Freeze-thaw stability of konjac glocomannane-potato starch gels: stability from macroscopic to microscopic scale, using image processing

Abstract

Freeze-thaw (FT) stability is often used to assess the ability of a gel to support the damage induced byfreezing; selected parameters such as drip loss, damage to structure etc can be used to assess the freezetolerance of a gel. Konjac glucomannan (KGM) is a very specific hydrocolloid able to trap 100 timesits weight in water; it has not been studied so far as an improver to enhance FT stability. The aim of the study was to show that the presence of a small quantity of konjac glucomannan (KGM)in potato starch suspension increased the stability of carvacrol antioxidant trapping. FT cycles wereused to accelerate the ageing of the product and to assess its stability. In addition to drip lossesdetermination, the stability of carvacrol trapping was evaluated by the quantification of carvacrol inthe syneresis liquid. Microscopic and macroscopic scales were considered with microscopy. Themoment of the addition of carvacrol and the presence of KGM both had an effect on the stability ofcarvacrol trapping and of the structure of the gel. KGM promoted amylose retrogradation but sloweddown amylopectin retrogradation. The stability of potato starch gels can be improved by the additionof a small quantity of KGM, which showed a “cryoprotectant” behaviour. New method to characterizethe micro and macrostructure from SEM images processing has also been proposed. The processing ofmicroscopy images was done using Generalized Fourier Descriptors and allowed the characterizationof each sample. The carvacrol addition lowered the physical stability of the gel with larger pores andincreased syneresis. On the contrary, the KGM addition increased the size of the pores but preventedthe formation of very large pores and reduced syneresis. The most stable system was obtained by theaddition of carvacrol at the end of heating, in a konjac glucomannane potato starch gel.