Development of carrot parenchyma softening during heating detected in vivo by dynamic mechanical analysis

• Softening of carrot parenchyma was both temperature and heating rate dependent.
• P1, P2 and second stage of LT shifted to high temperatures at high heating rate.
• P1, P2 and LT effect significantly decreased at high heating rate.
• Slow heating finally resulted in lower toughness of carrots at 90 °C.
• These changes linked to degradation of pore system and components of cell walls.

Dynamic mechanical analysis was employed to explore the development of carrot parenchyma softening during heating in the range of 30–90 °C. Results indicated that compared with raw carrots the toughness of carrots at 60 °C decreased significantly (P < 0.05) at 0.5 °C/min while there was no significant deference (P > 0.05) at 2 °C/min. Samples at 0.5 °C/min finally obtained the lower toughness. Besides, both loss tangent (LT) and two negative peaks on the temperature–slope plot were heating rate dependent. At 0.5 °C/min, distinct increases of LT started at about 48 °C while at about 67 °C at 2 °C/min. With increasing rate, LT effect decreased obviously. Increasing rates caused a shift of both peaks from low temperatures (49–60 °C) to high ones (67–75 °C). Values of both peaks were significantly higher (P < 0.05) at 0.5 °C/min than at 2 °C/min. Furthermore, the microstructure of samples at 0.5 °C/min was more rigid and irregular shrinkage, accompanied by more small pores as well as bigger cracks in comparison to that at 2 °C/min. And cell walls of samples at 0.5 °C/min seemed to thinner. Therefore, the development of carrot parenchyma softening during heating was both temperature and heating rate dependent. We suggested that the breakdown of pore proteins and middle lamella in cell walls induced the dramatic loss of cell turgidity and the subsequent changes of elastic modulus, collectively causing tissue softening.