Effect of Composition on Rheological Properties of Mango Jam Prepared with Sago Starch

The present study investigated the effectiveness of sago starch as a gelling agent on the rheological characteristics of mango jam. The rheological properties of modified mango jam containing sago starch in three concentrations (2%, 6%, and 10%) at the temperature range of (5, 25, and 45oC) were investigated. Mango jams containing sago starch behaved as a pseudoplastic fluid with yield stress. Modelling of the rheological data in three rheological models, namely, Power Law, Herschel-Bulkley, and MizrahiBerk showed that the most suitable model according to the R values was the HerschelBulkley model in each of the temperatures and concentrations studied. The apparent viscosity increased with sago starch concentration and decreased with temperature. The results obtained for the dynamic oscillatory shear revealed that the magnitude of storage modulus (G') of each of the mango jam samples were higher than the loss modulus (G′′) in most of the frequency window (10-100 Hz) studied. Results of this work indicated that from the rheological standpoint sago starch is a suitable alternative gelling agent to be used in jam industry. INTRODUCTION Fruit jam is a food product made from fruit pulp/puree, sugar, citric acid, and food hydrocolloids (Fugel et al., 2005). Hydrocolloids are used in fruit jams to thicken the consistency of the final product to achieve a reasonable texture (Basu and Shivhare, 2010a). The most common hydrocolloid added in fruit jam industry is high methoxyl pectin (HMP) because of its ease of gelatinization in low acidity and high soluble solid content conditions (Basu and Shivhare, 2010a). However, development in food formulations particularly fruit jams with cheaper food hydrocolloid substitutes such as starches would benefit both consumers and manufactures. In general starches are composed of two different types of glucose units, namely, amylose and amylopectin and are widely used in food applications due to their thickening ability (Ahmed and Williams, 1998). Sago starch is extracted from sago palm (Metroxylon spp.) and it is originated from South East Asia (Ahmed et al., 1999). Sago starch molecular structure contains amylose in the range of 25 to 31% respectively (Ahmed and Williams, 1999). Sago starch is a common product in numerous food applications in Malaysia (Kyaw et al., 2001, Karim et al., 2008, Rekha et al., 2008). In America and Europe sago starch is most likely utilized as thickening agent in soup (Cui M. Javanmard, J. Endan, S. Koohikamali 2 and Oates, 1996) and pudding production (Binky et al., 2007). Abd-Aziz et al. (2002) reported that in India and Indonesia sago starch is boiled with sugar and used in formulates jelly. Studies on sago starch have been reported by previous researchers (Cui and Oates, 1996, Ahmed et al., 1999, Nurul et al., 1999, Williams and Ahmad, 2000, Sopade and Kiaka, 2001, Maaurf et al., 2001, Abd-Aziz, 2002, Zaidul et al., 2003, Mohamed et al., 2008, Abdorreza et al., 2010). However, to our knowledge there in no information on replacement of starch with HMP in fruit jams. Knowledge of the rheological characteristics is important process design, quality control, and storage. Literature is replete with studies on the rheological behavior of fruit purees and jams such as strawberry, prune, peach, and apricot (Alvarez et al., 2006), fruit puree (Maceiras et al., 2007), pineapple jam (Basu et al., 2007), currents puree and jam (Speiciene et al., 2008), pistachio green hull marmalade (Moghaddam et al., 2009), and mango jam (Basu and Shivhare, 2010a). In a product like fruit jam changes in the ingredients composition could lead to changes in the products gel structure. In series of papers effect of composition on the rheological behaviour of mango jam prepared with HMP as the thickening agent were studied. Basu et al. (2010) reported that mango jam behaves as a pseudoplastic material and increase in HM pectin concentration would decrease the flow index behaviour (n), coefficient of consistency (k), and the yield stress (τ0). Although the same authors reported that with temperature increase the rheological parameters (n, k, and τ0 ) did not flow a descriptive trend (Basu and Shivhare, 2010b). Recently there have been studies on substitution of the sweetening agent in mango jams and studies on there gelling ability (Basu et al., 2011) but to our knowledge there is no study on the substitution of the thickening agent and its effect on the rheological parameters. Therefore, this study was conducted to evaluate the rheological (steady state shear and dynamic oscillatory shear) properties of mango jam containing sago starch as the gelling agent in mango jams. MATERIALS AND METHODS Sample preparation Mango jams were prepared with ripe mango Chokanan variety. The fruit was washed and skin peeled. To achieve a uniform consistency the fruit was blended using a highspeed laboratory blender (Warning blender 32BL80, New Hartford, USA) for 2 min. The fruit puree was cooled for 10 min in a water bath (Tsen and King, 2002). Sugar was in the form of commercial form. The acidifier, citric acid monohydrate (C6H8O7.H2O), was provided by ChemAR-SYSTERM, Malaysia. The sago starch (Metroxylon Sagu) used in this study was native and untreated obtained by NITSEI Sago industries Sdn. Bhd, (Province Wellesley, Penang, Malaysia). In this study sago starch was selected in three concentrations (2%, 6%, and 10% w/w) (Cui and Oates, 1996, M.Nurul I. et al., 1999). The mango jam samples were prepared following the procedure given by Basu etal. (2011) i.e. mixture of the ingredients, evaporation to reach the desired Brix level, finally cooling. The pH and the total soluble solid were monitored through the jam making process with a pH meter (S40 SevenMulti, Mettler Toledo, Swizerland) and digital refractometer (AR2008, ABBE Refractometer, Kruss, Germany) to reach the pH range (3± 0.2) of fruit jams and the total soluble solid of approximately 65-66 oBrix M. Javanmard, J. Endan, S. Koohikamali 3 (Basu and Shivhare, 2010). To achieve proper gel setting (68 oBrix) the samples were placed in refrigerator (SR-V57, Samsung Electronics, Korea) for up to 16 hours at (4 ±1 oC). The mango jam samples were removed from the refrigerator and were further analysed. Rheological measurements The rheological measurements were performed using a controlled shear rate Rheometer (ARG2, TA Instruments, New Castle, DE, USA) with computer control at different temperatures (5, 25, and 45oC). The geometry used was a hatched parallel plate with radius of 40 mm and the gap between two plates was fixed at 1000 μm. TA Rheometer Data Analysis software (version V5.7.0) was used in calculating the rheological measurements. For each test approximately 3 g of the mango jam sample containing sago starch was placed between the plates. The reproducibility of the results was investigated in three runs for each temperature and each time a new sample was loaded, the analysed results were the average values. Steady-state shear The steady-state shear was investigated at the temperature range of 5 to 45 oC with the shear rate between 1-500 s(S.Basu et al., 2007, Maceiras et al., 2007). The relationship between the shear rateshear stress was expressed by three rheological models namely, Power Law, Herschel-Bulkley, and Mizrahi-Berk.