Food oil-in-water (o/w) emulsions such as mayonnaise, some kind of dressing and milk are often destabilized during storage at low temperatures. The destabilization is considered to be induced by fat crystallization in a process of freeze-thawing. In this work, the relation between fat crystallization below freezing temperatures and oil separation was investigated for two types of o/w emulsions made from canola oil and soybean oil, using differential scanning calorimetry (DSC) and polarized optical microscopy. DSC measurements revealed the differences of crystallization temperature (T_c) between canola oil emulsion and soybean oil emulsion; that is, emulsified canola oil crystallized rapidly from -48℃. On the other hand emulsified soybean oil crystallized gradually in a wide range of subzero temperatures. Polarized optical microscopy showed a significant difference in the crystallization behavior in droplets. In the soybean oil droplets, fat crystals were located at oil-water interface while the fat crystals in the canola oil droplets were dispersed in the droplet. Freeze-thawing test showed that soybean oil emulsion exhibited higher stability than the canola oil. These results suggested that the crystals at the interface in the soybean oil droplets played a key role in its freeze-thaw stability.
