Influence of freeze-thaw cycles to frozen Zongzi (rice products): insights from water migration, starch deterioration, in vitro digestion properties, and product quality

Abstract

• Freeze-thawing (FT) led to a decrease in the quality of this independent system of glutinous rice, which is completely wrapped in plant leaves. • FT increased freezable water content and prompted ice recrystallization, resulting in a porous microstructure. • FT accelerated starch retrogradation and water migration, significantly increasing the hardness and chewiness of frozen Zongzi. • Rapidly digestible starch decreases while resistant starch increases after treatment. Frozen Zongzi, a convenient food with a significant market share in China, was examined in this study to explore the effects of multiple freeze-thaw cycles on its physical and chemical properties, as well as its in vitro digestion. The objective was to understand how temperature fluctuations influence the quality of frozen Zongzi. Results showed that with increasing freeze-thaw cycles, strongly bound water in the Zongzi gradually migrated outward. Compared to the control group, the content of weakly bound water in frozen Zongzi increased from 80.74% to 84.54% after eight freeze-thaw cycles, and the amount of freezable water also rose over time. Hardness, elasticity, and chewiness followed a trend of initial increase followed by a decrease with additional freeze-thaw cycles. Scanning electron microscopy (SEM) revealed honeycomb-shaped cross-sections of glutinous rice, with an increase in pores and a more porous, cracked texture. Meanwhile, after eight freeze-thaw cycles, the relative crystallinity of starch in glutinous rice increased from 16.65% to 21.44%, and short-range ordering also rose by 0.056. In vitro digestion of starch indicated that freeze-thaw treatment inhibited the digestion characteristics of frozen Zongzi, suggesting that starch had undergone aging. In conclusion, starch aging and ice recrystallization significantly impacted the quality of frozen Zongzi during freeze-thaw cycles. These factors were identified as the primary causes of quality degradation in frozen Zongzi. Therefore, minimizing temperature fluctuations during transportation and storage is essential to preserve the quality of frozen Zongzi products.