A comparison of the rheological behaviour of crude and refined locust bean gum preparations during thermal processing M. Samil Ko ¨k * , Sandra E. Hill, John R. Mitchell Division of Food Sciences, School of Biological Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK Received 3 March 1998; received in revised form 15 June 1998; accepted 6 July 1998 Abstract The behaviour, during thermal processing, of a higher quality analytical-grade (AG) locust bean gum (LBG) was compared with a lower quality technical grade (TG) LBG. The TG material contained a substantial amount of material (40%) of dry weight, which remained insoluble after heating to 70°C. Sugar analysis suggests that this insoluble material contained high levels of arabinose. The TG material showed low viscosity throughout the heating cycle and lower levels of degradation at high temperatures, as evidenced from viscosity measurements. The reason for this could have been that, in these samples, the viscosity is dominated by the non-soluble particulates in the system; however, on removal of particulates further rheological studies, made at comparable galactomannan concentrations, also showed differences between the degradation of the AG and TG LBG. Despite the difference in behaviour through the heating cycle, at equal galactomannan levels, the AG and TG materials had similar viscosities at the end of this cycle. This may explain why, after heat processing, the TG material interacts synergistically with carrageenan in a similar way to AG locust bean gum.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Thermal processing; Locust bean gum; Sugar analysis; Arabinose; Viscosity measurement; Galactomannan; Carrageenan 1. Introduction Polysaccharides derived from seed gums are widely used in the food industry as thickeners in dressings, sauces and frozen products because of their cold water dispersibility, compatibility with high acidic emulsions and low cost on a viscosity basis. In addition to increasing viscosity they inhi- bit ice crystal formation, modify texture and control product consistency with respect to changes in temperature (Fox, 1992). The seeds of many Leguminosae contain galacto- mannans in the cells of the endosperm and these have been studied extensively (Dea and Morrisson, 1975). The gum of the locust bean (LBG), Ceratonia siliqua, is derived from the endosperm of the seeds after removal of the testa (seed coat), and the quality of the gum is dependent on the degree of separation achieved. The structure has a linear backbone of b-1,4-d-mannose substituted to varying degrees at 1–6 with an a-d-galactose side groups (Fox, 1992). The LBG samples used in this study were a more refined analytical grade (AG) and a crude technical grade (TG). During many food sterilisation processes, gums are subjected to high temperatures. These processes cause the polymer to solubilise, but as other research has shown, can also cause degradation resulting in a lowering of viscosity (Owen et al., 1992). Different grades of LBG are already extensively employed in heat-sterilised foods, particularly when a mixed gel with carrageenan is required. The two polysaccharides are well known to show a synergistic inter- action (Morris, 1995). The objective of the work described in this paper is to compare the composition and rheological properties during thermal processing of TG LBG with an AG LBG, which is equivalent to a refined food grade (FG) preparation. To obtain information on the changes occurring during thermal processing, viscosities have been measured through the heat processing cycle using a Bohlin CS rheometer equipped with a high-pressure cell. This allowed the visc- osity to be assessed as the ‘suspension’ is heated from 20 to 121°C, held at this temperature and then cooled back to ambient. The resultant profile is a reflection of several phenomena occurring simultaneously throughout the experiment. The most important is: an increase in the concentration of polysaccharide in solution with increasing temperature and thermal degradation of the galactomannan. To obtain information on the latter, the change in viscosity with time at a constant temperature was monitored. A similar approach using a slit viscometer has been used by Carbohydrate Polymers 38 (1999) 261–265 0144-8617/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0144-8617(98)00100-3 * Corresponding author.