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Research on Rapid Freezing Process of French-fried Potatoes

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Tutor: LiZuo
School: Henan Agricultural University
Course: Of Food Science
Keywords: Potato,French fries,Polyphenol oxidase,Color fixative,Blanching,Frozen
CLC: TS215
Type: Master's thesis
Year:  2011
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The article deseribed the research on rapid freezing process of French-fried potatoes.In this study, three kinds of potatoes (Favorita, Chinese No.5 potato, Chinese No.6 potato,) produced in Henan Province were researched, the most suitable variety for processing frozen French fries was selected. Then, the browning mechanism of potato polyphenol oxidase was discussed, and the characteristics of potato PPO were studied. Meanwhile, the inhibitory effect of different inhibitors was researched. The best complex parameters were selected; the optimal blanching process temperature and blanching time were defined. The effect of different freezing methods on the French fries microstructure and nutritional quality was also studied, Main content as follows:1. Analysing major ingredients of three kinds of potatoes, and the results were analyzed, selected the most suitable potato variety for processing frozen French fries, which was the Chinese No.5 potato.2. With catechol as the substrate, determining the potato PPO absorbance value at different wavelengths, selected the best detection wavelength which was 420 nm. Under this condition, studied the characteristics of potato PPO, The results showed that: The optimum reaction temperature was 30¡æ; the potato PPO was very sensitive to heat, with heating at 90¡æfor 80s,Its activity had been basically inactive; Its optimum pH was 5.5,while when the pH was between 4.5 and 5.5, it had very strong activity, when the pH<5.0 or pH>6.0,its activity decreased significantly, when the pH¡Ü2 or the pH¡Ý8, the potato PPO had been basically inactive. Ascorbic acid, citric acid, calcium chloride and EDTA-2Na were significantly inhibited the activity of the potato PPO. Therefore, it could inhibit the activity of the potato PPO and reduce browning in the processing of French fries by heating with the instantaneous high temperature, adjusting the pH and adding the inhibitor.3. By choosing a variety of inhibitors to prohibit PPO¡¯s activity, researching on the effects of different inhibitors, the result showed that ascorbic acid, citric acid and calcium chloride could obviously decrease French fries¡¯s degree of browning , the optimal concentrations were 0.20%,0.40%,0.70%;oxalic acid, EDTA was also showed some inhibition, but inhibited effect was not obvious. Sodium dihydrogen phosphate, disodium hydrogen phosphate increased the PPO¡¯s activity, so they were not suitable for PPO inhibitors.4. Basing on single-factor test, with three factors and three levels by the quadratic regression experiment, researched the effects of the three inhibitors (ascorbic acid, citric acid and calcium chloride) . Established the regression model with experimental factors and optimized factors. By response surface analysis, determined the optimum combination (0.30% ascorbic acid, 0.50% citric acid and 0.76% calcium chloride) which could enormously inhibit French fries¡¯s PPO activity.5. Selecting different temperatures and time for blanching French fries, the result showed that: from 0.5 min to 2 min, the rate of enzyme inactivation was slow; from 2 min to 4 min, the rate of enzyme inactivation was increased dramatically; after 4 min, the rate of enzyme inactivation was 90%; 5 min later, the French fries¡¯s PPO has been almost completely inactivated. Due to increasing the temperature would affect the product quality, at the same time, with the heating time extended, the French fries¡¯s texture would soften, and it was not conducive to the final product quality assurance. Simultaneously, using microwave and ultrasonic assisted blanching, the experiment results showed that the effect of blanching with hot water was no significant difference compared with above two assisted treatments. While, using ultrasonic assisted blanching would make product soften and broken. Considering various factors, selected 90¡æhot water blanching, the best processing time was 5 minutes.6. By comparing with slow freezing and rapid freezing, also researching on these effects to quality of French fries¡¯s products, drawing the conclusion that rapid freezing was significantly better than slow freezing in inhibiting the French fries¡¯s PPO activity and insuring product quality. The reason was that it first formed ice crystals in the extracellular solution, intracellular water moved out, then formed large ice crystals, great mechanical damage happened on the cell and cell wall later, dripped more juice in the process of thawing, softened the product by slow freezing. While, by rapid freezing, it formed small and dense ice crystals intracellular, less mechanical damage happened on the cell later, dripped less juice in the process of thawing. The blanching process also had some damage to cell function, but rapid freezing was better than slow freezing in preserving the integrity of cells. Rapid freezing after blanching process would inhibit more than 90% activity of the French fries¡¯s PPO, while it could assure excellent quality of the product and enhance its commercial value.
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