molecules lie at the surface, with the glycosylated C-terminal sequences protruding to form an expanded “hairy layer” which acts as a barrier to aggregation[6]. Casein micelles have an average radius of ~100 nm containing approximately 3.4 g H 2 O g -1 dry matter which consist of, on a dry matter basis, ~ 94% protein and ~ 6% inorganic materials [7]. These inorganic materials are collectively referred to as micellar calcium phosphate (MCP) or colloidal calcium phosphate (CCP) including primarily calcium and phosphate, with lower levels of magnesium and citrate also present [8]. The stability of casein micelles can be divided into 2 categories: “inter-micellar” and “intra-micellar” stability. The inter-micellar, or colloidal, stability of casein micelles, denotes the stability of casein micelles against aggregation the three relevant types of interaction that play the most important roles in stability of casein micelles are van der Waals attraction, electrostatic repulsion, and polymer brush repulsion [9]. Such