Effects of unfreezing by autoclaving during freeze-thaw treatment on amylopectin breaking in different gelatinized starches

Abstract: Freezing and autoclaving-thawing were necessary steps to produce different food with starchy dough, in which the damage of starch granule and the rupture of some amylopectin into amylose brought many unpredictable effects to the physical properties of dough and food. Many studies had been carried out to hinder the detrimental effects of freeze-thaw treatment on food properties, but few of them were practical. The main reason was lack of the basic data about the property change of starch in dough during freeze-thaw treatment. Sweet potato, potato and maize starches were often added into dough to produce steam bread with a particular quality. The properties of those foods were traditionally controlled by empirical knowledge. It was difficult to investigate the effects of freezing-thawing treatment on properties of different starches when they were mixed together. So the starches were treated individually in the paper. The effects of heating temperature, heating time, freezing time and thawing time on change of amylose content in gelatinized wheat, sweet potato, potato and maize starches were determined, which indirectly reflected the fracture of amylopectin. The possible fracture modes of amylopectin in different starches during freezing-thawing process were presented based on analysis of the distribution of starch molecular weight, chain length distribution, optical and SEM micrographs before and after freeze-thaw treatment. The results showed that the amylose contents in all starches firstly increased and then decreased and the maximum amylose contents of those 4 starches were present when the freezing time was 48, 48, 48 and 72 h, respectively. So the freezing time of gelatinized wheat, sweet potato, potato and maize starches should not exceed 48, 48, 48 and 72 h, respectively. Light micrograph observations indicated that the freeze-thaw treatment made more gelatinized granules broken. There were 2 growths of amylose contents during freeze-thaw treatments, and the first was derived from amylose leaching of more broken granules and the second stemmed from fracture of amylopectin. For wheat starch, branched chains with 5, 6, 7 glucose residues were broke into 2+2+1, 2+2+2 and 2+2+2+1 residues respectively; for sweet potato starch, those chains with 10, 11, 13 glucose residues were broke into 3+3+4, 2+2+3+4 and 2+2+2+3+4 residues respectively; for potato starch, chains with 5, 6 glucose residues were broke into 2+3, 2+4 or 3+3 residues respectively; for maize starch, those with 7, 8, 9 glucose residues were broke into 2+5, 3+5 and 3+3+3 residues respectively. Freeze-thaw treatment made more amyloses leach out from granule, providing more amylose to involve in quick retrogradation, which led to the shortage of food shelf. Freeze-thaw treatment also could make amylose broken into polysaccharides and bring sweet taste to corresponding foods, which would deteriorate food properties. Those findings in the paper probably provided a way to screen seeds with freeze-thaw-stable starch. That is, genetic methods are used to control plants to reduce the synthesis of starch with side chains that are susceptible to breakage. Further studies in relation to interaction between those broken amyloses and proteins such as glutenin, gliadin should be performed deeply.