Changes in physical and thermo-physical properties of sugarcane, palmyra-palm and date-palm juices at different concentration of sugar

Journal of Food Engineering 90 (2009) 559–566

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Changes in physical and thermo-physical properties of sugarcane, palmyra-palm and date-palm juices at different concentration of sugar P.V.K. Jagannadha Rao a, Madhusweta Das b, S.K. Das b,* a b

Department of Agricultural Engineering, Agricultural College, Acharya N.G. Ranga Agricultural University, Naira 532 185, Andhra Pradesh, India Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur 721 302, West Bengal, India

a r t i c l e

i n f o

Article history: Received 26 March 2008 Received in revised form 13 July 2008 Accepted 24 July 2008 Available online 5 August 2008 Keywords: Jaggery Sugarcane Palmyra-palm Date-palm Palm juice

a b s t r a c t The process of making jaggery from three natural juices by boiling could be divided into three zones: rise in temperature to boiling (Zone I), slow rise in both boiling temperature and total soluble solids (TSS) (Zone II) followed by rapid rise in boiling temperature with concomitant increase in viscosity and TSS (Zone III). The juice samples in Zone III exhibited changes in boiling temperature, viscosity and TSS from 105 to 121 °C, 4.5 to 988 mPa s and 54.6 to 81.9 (% w/w) for sugarcane (Saccharum officinarum); from 104 to120 °C, 41.6 to 559 mPa s and 46 to 81 (% w/w) for palmyra-palm (Borassus flabellifer L.); and from 103 to 121 °C, 22.9 to 417 mPa s and 51 to 81 (% w/w) for date-palm (Phoenix sylvestris L.). Colour change  DE was rapid in Zone III. Difference in colour among these jaggery samples might be attributed D ð% w=wÞ to amount of reducing sugars present initially and respective changes in properties during juice concentration. Ó 2008 Elsevier Ltd. All rights reserved.

1. 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). * Corresponding author. Tel.: +91 3222 283112; fax: +91 3222 282244/255303. E-mail address: [email protected] (S.K. Das). 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.07.024

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) and 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

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P.V.K. Jagannadha Rao et al. / Journal of Food Engineering 90 (2009) 559–566

Nomenclature a* a b* C Cp Ecb DE k L* q

CIE colour values parameter for Chirife and Buera (1997) model CIE colour values concentration (% w/w) specific heat (kJ kg1 K1) parameter for Chirife and Buera (1997) model colour difference between two samples thermal conductivity (W m1 K1) CIE colour values heat produced per unit length per unit time (W m1)

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. 2. 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 measure-

Juice Extraction by Crushing or Tapping Juice Filtration

r T0 and T t0 and t Tb TSS X

a c q gr

radial distance (m) from the probe, correlation coefficient temperatures at initial and final time, respectively (K) initial and final time (s) boiling temperature (°C) total soluble solids (% w/w) mole fraction thermal diffusivity (m2 s1) Euler’s constant density (kg m3) relative viscosity

ments 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. 2.1. Preparation of sugarcane jaggery Fifteen hundred (1500) litres of juice was extracted from 1.25 tonnes of sugarcane (variety Co 85 A 298) in Harapalem village (83°10 E, 17°400 N), Visakhapatnam district, Andhra Pradesh, India. The initial total soluble solid concentration of the juice was 17.2 (% w/w) (average). After filtration with a fine muslin cloth, concentration of sugarcane juice was carried out by open atmosphere boiling in a large circular pan made of iron. Lime water (60 ml/100 kg juice) was added at the initial stage of boiling and also intermittently for clarification of juice. It was for clarification of juice. Boiling of juice continued in a regulated manner for more than 3 h till the concentrated syrup attained a total soluble solids concentration around 82 (% w/w) while the temperature rose slightly beyond 120 °C. The end point concentration level was decided manually by dropping a small aliquot of hot syrup into cold water taken in a container where it solidified. The concentrated juice was then transferred to moulds and allowed to cool gradually till it solidified.

Undesirable solid particles

2.2. Preparation of palmyra-palm jaggery

Partially clarified juice Boiling in a shallow tank/ pan in open atmosphere with/ without clarificant and removal of scum periodically Hot thick sugarcane syrup Air cooling & Moulding in various shape moulds

Final concentration or End point is examined by a skilled person

About fifty five (55) litres of juice (sap) was collected from several palmyra-palm trees at Kharar village (87°190 E, 22°250 N), East Midnapur District, West Bengal, India with initial total soluble solid content of 16 (% w/w). It was first filtered through a fine muslin cloth and boiling was carried out in an open shallow aluminum pan. A mild bleaching agent at the rate of 5 g/50 l juice was added intermittently during boiling for clarification of juice. Boiling of juice continued in a regulated manner for more than 2 h till the syrup attained total soluble solids around 81 (% w/w). At that time, temperature rose to about 120 °C. 2.3. Preparation of date-palm jaggery

Solid jaggery in different shapes

Storage Fig.1. Process flow chart for solid jaggery preparation.

About sixty (60) litres of juice was collected from different datepalm trees at Kharar village, East Midnapur District, West Bengal, India with initial total soluble solid content of 12.9 (% w/w). Boiling of juice for jaggery preparation was almost similar to that of palmyra-palm. However, no clarificant was added during boiling of juice. Boiling of juice continued more than 2½ h till the syrup attained total soluble solids concentration of 81 (% w/w), with corresponding end point temperature around 120 °C.

P.V.K. Jagannadha Rao et al. / Journal of Food Engineering 90 (2009) 559–566 Table 1 Composition of fresh sugarcane, palmyra-palm and date-palm juice (g/100 ml)



b ¼

Parameters

Sugarcane juice

Palmyra-palm juice

Date-palm juice

pH Total sugar Reduced sugar Protein Minerals as ash Calcium Phosphorous Potassium Iron Vitamin C

5.5 14 0.6 0.55 0.91 0.25 Trace 0.50 0.16 0.0046

6.8 10.93 0.96 0.35 0.54 Trace 0.14 Trace 0.4 0.013

6.0 11.50 0.85 0.40 0.46 Trace 0.11 Trace 0.35 0.0092

All the three juice samples were analyzed (AOAC, 1998) for total sugars, reducing sugars and other minerals present. Table 1 provides the composition of the three juice samples. 2.5. Measurement of temperature and TSS of juice during concentration For each of the processes, temperature of the boiling juice was measured periodically at an interval of 5 min using thermocouples (copper- constantan) coupled to a digital temperature indicator (50 to 200 °C) with resolution of 0.1 °C. Measurement of temperatures of the boiling juice was taken at three locations in the boiling pan and their average value was taken. Samples were collected at regular interval to determine the TSS (% w/w) using three portable refractometers (Models 0–32, 28–62 and 58–92 °Brix, Erma Optical Works Ltd., Tokyo, Japan) having resolution 0.1 °Brix (% w/w) for each. 2.6. Measurement of physical and thermo-physical properties of the collected juice samples 2.6.1. Density Density of sugarcane, palmyra-palm and date-palm juice and syrup at different total soluble solids were determined at 25 °C, by weighing the juice contained in a 25 ml pycnometer (Constenla et al., 1989). For all these measurements, three replicates per sample were made and their respective average was taken. 2.6.2. Viscosity A Brookfield Viscometer (Model DV-1, Brookfield Engineering Laboratories, MA, USA) was used to determine the viscosity of the collected juice samples at 25 °C using UL adopter as well as specific spindles. 2.6.3. Colour Colour change was initially judged by visual examination of juice collected in clear test tubes at different interval of time. In addition to this, a high-resolution digital camera (Model G1, Canon Digital Lab, Singapore, Malaysia) was used to measure colour by capturing the colour image of the juice sample as per the method described in the literature (León et al., 2006). The colour images were then analyzed qualitatively and quantitatively using Adobe Photoshop (Yam and Papadakis, 2004). The lightness, a, and b in the Histogram Window (Adobe Photoshop) are not standard colour values. However, they were converted to CIE L*, a*, b* values using the following equations (Yam and Papadakis, 2004):

Lightness  100 255 240a a ¼ —120 255

ð1Þ ð2Þ

ð3Þ

The colour difference between two samples was estimated (Siddiqui and Nazzal, 2007) using the following equation:

DE ¼

2.4. Composition of different juice samples

L ¼

240b  120 255

561

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ðDLÞ2 þ ðDa Þ2 þ ðDb Þ2

ð4Þ

2.6.4. Thermal conductivity, thermal diffusivity and specific heat 2.6.4.1. Theory of thermal properties measurement by line source heating. Thermal conductivity and diffusivity of juice samples were determined using the line heat source theory of transient heat transfer analysis. The detailed theory of the thermal conductivity probe or line heat source technique has been described in literature (Fontana et al., 2001). In brief, a line heat source probe was inserted into the juice sample (initially at a uniform temperature) and then heated at a constant rate. Temperature adjacent to the line source was monitored. A plot of logarithm of time versus temperature was linear, and its slope used to calculate the thermal conductivity of the sample. The final equation for the thermal conductivity may be written as

T  T0 ffi

  2  r lnðtÞ  c  ln 4p k 4a q

ð5Þ

where q is heat produced per unit length per unit time (W m1); k is thermal conductivity of the medium (W m1 K1); a is thermal diffusivity (m2 s1); t is time (s); c is Euler’s constant; and r is radial distance (m) from the probe. The diffusivity was obtained from the intersection of the regression line (Eq. (5)) with the t-axis (DT = 0). Thus analy tically

 lnðt0 Þ ¼

c þ ln



 r2 ; 4a

T  T 0 ¼ DT ¼ 0

ð6Þ

Taking the value of t0 [from the intercept of DT versus ln(t)] and finite r, the diffusivity was calculated. 2.6.4.2. Measurement system. The line heat source thermal property analyzer (Model KD2, Decagon Devices Inc., WA, USA) consisted of a 0.9 mm diameter, 60 mm length stainless steel needle, with a line heat source element and a temperature sensor. A micro-controller regulated the power to the heating element and measured the probe temperature. The thermal conductivity and diffusivity of the test sample were computed from its own in-built software on the basis of the theory described above. The least count of this instrument was 0.02 W m1 K1 for thermal conductivity and that for diffusivity it was 0.1  106 m2 s1 with the corresponding accuracy of 5% and 10%. The instrument was initially calibrated against distilled water and castor oil as per the procedure described by the manufacturer. Both values were compared with the corresponding reported data. The instrument took about 2 min to attain a stabilized value. Each of these measurements was replicated thrice and average values were taken. The specific heat (Cp) of the juice sample was estimated from the measured values of thermal conductivity (k), diffusivity (a) and density (q) of the same sample (Sweat, 1986) using the following equation:

Cp ¼

k

qa

ð7Þ

2.6.5. Statistical analysis Effect of TSS on various properties of the juice during concentration was analyzed (ANOVA) following the method of single factor experiment with completely randomized design (Gomez and Go-

P.V.K. Jagannadha Rao et al. / Journal of Food Engineering 90 (2009) 559–566

ð8Þ

90 80

120

70 100

40 30

40

20 20

10 Zone-I

0 0

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 boiling temperature occurred in Zone III. This fast increase in TSS in this zone is due to rapid evaporation of juice at elevated temperature (higher boiling point) with lesser volume and depth of juice in the boiling pan. For making quality jaggery from these juice samples, controls of parameters corresponding to Zone III are most critical. Nevertheless, many other changes also occurred in the latter part of Zone II. Table 2 gives a range of values of boiling temperatures and TSS for Zones II and III. 3.2. Change in boiling temperature with the change in TSS Fig. 3 shows boiling point rise (DTb) of different juice samples at different concentration TSS (comprising of Zones II and III). All the juice samples showed a sharp increase in boiling point after attaining a particular zone of concentration of sugar; exactly similar to the trend reported for pure sucrose solution (Junk and Pancoast, 1973) as shown in Fig. 3. However, it may be noted that at any corresponding concentration value (x-axis), all these juice samples had higher boiling point rise than that of pure sucrose solution. This might be possibly due to presence of other low molecular weight soluble substances in the juice samples (Table 1) that gives higher boiling point rise (Junk and Pancoast, 1973). With the progress of evaporation, the concentration of reducing sugars increases which accounts further rise in boiling point of the juice samples in comparison to that of pure sucrose solution. The boiling point rise versus TSS for all these juice samples showed exponential trends

DT SGJ ¼ 0:2209e0:0557C

ðr ¼ 0:969Þ

ð9Þ

0:0558C

ðr ¼ 0:971Þ

ð10Þ

DT DPJ ¼ 0:4754e0:0469C

ðr ¼ 0:965Þ

ð11Þ

DT PPJ ¼ 0:3433e

At any particular TSS beyond 40 (% w/w), the boiling points followed the order: DTb-palmyra-palm juice > DTb-date-palm juice > DTb-sugarcane juice corresponding to the order of reducing sugars present in these samples (Table 1).

Zone-II

20

40

Zone-III

0 60 80 100 120 140 160 180 200

140

90

120

80 70

100 60 80

50

60

40 30

40

20 20

10 Zone-III

Zone-II

Zone-I

0

0 0

c

20

40

60

80

100

120

140 90

140

80

120

Juice temperature (°C)

3.1. Temperature and TSS concentration profiles during concentration of juice samples

Juice temperature (°C)

b

50

Total soluble solids,°Brix

60

where Ypre,i,i and Yexp,i are the observed and predicted values and N is the number of data.

3. Results and discussion

60

Temperature,°C

80

Total soluble solids (%w/w)

# N 100 X ðY pre;i  Y exp;i Þ MRPD ¼ N i¼1 Y exp;i

140

Total soluble solids (%w/w)

"

a

70 100 60 80

50

60

40 30

40 20 20

10 Zone-I

0 0

20

Zone-III

Zone-II

40

60

80

Total soluble solids (%w/w)

mez, 1984) using Microsoft Excel (Anonymous, 2003). Regression analysis for correlating various properties of the juice and total soluble solids was carried out with the Origin 6.1 package (Anonymous, 2000). Statistical validity of the predictive models was evaluated using statistical parameters such as mean relative percentage deviation (MRPD) (McLaughlin and Magee, 1998) and correlation coefficient (r) according to the following equation:

Juice temperature (°C)

562

0 100 120 140 160 180

Heating time (min) Fig. 2. Time–temperature and time–TSS profiles of different juices during jaggery making process (a) sugarcane, (b) palmyra-palm, and (c) date-palm juice.

Table 2 Range of boiling temperatures and TSS concentration for Zones II and III for three different juice samples Sample name

Sugarcane juice Palmyrapalm Date-palm

Zone II Boiling temperature (°C) 100.5–104 101–103 100–102

Zone III TSS concentration (% w/w)

Boiling temperature (°C)

TSS concentration (% w/w)

21.2–54.5

105–121

16.8–45

104–120

46–81

103–121

51–81

14–50

54.6–81.9

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P.V.K. Jagannadha Rao et al. / Journal of Food Engineering 90 (2009) 559–566

35

a

90

SGJ

30

Colour values (L*,a*,b*)

PPJ

boiling point rise(0C)

DPJ Sucrose

25

Expon. (Sucrose) Expon. (SGJ) Expon. (DPJ)

20

100

Expon. (PPJ)

15

L a b

L

70 60 50 40

b More yellow

30 20 a

10 0

10

Poly (L) Poly (a ) Poly (b)

More Dark

80

More red

-10 10

5

b 0 30

40

50

60

70

80

90

100

Total soluble solids (%w/w) Fig. 3. Boiling point rise of sucrose solution (Junk and Pancoast, 1973), sugarcane juice, palmyra-palm juice and date-palm juice with the increase in total soluble solids.

3.3. Change in colour during concentration of juice Fig. 4a–c shows the change in colour of three juice samples with the increase in concentration of the juice samples. All these three juice samples showed similar trends; the colour of the juice changes from a dull colour (low a*, b*, and high L-values) to a dark golden yellow followed by dark-brown (high +a* values referred to red and high +b* values referred to yellow as per CIE colour space) (Siddiqui and Nazzal, 2007). Initially there was rapid change in L-value (lightness–darkness) up to 30 (% w/w) that includes Zone I and Zone II of boiling. However, a* and b*-values changed slowly. Further boiling caused little difference in L-values but both a* and b* continued to change, and the colour of the juice changed towards orange yellow – suggesting onset of caramelization process. At around 60 (% w/w), there was rapid change in all L, a* and b* values, that confirms high rate of caramelization of sugars. At this point, zone or localized heating would be more common with reduced heat transfer in the entire volume of highly viscous mass. In sugarcane juice, extend of change in a* values were found more compared to that of b* values in the latter part of Zone II and early Zone III. This, however, was not true for other two juices, viz., palmyra-palm and date-palm juices; both showed rapid change in both a* and b* values leading to brown to dark-brown colour of the product. The successive change in colour of the juice with the change in   concentration D ð%DEw=wÞ is presented in Fig. 5. These trends are also similar for all three juice samples. From an initial high value,   DE attained a minimum at around 25–50 (% w/w) followed D ð% w=wÞ

Colour values (L*,a*,b*)

20

c

30

40

70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5

50

60

70

80

90

L a b Poly (L) Poly (a) Poly (b)

More dark

More yellow

More red

10

Colour values (L*,a*,b*)

10

20

20

65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5

30

40

50

60

70

80

90

70

80

90

More dark

L a b Poly (L) Poly (a) Poly (b) More yellow

More red

10

20

30

40

50

60

total soluble solids (%w/w) Fig. 4. Change in colour values for (a) sugarcane, (b) palmyra-palm, and (c) datepalm juice samples with the increase in total soluble solids at 25 °C.

by a steady increase. At this point, the reaction rate with reduced volume and higher temperature facilitates the caramelization reaction. During this time, rapid and continuous scrapping the mass with ladle and reduced heat input were necessary for avoiding   excessive charring and burning. Successive changes in D ð%DEw=wÞ

3.4. Change in viscosity during concentration of juice

for palmyra-palm and date-palm were found to be more compared to that of sugarcane juice. This might be attributable to the higher amount of reducing sugars in the former two juice samples compared to that of sugarcane juice. It may be pertinent to mention

Fig. 6 shows changes in apparent viscosity of three juice samples with increasing TSS concentration at shear rate of 122.3 s1. All of them showed a similar trend, i.e., viscosity remained almost

that, the rate of caramelization of fructose and glucose are more than that of sucrose when the medium temperature is progressively increased keeping the pH constant (Buera et al., 1987).

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P.V.K. Jagannadha Rao et al. / Journal of Food Engineering 90 (2009) 559–566

12

1400 Sugarcane juice Palmyra-palm juice Date-palm juice Linear regression

Sugarcane juice

10

1350

Palmyra-palm juice

Density (kgm-3)

ΔE / Δ(%w/w)

Date-palm juice

8

6

4

1300 1250 1200 1150 1100 1050

2

1000 10

0 10

20

30

40

50

60

70

80

20

Average total soluble solids ( %w/w) Fig. 5. Successive change in total colour (DE) with the corresponding change in concentration (D (% w/w)) for three juice samples at different concentration.

30

40

50

60

70

80

90

Total soluble solids (%w/w)

90

Fig. 7. Change in density of sugarcane juice, palmyra-palm juice and date-palm juice with the increase in total soluble solids at 25 °C.

10000 -1 -1

Thermal conductivity (Wm K )

0.60 Sugarcane juice Palmyra palm juice

Viscosity (mPa s)

1000

Date-palm juice Sucrose

100

10

Sugarcane juice Palmyra-palm juice Date-palm juice Linear regression

0.55 0.50 0.45 0.40 0.35 0.30 10

1 20.0

40.0

60.0

80.0

20

100.0

Total soluble solids (%w/w) Fig. 6. Change in viscosity of sucrose solution (Junk and Pancoast, 1973), sugarcane juice, palmyra-palm juice and date-palm juice with increase in total soluble solids at 25 °C.

invariant in the Zone I, slow increase in mid Zone II and rapid variation in the tail end of Zones II and III. Similar variation in viscosity of sucrose solution has been reported by Junk and Pancoast (1973) as shown in the same figure. At any corresponding value of TSS concentration (>60 (% w/w)), the viscosity of date-palm juice was higher followed by sugarcane and palmyra-palm juice in this order. The higher levels of viscosity of these juice samples might be attributed to different high molecular weight organic compounds present in these juice. The nature of sugars in these juice samples might have some contribution for high viscosity (Junk and Pancoast, 1973). Table 3 Estimated model parameters a and Ecb for three juice samples at 25 °C (Chirife and Buera, 1997) Parameter

Sugarcane juice

Palmyra-palm juice

Date-palm juice

a Ecb Correlation coefficient MRPD

1.308 35.841 0.998 0.576

1.458 34.045 0.986 0.352

1.077 50.425 0.995 1.016

30

40

50

60

70

80

90

Total soluble solids (%w/w) Fig. 8. Change in thermal conductivity of sugarcane juice, palmyra-palm juice and date-palm juice with the increase in total soluble solids at 25 °C.

4.2 4.0

Specific heat (kJkg-1K-1)

0.0

Sugarcane juice Palmyra-palm juice Date-palm juice Linear regression

3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 10

20

30

40

50

60

70

80

90

Total soluble solids (%w/w) Fig. 9. Change in specific heat of sugarcane juice, palmyra-palm juice and datepalm juice with the increase in total soluble solids at 25 °C.

565

P.V.K. Jagannadha Rao et al. / Journal of Food Engineering 90 (2009) 559–566 Table 4 Mean relative percent deviation (MRPD) for various models Sample

Density

Thermal conductivity

Choi and Okos model Sugarcane juice MRPD 0.121

Specific heat

Linear equation (12)

Pharm and Willix parallel model

Pharm and Willix series model

Maxwell–Eucken model

Linear equation (13)

Summation model

Linear equation (14)

0.011

4.813

10.258

4.897

0.229

0.664

0.061

Palmyra-palm juice MRPD 0.183

0.215

7.301

6.816

1.652

2.881

2.544

3.129

Date-palm juice MRPD 0.901

0.622

4.583

9.202

4.579

0.517

1.850

2.583

It is apparent from the above result that, the increase in viscosity (50–60 mPa s) of the juice samples was rapid and became critical when the TSS concentration exceeded 60 (% w/w). Thus, monitoring of viscosity change (50–60 mPa s) of the juice could be employed to identify the critical end point of the process. Chirife and Buera (1997) have reported a simple exponential model (gr ¼ a expðEcb XÞ) for predicting the viscosity of sugar and oligosaccharide solutions; where a and Ecb are the model constants and X is the mole fraction of sugar in the solution. The experimental data of three juice samples were found to fit well with this exponential model (MRPD < 10%; correlation coefficient 0.995– 0.998). The model parameters, estimated for three juice samples, are shown in Table 3. 3.5. Change in density, thermal conductivity and specific heat of juice during concentration Variation of density, thermal conductivity and specific heat of different juice samples at different levels of concentration are shown in Figs. 7–9. The density increases from 1023 to 1445; 1072 to 1359 and 1051 to 1413 kg m3 for sugarcane, palmyrapalm and date-palm juice, respectively, when the corresponding TSS concentration levels varied from respective initial level to final level around 81 (% w/w). Both thermal conductivity and specific heat of juice samples decreased with the increase in solid concentration (or decrease in water fraction). Thermal conductivity values decreased from 0.59 to 0.35; 0.54 to 0.35 and 0.587 to 0.26 W m1 K1 for sugarcane, palmyra-palm and date-palm juice, respectively, while the variation in concentration was from 7.6 to 81.9; 16 to 81 and 10.4 to 81 (% w/w) in that order. Corresponding variation in specific heat of juice samples were from 3.97 to 2.42; 3.87 to 2.57 and 3.88 to 1.99 kJ kg1 K1. The increasing trend for density and decreasing trend for thermal conductivity and specific heats have been observed for many fruit juices and liquid foods (Ramos and Ibarz, 1998; Cepeda and Villaran, 1999; Zainal et al., 2000; Riedel, 1949; Gratao et al., 2005). It may be noted that all the properties of three juice samples followed a linear trend with the concentration of solid. Eqs. (12)–(14) give generalized expressions for each of these properties

q ¼ 997:39 þ 4:46C ðr ¼ 0:995Þ 3

k ¼ 0:603  3:3  10 C

ðr ¼ 0:989Þ

C p ¼ 4:149  2:46  102 C

ðr ¼ 0:983Þ

ð12Þ ð13Þ ð14Þ

The effect of concentration on these properties was found to be highly significant (p < 0.01). Different models for density (Nesvadba, 2005), thermal conductivity (Carson, 2006) and specific heat (Nesvadba, 2005) along with the linear equations (Eqs. (12)–(14)) were tested with each juice sample for prediction of these parameters. All these models and the equations showed MRPD values < 10% (Table 4) suggesting good fit.

All these properties corresponding to start of the critical zone (Zone III) was estimated as 1223–1363 kg m3, 0.436– 0.336 W m1 K1 and 2.91–2.16 kJ kg1 K1 for density, thermal conductivity and specific heat respectively. 4. 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 palmyra-palm juice; and 103–121 °C, 51–81 (% w/w) and 23–417 mPa s, for date-palm juice. These values could be monitored conveniently by measuring the temperature, TSS concentration or viscosity of concentrated juice using handy instruments at the site. Viscosity of all three juice samples fitted well to simple exponential relationship. Density, thermal conductivity and specific heat of each juice were found to be linear function of concentration of solids. Measurement of these parameters could also be used to decide the end point once the critical concentration range is identified and applied in the respective equation obtained. Although measurement of colour and their analyses could be a preferred method, it is difficult to employ this methodology at site or to obtain the decisive results within reasonable time. References Ali, S.D., Ramaswamy, H.S., Awuah, G.B., 2002. Thermo-physical properties of selected vegetables as influenced by temperature and moisture content. Journal of Food Process Engineering 25, 417–433. Anonymous, 2003. Microsoft Office Professional Edition 2003. Microsoft Office Excel. One Microsoft Way. Microsoft, Inc., Redmond, WA 98052-6399, USA. Anonymous, 2000. Origin 2000. Non-Linear Curve Fit. Version 6.1. Origin Lab, Inc., Northampton, MA 01060, USA. AOAC, 1998. Official Methods of Analysis. Association of Official Analytical Chemists, Washington, DC, USA. Buera, P., Chirife, J., Resnik, S.L., Lozano, R.D., 1987. Nonenzymatic browning in liquid model systems of high water activity: kinetics of colour changes due to caramelization of various single sugars. Journal of Food Science 52 (4), 1059–1062. Carson, J.K., 2006. Review of effective thermal conductivity models for foods. International Journal of Refrigeration 29, 958–967. Cepeda, E., Villaran, M.C., 1999. Density and viscosity of Malus floribunda juice as a function of concentration and temperature. Journal of Food Engineering 41, 103–107. Chirife, J., Buera, M.P., 1997. A simple model for predicting the viscosity of sugar and Oligosaccharide solutions. Journal of Food Engineering 33, 221–226. Constenla, D.T., Lozano, J.E., Crapiste, G.H., 1989. Thermo-physical properties of clarified apple juice as a function of concentration and temperature. Journal of Food Science 54 (3), 663–668. Dalibard, C., 1999. Overall view on the tradition of tapping palm trees and prospects for animal production. Livestock Research for Rural Development 11 (1), 1–37. Fontana, A.J., Wacker, B., Campbell, C.S., Campbell, G.S., 2001. Simultaneous thermal conductivity, thermal resistivity, and thermal diffusivity measurements of

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