Modeling textural changes in raw and cooked potatoes | |
| Author | Kidane, Solomon Worku |
| Call Number | AIT DISS. no. PH-03-04 |
| Subject(s) | Potatoes--Storage Food texture |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering. |
| Publisher | Asian Institute of Technology |
| Abstract | Storage of potatoes in bulk is essential for continuous supply of raw product for household consumption as well as potato processing industry. However, the potatoes undergo many changes during storage and the quality of the final product may be adversely affected. Though several studies have reported the deterioration in the quality of potatoes during storage, detailed information on the quality changes under variable storage conditions, and quantification of these changes both in raw and cooked potatoes and their interrelationships is lacking. This study was initiated to investigate and quantify the changes in the texture of raw and cooked potatoes induced by long term storage of raw potatoes under constant and variable-temperature conditions. Moreover, the interrelationship between the texture of raw and cooked potatoes was studied for predicting the texture of cooked potatoes directly from the measurements on texture of raw potatoes using quasi-static and viscoelatic test techniques. The first phase of the study consisted of the comparison of different quasi-static and viscoelastic test techniques for their sensitivity to detect textural changes in potatoes due to thermal softening under selected experimental conditions. Thermal softening experiments were conducted for monitoring the changes in the texture of cylindrical (15 mm diameter and 30 mm long) samples of potato tubers immersed in water at 60, 70, 80 and 90°C for up to 80 minutes in terms of tangent modulus of elasticity in axial and radial compression tests, and elasticity and viscosity parameters in creep and stress relaxation tests. The creep and stress relaxation responses of individual potato samples were adequately represented by four-element (Burger's) model and two Maxwell models arranged in parallel, respectively indicating high values of the coefficient of determination (R2 = 0.94 - 0.99). All rheological parameters obtained from quasi-static compression and visco-elastic tests decreased exponentially with an increase in heating duration and heating medium temperature approaching steady levels corresponding to the remaining texture after prolonged exposure. However, the elasticity and viscosity parameters of the second Maxwell model in the stress relaxation test did not show systematic trends with heating time and temperature. A single first-order kinetic model described the changes in various rheological parameters due to thermal softening of potatoes using fractional conversion technique indicating R2 values higher than 0.97. Based on the values of rate constant, the visco-elastic test parameters appeared to be relatively more sensitive in detecting texture degradation in potatoes in comparison with the parameters from quasi-static tests. Overall comparison showed that elasticity and viscosity parameters in Kelvin component of the 4- element model (E2 and 112) from creep tests and elasticity moduli from axial and radial compression tests (Ea and Er) best represented the texture degradation kinetics in potatoes with R2 ranging from 0.98 to 0.99. Based on these results and other subjective judgments, axial compression and creep tests were selected for texture evaluation in raw and cooked potatoes during storage experiment. In the second phase of the study, the changes in the texture of raw and cooked potatoes due to storage of raw potatoes were investigated. Raw potatoes were stored at 5, 15, and 25°C, and under different temperatures over fixed time intervals for 16 to 26 weeks. Potatoes were subjected to textural evaluation periodically in the raw state as well as after cooking using axial compression and creep tests. The tangent modulus elasticity (Ea) of raw potatoes in axial compression and elasticity and viscosity parameters (E1, E2, 11 i, and ri2 in creep tests decreased with storage time at all temperatures (p<0.05). Except viscous parameter of the Maxwell component (ri 1) of the four-element model, all parameters exhibited systematic trends in both tests indicating a faster decrease at higher storage temperature. The changes in rheological parameters of potatoes during fixed storage conditions were adequately described by a modified exponential model with R2 ranging from 0.89 to 0.99. The logarithm of the texture degradation rate constant (k) was linearly related to storage temperature whereas the exponent (n) was found to be a linear function of storage temperature. In storage of potatoes under non-uniform storage temperatures, it was necessary to account for the cumulative effect of different combinations of time and temperature on the quality of stored potatoes. Accordingly, a bulk mean temperature (T hm) was calculated by taking into account the various timetemperature combinations up to the selected duration of storage. Such values of T bm were assumed to be equivalent to pseudo constant temperatures responsible for inducing textural changes in potatoes during storage under non-uniform conditions. Results of stepwise multiple regression showed that the changes in the texture of raw potatoes could be described adequately as a function of T hm and storage time for textural parameters such as Ea, E1 , E2 and ri2 except Y]1 with R2 values ranging from 0.84 to 0.99. The magnitudes of all textural parameters of cooked potatoes were considerably lower than that of raw potatoes. There was a significant decrease in the texture of both raw and cooked potatoes (p<0.05) with storage time at all temperatures. However, changes in the texture of cooked potatoes did not exhibit systematic trends with storage temperature. In axial compression test, the values of Ea showed a decrease of about 49 to 70% for raw potatoes and about 40 to 70% for cooked potatoes. Similarly in creep testing, values of visco-elastic parameters decreased by about 30 to 55% for raw potatoes and about 25 to 70% for cooked potatoes. As observed in case of raw potatoes, the viscosity parameter of Maxwell component in the four-element model did not show a systematic trend for cooked potato samples also. The overall comparison of results indicated that there was an apparent correlation between the textural parameters of raw and cooked potatoes irrespective of storage conditions (p<0.05). Such interrelationships led to the prediction of textural parameters of cooked potatoes directly from that of raw potatoes regardless of the storage history with R2 ranging from 0.70 to 0.87. This could help to develop techniques to predict texture of cooked potatoes from measurements on the raw potato texture. |
| Year | 2003 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Food, Agriculture and Natural Resources (Former title: Department of Food Agriculture, and BioResources (DFAB)) |
| Academic Program/FoS | Postharvest and Food Process Engineering (PH) |
| Chairperson(s) | Jindal, Vinod K. |
| Examination Committee(s) | Athapol Noomhorm;Rakshit, Sudip Kumar;Ranamukhaarachchi, S. L.; |
| Scholarship Donor(s) | Agricultural Research and Training Project (ARTP/AUA);Government of Ethiopia |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2003 |