Abstract

The suitability of an oscillatory shear technique to distinguish the rigidity of carrot tissue of different maturity and morphological origin has been evaluated. The rigidity was related to apparent G' (G' = shear modulus) of tissues and growing regions showed greater rigidity than mature tissue. Despite these differences in raw tissue, all samples exhibited a rapid decline in apparent G' around 55–60°C when small discs of these tissues (2 mm thick × 13 mm dia) were heated from 20 to 80°C. Force-compression studies on carrot tissue were performed using a TA.XT2 texture analyser. Carrot discs (3 mm thick × 20.6 mm dia) showed a linear elastic region up to 37% of compression at 20°C. When discs of carrot tissue were heated in the range 20–80°C, they showed a gradual decline in modulus of elasticity (E) unlike apparent G'.

Heated-stage microscopic studies were carried out to examine the reasons for rapid changes in G' of carrot tissue during heating. As the temperature increased from 20 to 60°C, thickening of the cell wall was observed with disruption of plasma membrane and loss of cellular integrity evident around 60°C.

The relationship between sensory analysis and the rheological parameters was also studied. There was a gradual decline of both the apparent G' and E values as well as the sensory score as carrots were cooked in boiling water but, after 20 min. plateau values were observed. There was a linear relationship between sensory score and apparent G' and E values (r = 0.86 to 0.93) suggesting the instrumental methods used in this investigation were able to measure the same intensity of textural characteristics as perceived by a taste panel.