Hamed Beyki; Nasser Hamdami; Jalal Pourtaghi
Abstract
Introduction: Heat treatment of potato tissue involves chemical, physical and structural changes. The magnitude of these changes will depend on the extent of thermal processing in terms of time and temperature. Blanching is a heating process that used to inactivate enzymes, de-aeration and modification ...
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Introduction: Heat treatment of potato tissue involves chemical, physical and structural changes. The magnitude of these changes will depend on the extent of thermal processing in terms of time and temperature. Blanching is a heating process that used to inactivate enzymes, de-aeration and modification of texture, preserving color, flavor and nutritional value of fruits and vegetables prior to freezing, canning, drying and frying. Temperature and time of blanching is largely determined by the amount of reducing sugars and textural characteristics of the species. In the previous studies performed on the changes of reducing sugars content in potatoes during blanching, some researchers found evidence that besides the mass transfer to the bath there exists an internal generation of reducing sugars, probably due to an enzymatic hydrolysis of starch. Therefore when predicting the concentration of reducing sugars changes, both mechanisms should be considered. Considering that no study has been done on the effect of internal generation of reducing sugars on their effective diffusivity during blanching and so far there is no comprehensive study on the kinetic of texture changes during blanching of potato at different temperatures and also direct relationship of reducing sugars with Maillard reaction and acrylamide formation in potato, therefore the aims of this study are: 1) determination of effective diffusivity of reducing sugars with and without consideration of them generation and 2) investigation of kinetic of texture changes during blanching of potato strips at the temperature range of 50-90 ºC and modeling of these phenomena.
Materials and methods: Potatoes (variety Agria, ~23 g/100g (dry basis)) was the raw material used in this study. Potatoes stored in a dark room at 20 ºC for 2 weeks were washed and peeled. Then, were striped into dimensions of 0.8×0.8×8 cm using an electric striping machine (Halldeh, model RG-100). Raw potato strips were rinsed immediately after cutting for 1 min in distilled water to eliminate the starch material adhering to the surface prior to blanching. Then, blanching was accomplished by immersing the potato strips in hot distilled water (ratio of mass of potato to water of 1:20) at 50, 60, 70, 80 and 90 ºC. To investigate the kinetic of generation of reducing sugars, strips packaged in duplex in the polyethylene (HDPE with 10 micron thickness) film were immersed in hot distilled water to blanching at the temperatures. Samples were withdrawn after 5, 10, 15, 20, 40, 60, 80, 100 and 120 minutes to measure the reducing sugars content and texture firmness. All experiments were done in triplicate. Maximum stress caused by penetrating of the probe (diameter of 2 mm) into strip texture was the texture evaluation index. To describe the mass transfer (extraction of reducing sugar) during blanching, a three dimensional numerical model based on finite difference method of solving Fick's Second Law has been developed in MATLAB software. The first order kinetic with production rate limiting effect was used to describe the potato strips texture changes during blanching. All experiments were done in triplicate as factorial based on completely randomized design. Data analysis and plotting of figures were performed in SAS and Excel software respectively.
Results and Discussion: The results showed that increasing of blanching temperature increase the reducing sugars effective diffusion coefficient significantly (P
