Tetsuo Koyano’s research while affiliated with Japan Food Research Laboratories and other places

Rheological analysis was conducted to study the physical properties of the mixtures of high-melting (HM) and low-melting (LM) fats. The HM and LM fats examined in the present work were fully hydrogenated rapeseed oil with a high amount of behenic acid (FHR-B), and sal fat olein (SFO). Gel-like behavior of the FHR-B/SFO fat mixture with FHR-B concentration of 1.5–4.0 wt% indicated the existence of many small FHR-B crystals of β polymorph that were formed by the following thermal treatment: rapid cooling (Vc=10 °C/min) from 70 °C to crystallization temperature (Tc=20 °C), and heating from Tc to final temperature (Tf=38 °C) where the fat mixture was held for 60 min. These results were obtained: (1) FHR-B/SFO fat mixture exhibiting gel-like behavior formed by Vc=10 °C/min exhibited pseudo-plastic flow properties with the apparent yield value at Tf=38 °C. Storage moduli G′ showed approximately the same values during the frequency scanned. Loss moduli G″, which were smaller than G′, decreased with increasing frequency in the measured range, whereas the gel-like fat was not formed at Vc=1.5 °C/min. In this mixture, G′ and G″ were increased with increasing frequency, resulting in smaller values than those of the rapid cooling. Additionally, G″ dominated the viscoelasticity, which corresponds to liquid-like behavior. (2) At the FHR-B concentration of 1.5 wt% or higher, the FHR-B/SFO fat mixtures exhibited gel-like behavior with proper thermal treatment, whereas G′ and G″ showed liquid-like behavior when FHR-B concentration was less than 1.5 wt%. (3) G′ and G″ of gel-like behavior of FHR-B (2.0 wt%)/SFO (98.0 wt%) decreased drastically around Tf=38 °C, and rheological behavior changed from gel-like to liquid-like at 43 °C. (4) The many small β crystals of FHR-B crystallized by proper thermal treatment may play decisive roles in viscoelastic behavior of β-fat gel.

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 β-fat gel. (i) Gel-like behavior occurred when rapid cooling (10 °C/min) of a high-temperature liquid mixture formed many small crystals of α polymorph of HM fats, and the α crystals converted to the most stable β form in crystals of very small size after heating from 10 to 38 °C through the α-melt mediated transformation. Slow cooling crystallized the β′ and β forms with large, aggregated patterns. After subsequent heating, the β′ crystals were converted to large β 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 (Tc) even when rapid cooling was applied. When Tc was <20 °C, the first crystallizing form was α form that converted to the β form. However, the β′ and β forms were crystallized at Tc>20 °C, and did not exhibit gel-like behavior. (iii) Mixtures of FHR-S/SFO and SSS/SFO caused the α-melt mediated transformation and formed the most stable β 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 β-fat gel.

Gel-like fat mixtures of high-melting (HM) and low-melting (LM) fats were formed by means of rapid cooling and subsequent heating. No “non-fat” ingredients such as emulsifiers, water, or waxes were added to the mixtures. The gel-like fats having solid fat content (SFC) values below 2.0 wt% formed crystal networks of HM-fats that entrapped the liquid oil fraction of LM-fats. In a search for optimal fat combinations exhibiting gel-like behavior, 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), tristearoylglycerol (SSS), triarachidonoyl-glycerol (AAA), and tribehenoylglycerol (BBB) were examined as the HM-fats. For LM-fats, sal fat olein (SFO), cocoa butter (CB), palm super olein (PSO), and olive oil were examined. The following results were obtained: (i) the gel-like behavior was observed in mixtures of FHR-B/SFO and FHR-B/CB with initial concentrations of FHR-B of 1.5–4.0 wt%. (ii) Rapid cooling to T c (crystallization temperature) from 70°C and subsequent heating to T f (final temperature) were necessary to reveal the gel-like behavior, whereas simple cooling without a cooling/heating procedure did not form the gel-like fat mixture. (iii) Optimal values of T c and T f were related to the m.p. of the LM-fat and HM-fat, respectively. (iv) Temperature variations of SFC as well as X-ray diffraction spectra showed that the melt-mediated transformation from α to β of the HM-fat crystals was a prerequisite to reveal the gel-like behavior. Consequently, the fat mixture revealing the gel-like behavior might be called β-fat gel.

Effects of application of ultrasonic power (20 kHz, 100 W) on the crystallization behavior of tripalmitoylglycerol (PPP) and cocoa butter have been examined in terms of rate of nucleation and polymorphic control. High-purity PPP (>99%) and low-purity PPP (>80%) samples were employed to mimic real fat systems, which usually have higher concentrations of minor components in addition to the main component. For both the high-purity and low-purity PPP, the application of ultrasonic power accelerated the rate of nucleation as measured by induction time for the occurrence of crystals and by the number of crystals nucleated. As for the polymorphic influences, the nucleation of both the β′ and β forms was accelerated by the ultrasound, yet the β′ form nucleation was more accelerated when the low-purity PPP samples were employed. As for cocoa butter, sonication for a short period accelerated the crystallization of Form V. The present results indicate that ultrasound irradiation is an efficient tool for controlling polymorphic crystallization of fats.

The crystallization behavior of ternary triacylglycerol (TAG) mixtures of POP (2-oleoyl-1, 3-dipalmitoylglycerol), POS (2-oleoyl-1-palmitoyl-3-stearoylglycerol), and SOS (2-oleoyl-1, 3-distearoyl-glycerol) was examined using high purity materials obtained by preparative HPLC. Seventy three samples were prepared by mixing TAGs and dissolved in acetone. Crystallization was carried out by evaporation of acetone at low pressure, and the crystals were aged at 30°C for 6 months. From the result of DSC analysis of the aged samples, the melting behavior of ternary mixtures was clarified and found related to optimum usage of vegetable fats in chocolate manufacturing.

Explanation has been made for thermo-rheological properties of mixture systems of 1, 3-dioleoyl-2-stearoylglycerol (OSO) /cocoa butter, and dark chocolate involving OSO. A OSO/cocoa butter mixture was shown to form a compound at a ratio such that the sum total amount of POP (P : palmitoyl), POS and SOS in cocoa butter was the same as that of OSO equal. X-ray diffraction patterns showed the stable form of the compound to possess a double chain length structure. Stress values required for constant strain were determined with a rheometer at different temperatures. The OSO/cocoa butter compound showed stress values much less than that of the compound formed by OSO/SOS. Lowering stress values in OSO/cocoa butter would thus possibly be due to interactions between compound crystals and the liquid oil fraction in the cocoa butter. Possible molecular mechanisms of the interactions are discussed based on the relationship of double chain length lamellar structures to lamellar slipping movement induced by the incorporation of liquid oil at the lamellar interface.

The phase behavior of mixed systems of SOS (sn-1,3-distearoyl-sn-2-oleoylglycerol) and OSO (sn-1,3-dioleoyl-sn-2-stearoylglycerol) was studied with X-ray diffractometry and thermal analysis by using pure samples (>98.0%). A molecular compound was formed at the mixture of equal weight ratio, giving rise to two monotectic phases of SOS/compound and compound/OSO in a juxtapositional way. The stable form of the compound, beta(c), has an interlamellar distance of 44.72 angstrom of double chain length structure, which is in contrast to the triple chain length structure of the stable forms of SOS and OSO having the same interlamellar distance of 65 angstrom. Beta(c) was formed through specific molecular interactions between stearoyl and oleoyl chains connected to glycerol groups: stearoyl chains at the 1,3-positions of SOS and at the 2-position of OSO are packed together in one leaflet of the double chain length lamellae, whereas another leaflet contains oleoyl chains of the two molecules. In the monotectic region of SOS/compound, two crystals are present below the melting point of beta(c), 36-degrees-C. Above that temperature, the stable crystals of SOS equilibrated with mixed liquid are present. In the monotectic region of compound/OSO, beta(c) is present with mixed liquid above the melting point of the stable form of OSO, 25-degrees-C, below which two crystal fractions of the compound and OSO are present. Beta(c) was expected to have a congruent melting point, according to its unique melting and crystallizing behavior. The compound formation having the double chain length structure was also observed in the mixtures of POP/OSO and POS/OSO, in which P is a palmitoyl chain. A molecular model of the compound crystal is proposed, based on the molecular structures of SOS, OSO, and principal monounsaturated fatty acids.

Melt crystallization of four polymorphs of POS, α,δ, pseudo-β′ andβ, was examined with pure samples (>99.9%). Induction time, τ, for newly occurring crystals was measured with a polarizing microscope equipped with a temperature-controlled growth cell. Rate of crystallization, 1/τ, was obtained for each polymorph, whose identification was done with x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Two modes of crystallization, melt cooling and melt mediation, were applied. From these experiments, the following conclusions were obtained: (i) The rate of melt-mediated crystallization was always higher than of simple melt cooling; (ii) the pseudo-β′ form was crystallized in a wider range of temperature than the less stable δ form; (iii) the occurrence behavior of the polymorphs differed between simple melt cooling and melt mediation; (iv) the δ form was crystallized only by simple melt cooling in a narrow range of temperature, 25.5°C∼28.3°C. This means that there is a possibility that δ may result from racemic compounds that are crystallized in a specific manner. The experimental results are discussed in comparison to 1,3-dipalmitoyl2-oleoylglycerol (POP), 1,3-distearoyl-2-oleoylglycerol (SOS) and cocoa butter.

Effects of seeding of fat crystals on the crystallization kinetics of cocoa butter and dark chocolate were examined with a rotational viscometer. The seed crystals employed were cocoa butter, 1,3-distearoyl-2-oleoylglycerol (SOS), 1,3-dibehenoyl-2-oleoylglycerol (BOB) and 1,2,3-tristearoylglycerol (SSS). The seed powders were prepared by pulverization below —50°C, the dimensions being in a range from 20–70 μm. Particular attention was paid to the influence of polymorphism of the seed crystal. We found that all of the above seed materials accelerated the crystallization, the degree of acceleration being in a following order; SOS (β 1) > cocoa butter (Form V) > SOS (a mixture ofβ’ andβ 2) > BOB (β 2) > BOB (pseudo-β’) > SSS (β). Precise measurements of the crystallization kinetics showed that the most influential factors in the seeding effects are the physical properties of the seed materials—above all, thermodynamic stability, and similarity in the crystal structure to cocoa butter are the most determinative.