In situ optical observation of microstructure of beta-fat gel made of binary mixtures of high-melting and low-melting fats

Optical observation was conducted to study the size, morphology and aggregation patterns of high-melting (HM) fat crystals that exhibit gel-like behavior in mixtures of HM and low-melting (LM) fats. The HM fats examined were fully hydrogenated rapeseed oil with a high amount of behenic acid (FHR-B), fully hydrogenated rapeseed oil with a high amount of stearic acid (FHR-S), and tristearoyl-glycerol (SSS). The LM fats examined were sal fat olein (SFO), cocoa butter (CB), palm super olein (PSO) and olive oil. Our previous study [K. Higaki et al., J. Am. Oil Chem. Soc. 80 (2003) 263] reported that two mixtures, FHR-B/SFO and FHR-B/CB, having solid fat content (SFC) values of 1.5-4% exhibited gel-like behavior when proper tempering procedures were applied. In the present study, we focused on the crystallization behavior of the same two mixtures during different thermal treatments, and compared them with those of other mixture systems. The results obtained indicated that the size and morphology of HM fat crystals play decisive roles in the formation of beta-fat gel. (i) Gel-like behavior occurred when rapid cooling (10 degrees C/min) of a high-temperature liquid mixture formed many small crystals of alpha polymorph of HM fats, and the alpha crystals converted to the most stable beta form in crystals of very small size after heating from 10 to 38 degrees C through the alpha-melt mediated transformation. Slow cooling crystallized the beta' and beta forms with large, aggregated patterns. After subsequent heating, the beta' crystals were converted to large beta crystals by solid-state transformation. This thermal treatment did not produce gel-like behavior. (ii) The polymorphism, crystal size and morphology of HM fats were influenced by crystallization temperature (T(c)) even when rapid cooling was applied. When T(c) was <20 degrees C, the first crystallizing form was alpha form that converted to the beta form. However, the beta' and beta forms were crystallized at T(c) > 20 degrees C, and did not exhibit gel-like behavior. (iii) Mixtures of FHR-S/SFO and SSS/SFO caused the alpha-melt mediated transformation and formed the most stable beta form, similar to the FHR-B/SFO mixture. However, the two fat mixtures did not exhibit gel-like behavior since the crystals of FHR-S/SFO and SSS/SFO were larger than that of FHR-B/SFO, and the HM fat crystals were distributed non-uniformly and possessed crystal aggregates. To conclude, we found that the size and morphology of HM fat crystals play decisive roles in forming beta-fat gel.