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This study was undertaken to investigate 1) the effect of low temperature blanching on the firmness of processed whole new potatoes and 2) to determine the extent to which the activity of pectin methyl esterase (PME), a naturally occurring enzyme in fruits and vegetable (proposed to play a role in firming using low blanching temperatures), contributes to improved firmness in processed pre-blanched potatoes. Whole new potatoes (var. Maris peer from Portugal and Nicola from England) were blanched at temperatures of 60, 65, 70, 75, 80, 90 and 100°C for times of 5 up to 60 minutes to investigate texture changes with time and temperature. Texture was measured by puncture testing using a probe attached to an Instron Universal Testing Machine. Results showed that blanching temperatures could be grouped according to the rate of texture changes at 80, 90 and 100°C were found to be temperature dependant on the application of the Arrhenius equation, which is used to show the temperature dependence of a reaction. PME activity was studied by blanching whole new potatoes at temperatures of 65, 75, 80 and 90°C for times of 5 up to 30 minutes. Optimum activity was recorded after blanching at 65°C for 15 minutes. The enzyme was rapidly inactivated after 15 minutes at 75°C and after 5 minutes at both 80 and 90°C. The effect on potato texture of using a low temperature pre-treatment before processing was investigated at processing temperature of 95 and 100° for 5 up to 25 minutes with pre-processing temperatures of 65 and 750° for 5 to 30 minutes. Texture was measured by shearing the whole potato with a single blade. It was found that using a low temperature pre-treatment at 65°C before processing improved the firmness of the processed potatoes at both 95 and 100°C. However, although the firmest overall product was achieved with a pre-treatment at 65°C followed by processing at 95° (highest shear force values) the best result in terms of texture retention when using a low temperature pre-treatment before processing compared to when no pre-treatment was used, was for potatoes pre-processed at 65°C then processed at 100°C. This improved firmness may be related to PME activity at 65°C since blanching at 75°C did not significantly improve the firmness of the processed potatoes (P>0.05). Calculated instrumental parameters of work to fracture (J) and maximum slope before fracture (N/mm) showed that using a low temperature pre-treatment in combination with a high processing temperature increases the energy required to fracture the product and makes it more elastic thereby strengthening the potato and making it less breakable and more suitable for further processing. Sensory analysis of a selection of blanching and processing treatments was carried out to see if there was a noticeable difference after processing between the texture of potatoes that had received a pre-processing treatment and those that had not. Panelists tested for hardness, denseness, chewiness, moistness, fibres and palatability and were asked to comment on each sample and give their preference. Overall, potatoes that were processed without a pre-treatment were preferred. However, although potatoes that had received a pre-treatment prior to processing were considered to be inedible and undercooked they would be ideal for further processing. Instrumental parameters were found to be good indicators of the hardness and denseness of potatoes.
Crowley, H.:The Effect of Low Temperature Blanching on the Texture of Whole Processed New Potatoes. Masters Thesis. Dublin Institute of Technology, 2001.