210 JOURNAL OF FOOD SCIENCE—Vol. 68, Nr. 1, 2003 © 2003 Institute of Food Technologists Further reproduction prohibited without permission Food Engineering and Physical Properties A Process for Increasing the Free Fat Content of Spray-dried Whole Milk Powder A.B. KOC, P.H. HEINEMANN, AND G.R. ZIEGLER ABSTRACT: Exposing spray-dried whole milk powder to high shear and elevated temperature in a twin-screw continuous mixer increased the free fat content. The effects of operating conditions (powder feed rate, processor screw speed, and process temperature) on lactose crystallinity, particle size distribution, color, and moisture content of spray-dried whole milk powder were investigated using response surface methodology. Exposure to elevated temperatures and high shear: (a) increased the free fat to more than 80%, (b) crystallized the lactose, (c) reduced the average volume-based particle size, and (d) broadened the particle size distribution. The raw whole milk powder with creamy-white color turned into an oily paste with bright-yellow color. Processing enhanced the functional properties of spray-dried whole milk powder for milk chocolate manufacture. Keywords: dry whole milk (DWM), milk chocolate, lactose, crystallization, free-fat Introduction M ANUFACTURING QUALITY MILK CHOCOLATE WHILE REDUCING PRO- duction costs are major goals of the chocolate industry. The cost of milk chocolate results, in part, from the amount of energy, cocoa butter, and lecithin used in manufacturing; of these ingredi- ents, cocoa butter is the most expensive. The amount of cocoa but- ter required to achieve a flowable product can be reduced using whole milk powder with high free fat content. The term “free fat” describes the available milk fat that exists in the continuous fat phase in milk chocolate with cocoa butter (Verhey 1986), and that reduces the chocolate viscosity during conching. Another definition of “free fat content” is the fat that can be extracted with organic solvents under standardized conditions (Buma 1971). The free fat content of spray-dried whole milk powder typically is less than 10%. Increasing the free fat content makes the milk fat in whole milk powder more functional in milk chocolate manufacture by reduc- ing the amount of cocoa butter required to obtain a desired viscos- ity. In milk chocolate manufacturing, whole milk powder is mixed with chocolate liquor, sugar, lecithin, and cocoa butter and refined into a chocolate mass. The chocolate mass is further mixed (conched) at a certain temperature, and a homogeneous liquid chocolate with a distinct flavor is developed. For standardization of flow properties, more cocoa butter and lecithin are added to the chocolate mass toward the end of the conching process. Lecithin is a surface-active lipid that helps reduce the viscosity when used in proper amounts. Binding to sugar makes lecithin an effective emul- sifier in chocolate (Bouzas and Brown 1995). The amount of lecithin in chocolate is typically less than 0.3%. Exceeding 0.3% may result in an increase in apparent viscosity. Since its viscosity reducing effect is 10 times greater than the same amount of cocoa butter, lecithin is used to reduce the amount of cocoa butter in chocolate (Chevalley 1994). Spray-dried whole milk powder contains more than 26% milk fat. Milk fat contributes to the smooth texture and glossy appearance and acts as a flavor carrier in milk chocolate (Campbell and Pavlasek 1987; Bouzas and Brown 1995). The majority of the fat in spray- dried whole milk powder is entrapped in a lactose and protein matrix during spray drying. Traditionally, roller-dried whole milk powder was preferred for manufacturing milk chocolate because of its high free fat content of greater than 95% (Reimerdes and Mehrens 1994). The high free fat content results in lower energy usage during blending (Reimer- des and Mehrens 1994). When spray-dried whole milk powder is used, 2% to 2.5% more cocoa butter is required to provide the same flow properties compared to roller-dried powder (Verhey 1986). Milk powder with high free fat content, such as roller-dried whole milk powder, is more susceptible to oxidation than spray-dried whole milk powder; consequently, its shelf life is short. One way of making the milk fat 100% available is to use anhydrous milk fat with skim milk powder at desired amounts for manufacturing less ex- pensive milk chocolates (Minifie 1989). However, the quality of milk chocolate manufactured with this process is typically not as good as the milk chocolate produced from whole milk powder, and the an- hydrous milk fat is still susceptible to oxidative rancidity. Using spray-dried whole milk powder is advantageous because of its longer shelf life and positive impact on the chocolate quality. Lactose is the major carbohydrate in whole milk powder. Lactose is a milk sugar composed of D-glucose and D-galactose (Walstra and others 1999). The sweetness of lactose is about 0.3 times as sweet as sucrose (Kruger 1994; Walstra and others 1999). Lactose in spray-dried whole milk powder forms a continuous medium where fat globules, proteins, and air vacuoles are dispersed (Saito 1985). Drying, handling, and the storage conditions of milk powder affect the form of lactose present. Lactose may exist in an amorphous glassy state, -crystalline, and/or -crystalline states in spray-dried whole milk powder (Aguilar and Ziegler 1993). The structure of lac- tose affects physicochemical and structural properties of the milk powder (Mistry and others 1992; Kedward and others 2000) and attributes—such as texture, flavor, and general acceptability—of the milk chocolate formulated with it. Milk powder with amorphous lactose is stable at temperatures below its glass transition temper- ature (Tg) (Jouppila and Roos 1994). Above Tg, lactose tends to absorb moisture from the environment and crystallizes in the form of -lactose hydrate (Saito 1985). Crystallization of lactose due to moisture uptake causes lumping and caking of milk powder (Roet- man 1979; Mistry and others 1992). The glass transition temperature can be defined as the temper- JFS: Food Engineering and Physical Properties