Stability of palace halva in relation to storage temperature and relative humidity

Palace halva is a traditional confectionery product of Turkey. Sugar, wheat flour and milk fat are the main components of this halva. It has a special fibrous, soft and brittle structure, which melts in the mouth. The product has low water activity hence tendency to adsorb water. Physical deterioration is observed in the product during storage. Storage relative humidity and temperature are the major factors affecting quality and shelf life of the product. Changes in quality of palace halva were investigated in this research at different storage relative humidities and temperatures. Halva samples were stored at different relative humidity values in the range of 22-90% at 25 and 35°C until equilibrium was reached. Changes in quality of palace halva were followed by measurements of volume change, hardness, lipid oxidation and color. Physical state of the product was determined by X-ray diffractometry. Adsorption isotherms were determined for 25 and 35°C and modelled and compared by using isotherm models published in the literature. Adsorption isotherms were established by applying isopiestic method. Halva samples were stored in eight different jars with different relative humidity. Different relative humidities were obtained by preparing supersaturated solutions of salts including CH3COOK, MgCl2, K2CO3, Mg(NO3)2, NaBr, NaCl, KCl and BaCl2 in the jars to have relative humidities in the range of 22-90% in the headspace. Halva samples were kept on a stand in the headspace of the jars. Jars were stored in incubators at 25 and 35°C. Gravimetric method was used to determine the time when the samples reached equilibrium. Eight different isotherm models were tested to choose the best fitting model to water adsorption data. These models were Oswin, Henderson, Halsey, Iglesias-Chirife, Caurie, Chung-Pfost, Kuhn and Bradley models. Regression coefficients (R²), mean relative error (MRE) and standart of the mean (SEM) were calculated by using a computer program and best model was chosen by comparing the results. Hardness of the samples were measured by applying penetration method with a 2 mm-diameter probe. The color was mesured by using a colorimeter. L*, a* and b* values were measured and total color change (ΔE*) relative to color values of the original sample was calculated. Volume of the samples was calculated from their heights and base areas. Lipid oxidation was measured by using thiobarbituric acid (TBA) method. In this method, oxidation products were seperated by Kjeldahl distillation and reacted with TBA reagent. Amount of TBA reactive substances (TBARS) in the samples was determined spectrophotometrically. X-ray diffractometry was used to determine physical state of palace halva samples. The instrument was run with a 1 minute dwell time at each 1° from 5° to 27° 2-theta angles. Computer software was used to generate the X-ray diffraction plot of intensity vs. 2-theta angle. Samples stored at three relative humidity levels at two temperatures were chosen for X-ray diffraction analysis to explain phsyical changes observed in the product. Adsorption isotherms of palace halva were found to be similar to those of other foods with high amounts of sugar. Halsey, Henderson and Iglesias-Chirifie models were found to be suitable for adsorption isotherm of palace halva. Hardness of palace halva increased when stored at 25°C and higher than 21%relative humidity. Highest hardness value was obtained at 44%relative humidity at 25°C. Halva samples softened at relative humidities higher than this value. A similar trend was observed in halva samples stored at 35°C. Highest hardness value was measured at 32% relative humidity at this temperature. Changes in the hardness value of halva was found to be more at 35°C compared to those occurred at 25°C. There was no difference in the volume of palace halva at both temperatures upto 75% relative humidity. At relative humidities higher than this value, samples' height and volume decreased. Halva samples could not maintain their original shape and they melted at relative humidity values higher than 75%. They lost their brittle and fibrous structure. Color of the palace halva tended to darken as relative humidity was increased. The L* indicating the lightness of the color of the samples were found to decrease at relative humidity values higher than 64% at both temperatures. The a* value, indicating green-red tones of the color, was found to increase as relative humidity was increased and the increase was higher at relative humidity values higher than 64%. The b* value, showing the blue-yellow tones, was found to increase upto 64% relative humidity but it decreased after this value. ΔE* value slightly changed upto 64% relative humidity, but it increased dramatically at higher relative humidities at both temperatures. Amount of TBARS in palace halva samples was found to be at a minimum at 44-54% relative humidity values at both temperatures. At higher and lower relative humidities, TBARS tended to increase. The increase was more at relative humidities higher than 54%. The trend observed in the amount of TBARS with increasing relative humidity was in accordance with the lipid oxidation-water activity relationship at both temperatures. Changes in the physical state of the palace halva were found to be related with the changes in the hardness of the product. Physical state of halva was found to be changed from glassy to crystalline with increasing relative humidity. Fresh palace halva was found to be in glassy state by X-ray diffraction analysis. Crystalline state was observed in the halva samples stored at 54% relative humidity at 25°C and 32% relative humidity at 35°C. Temperature increase was found to accelerate this change and allowed the state transition to take place at a lower relative humidity. These findings also showed that physical state of the samples changed within the time required for reaching equilibrium at certain relative humidities during the determination of adsorption isotherm. Stability of palace halva was determined in relation to storage temperature and relative humidity. It was found that storage temperature and relative humidity had important influence on the quality of palace halva. Changes in storage temperature and relative humidity may cause changes primarily in the structure of the product. In addition, chemical reactions may cause changes in color and taste of the product. For this reason, palace halva should be packed in moisture-proof packaging and stored at controlled temperature and relative humidity to avoid quality loss during storage.