Changes in physical and thermo-physical properties of sugarcane, palmyra-palm and date-palm juices at different concentration of sugar
Introduction
Jaggery, a sugar rich food product is produced all over the world under different names, such as Gur (India), Desi (Pakistan), Panela (Mexico and South America), Jaggery (Burma and African countries), Hakuru (Sri Lanka), Htanyet (Myanmar), Panocha (Philippines), Rapadura (Brazil), and Naam Taan Oi (Thailand) (Thakur, 1999). It is consumed directly or used for preparation of sweet confectionery items and ayurvedic/traditional medicines (Pattnayak and Misra, 2004), and it may have a role to reduce the chance of lung cancer (Sahu and Paul, 1998). Jaggery is prepared traditionally by concentrating sugarcane juice (Saccharum officinarum) in open atmosphere boiling. In addition, sap collected from palm trees such as palmyra-palm (Borassus flabellifer L.), coconut palm (Cocos nucifera L.), wild date-palm (Phoenix sylvestris L.) and sago palm (Caryota urens L.) are also used for preparation of jaggery (Pattnayak and Misra, 2004). The sap or juice collected from these trees contains around 10–12% total sugars; mainly comprised of sucrose, less amount of reducing sugars, and other minerals and vitamins (Dalibard,1999). All these jaggery products have their own characteristic taste and aroma and their production is seasonal. India produces about 6 million tonnes of jaggery annually, which accounts 70% of the total production in the world; 65–70% of the total jaggery is from sugarcane, the remaining 30% is from palms (Kamble, 2003).
The production of solid forms of jaggery involves: collection of juice by crushing canes or tapping the sap from palm trees; its filtration and concentration by boiling, cooling of concentrated juice followed by moulding, drying and storage (Fig. 1). The quality of the prepared jaggery, such as aroma, texture, colour and taste, is largely dependent on monitoring and controlling of various physical and chemical changes occurring during concentration, particularly when the process approaches the end point (high total soluble solid concentration). In absence of scientific data, this stage becomes critical, and is mostly tackled by the skill of the processor.
Variations in density, viscosity and boiling point rise for pure sucrose solution has been reported to be a function of concentration (Junk and Pancoast, 1973). Buera et al. (1987) have reported kinetics of colour changes due to caramelization of various single sugars with heating time. Physical and thermo-physical properties of the different juices have been found to exhibit a close relationship with temperature and water content (Ali et al., 2002). Singh, 1992, Sweat, 1974 have reviewed thermo-physical properties of different vegetables, fruits and its juices. Several workers (Constenla et al., 1989, Telis-Romero et al., 1998, Patricia et al., 2005, Zuritz et al., 2005, Shamsudin et al., 2005) have reported mathematical models correlating thermo-physical properties of fruit juices, soluble solids content and temperature. According to Telis-Romero et al. (1998), in Brazilian orange juice, total soluble solids exhibited a significant role on its density, thermal conductivity, thermal diffusivity and specific heat compared to temperature when concentration and temperature were varied. However, no information on physical and thermo-physical properties of sugarcane, palmyra-palm and date-palm juice is available in the literature.
The present study discusses the variations in some physical (boiling point, density, viscosity and colour) and thermo-physical (thermal conductivity and specific heat) properties of sugarcane, palmyra-palm and date-palm juices with changes in soluble solid (increase in total soluble solids). Furthermore, correlations among these changes with solid concentration in the syrup were explained.
Section snippets
Materials and methods
This study comprises of two parts: preparation of jaggery type from their respective juices was monitored at the site (while the evaporation process was in progress) with periodical measurements of temperature of the juice and total soluble solids (TSS). Other properties, i.e., density, viscosity, thermal conductivity, thermal diffusivity, specific heat and colour of the juice or syrup, collected at different stages of boiling, were measured at room temperature.
Temperature and TSS concentration profiles during concentration of juice samples
Fig. 2a–c shows the time–temperature profile and corresponding change of total soluble solids (TSS) for three juice samples. All these temperature profiles and change in solids concentration were noted to contain three distinct zones. The temperature of the juice varied more rapidly in Zone I till it attained the boiling temperature followed by elevation of boiling point very slowly (Zone II) arising from corresponding increase in TSS in the juice. Rapid increase in solids concentration and
Conclusions
The change in temperature and TSS concentration with time of heating showed similar trend for three juices-having three distinct zones: Zone I, Zone II, and Zone III. Zone III is very critical and heating must be regulated in this zone to maintain the quality of final jaggery. The critical ranges of temperature, concentration and viscosity in this zone are estimated to be, respectively, 105–121 °C, 54.6–81.9 (% w/w), 4.5–988 mPa s, for sugarcane juice; 104–120 °C, 46–81 (% w/w) and 42–559 mPa s for
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