Decrease casein size of reconstituted nonfat dry milk.

The quality of a beverage is often directly correlated to the size of the particles it contains, as larger particles will precipitate faster. Research indicates that mixing equipment, water temperature and shear rate affect particle size. They directly contribute to a beverage's quality and how well it is accepted by consumers.

Scientists at Kansas State University determined the best mixing conditions that would lead to the smallest casein size, the least sedimentation and the least foam produced by reconstituted nonfat dry milk (NFDM). The best mixing procedure that would improve dairy-based beverage shelf life appears to be high-shear blending at 22 C.

In their work, the researchers took into account a number of variables, including the type of processing equipment used: high-shear blending systems compared with recirculating systems, water temperature (4 C compared with 22 C), and high and low shear rates.

NFDM was reconstituted with distilled deionized water to 11% (w/v) at 4 C for all treatments. A randomized block design was used with four replications. The researchers measured particle size on a commercial laser-scattering particle size distribution analyzer on the day of mixing. They monitored sedimentation for seven days after the mixing was completed. Foam volume and stability were measured for 2 hours after mixing had been performed.

High-shear blending at 22 C produced the smallest casein size (0.155 [micro]m), no sedimentation, but a moderate amount of stable foam. Overall, higher shear rates resulted in smaller (2%) casein sizes than did the lower shear rates in the mixing equipment. NFDM reconstituted at 4 C had larger (3.6%) casein sizes than NFDM reconstituted at 22 C in the mixing equipment.

No sedimentation was observed for any treatment. Foam was most stable when high-shear blending was used at 22 C. Foams produced using recirculation equipment were low in volume and unstable.

Further information. Karen Schmidt, Food Science Institute, Department of Animal Sciences and Industry, Kansas State University, 224 Call Hall, Manhattan, KS 66506; phone: [phone]; email: [e-mail].