Determination of Fruit Juice Freezing Curves to Optimize Multi-Plate Freeze Concentration

DETERMINATION OF FRUIT JUICE FREEZING CURVES TO OPTIMIZE MULTI-PLATE FREEZE CONCENTRATION PROCESS

Hernández, E1, Auleda, J.Mª1, Raventós, M1

1Dept Enginyeria Agroalimentària i Biotecnologia. Universitat Politècnica de Catalunya (UPC). Avda. del Canal Olímpic s/n 08860 Castelldefels (Barcelona) e-mail: [email protected]

Abstract—The aim of this study was to determine the freezing curves of fruit juices typically concentrated by the Freezing food industry. These results were used to optimize the Unit freeze concentration process performed using a semi- Hydraulic Chilling industrial equipment designed at the food processing pilot System System plant of Universitat Politècnica de Catalunya. Standard industrial apple, pear, orange and peach juices

(10 to 40ºBrix) were used for this research and their level of Fluido a concentrar Fluido frigorígeno sugars, densities, specific heats, thermal conductivities and Electric freezing points were assessed. We found that the freezing System point depended on the concentration and composition of sugars. The appropriate temperatures to perform the freeze concentration process were -1.0ºC and -10.0ºC and depended on the initial concentration of the fruit juices. In a preliminary study, sugar solutions of sucrose, Freezing temperatures decreased quadratically as glucose and fructose were freeze concentrated with this concentration increased. semi-industrial equipment obtaining concentrations of Keywords— freeze concentration, fruit juice, freezing soluble solids from 5 to over 30ºBrix. These point concentrations could be useful for the fruit juice producers [8] [10]. I. INTRODUCTION Before starting the freeze concentration process of fruit juices, a characterization of their physical-chemical Freeze concentration is a new technology for properties (freezing curves, density, viscosity, specific concentrating natural products at freezing points by heat and thermal conductivity) was performed. These means of crystal formation and following separation of parameters were analyzed to allow tracking of the freeze the ice crystals. This technology has been proposed as a concentration process: viable alternative to conventional concentration * Freezing curves at different fruit juice concentrations. techniques, such as evaporation and membrane, The freezing temperature of each solution depends, traditionally used in the food industry. The greatest among other factors, on the type of soluble solids present advantage offered by the use of freeze concentration has in the solution and their concentration. The freezing to do with the low temperatures reached in the process point decreases as the concentration increases until the and the non-existence of a liquid-vapour interface. There Eutectic point is reached. At temperatures below this is no loss of volatiles, making this technique very point, only ice and soluble solid crystals are formed. suitable for the concentration of thermosensitive fluids, Experimental freezing curves can be used to optimize and avoids the problems of frequent membrane processing conditions of a freeze concentration replacements. The freeze concentration equipment equipment: operating conditions and chilling designed at the food processing pilot plant of Universitat requirements are mainly influenced by freezing Politècnica de Catalunya has been described in a temperature [6]. previous paper [10]. The general basic scheme is showed * Density. Solutions with high concentrations of soluble in the following figure: solids show high densities values. This behaviour needs to be considered during the pipes design [1]. * Viscosity is one of the limiting factors in the freeze concentration process. Working temperatures decrease during the process and fluids increase their concentration producing, therefore, and increase in their viscosity value. The hydraulic system of the freeze concentration equipment can present processing problems (enough different concentrations (10, 20, 30, 40 ºBrix) and 20ºC power to impel the fluid) with high viscosity values. was determined. This parameter was measured using Reynolds number is directly proportional to density and four PROTON densimeters calibrated at 20ºC, which indirectly proportional to viscosity of fluids. Moreover, measured the following intervals: 1.000 to 1.050 g/cm3, viscosity also affects the formation of ice crystals on the 1.050 to 1.100 g/cm3, 1.100 to 1.150 g/cm3, 1.150 to plates increasing the difficulty of crystals nucleation and 1.200 g/cm3. growth processes [6].

* Specific heat and thermal conductivity play an F. Specific heat (Cp) and thermal conductivity (k) important role in the heat transmission processes [11]. Choi and Okos mathematical equations [5] were used Factors such as the amount of heat to eliminate and the for liquid foods: freezing estimated time are related with these w C p  C pi  X i (1) characteristics. i v II. MATERIALS AND METHODS k   ki  X i (2) i A. Sample preparation Cp = fruit juice specific heat (kJ/kgºC) The original fruit juices (apple, pear, orange and Cpi = i component specific heat (kJ/kgºC) w peach with 70ºBrix) were provided by local juice Xi = i component massic fraction producers (Lleida, Spain) and obtained by multiple- k = fruit juice thermal conductivity (W/mºC) effect evaporator and mechanical vapour recompression ki = i component thermal conductivity (W/mºC) v (MVR). Fruit juices samples were prepared at different Xi = i component volumic fraction concentrations (10, 20, 30 and 40 ºBrix), by diluting them with distilled water and were kept under refrigeration conditions at 0ºC III. RESULTS AND DISCUSSION

B. Sugar content in fruit juice Fruit juice characterization The levels of fructose, glucose and sucrose in fruit juices The levels of glucose, fructose and sucrose in the fruit were determined using High Performance Liquid juices samples used in this research and determined by Chromatography (HPLC). The analyses were repeated HPLC are shown in Table 1: three times. Table 1. Proportions of sugars in the fruit juices samples

C. Concentration measurement (ºBrix) % Apple Pear Orange Peach ºBrix is a measure of soluble solids present in fruit pulp and juice expressed as percentage of sucrose. Glucose 27 21 45.3 15.7 Soluble solids were measured using a ATAGO Fructose 50.6 71.3 54.7 19.3 refractometer (model DBX-55A). with a precision of ± 0.1 ºBrix and with a measurement range of 0 to 55 ºBrix. Sacarose 22.4 7.7 - 65

D. Freezing point depression (∆Tf). There are different methods to determine the freezing point depression: Differential Scanning Calorimetry Freezing point (∆Tf) (DSC), Hortvet method (British standard for milk) or The freezing points depend on the concentration of cryoscopic termistor [4]. These are expensive techniques soluble solids. To determine them it is necessary to work and require a long training period. For this research, 10 with the freezing curves of each sample, as shown in ml of sample and a freezer at -30ºC were used to obtain Figure 2 in the case of peach juice: the freezing curves of fruit juices at the different concentrations of soluble solids. Temperature value was registered every 15 s using a K Termopar sensor TESTO model 177-T4, with accuracy 0,1ºC, previously calibrated with distilled water and placed in the middle of the tube, connected with a data logger. The experiments were repeated four times

E. Density The density of apple, pear, orange and peach juice at 44,94 -11,4 46,3 -12,8 44,9 -11 46,2 -14,1

4,00 Cooling Crystallization Freezing point values of juices at different

2,00 concentrations of soluble solids are shown in Figure 4.

0,00 The freezing point values of fruit juices decreased as -1,41 concentration of soluble solids increased. The -2,00 differences between fruit juice samples were produced -4,00 Nucleation by the level and type of sugars content. The lowest -6,00 1 11 21 31 41 51 61 freezing point was found in orange juice due to the time (min) presence of glucose and fructose and the absence of sucrose. Instead, peach juice had the highest freezing Fig. 2 Cooling curve of pear juice at 10ºBrix points, probably due to the presence of higher levels of sucrose. Moreover, it is important to note that pear juice The lowest temperature of the freezing curve shows had a greater proportion of fructose than apple juice. the starting process of ice crystals formation The freezing temperature of a solution is mainly (nucleation). It is followed by a temperature increase due affected by the presence of soluble solids with low to latent heat of the phase change. The highest molecular weight (glucose and fructose), as the molar temperature reached at that point corresponds to the fraction of these compounds is higher than the molar freezing point of the sample. It is linked to the ice fraction of a soluble solid with a higher molecular crystals growth phase (crystallization). The difference weight (sucrose). Therefore, when the solutions have between the freezing point and the lowest temperature equal ºBrix value, the freezing point of glucose and reached is called supercooling. fructose is lower than that of sucrose. [10] Freezing point - Apple 24.93ºB The freezing point values of fruit juices at different Termocouple 1 Termocouple 2 Termocouple 3 Termocouple 4

10 concentrations of soluble solids and the freezing curves of sucrose and fructose, found in previous experiments 5 [8], are shown in Figure 4. It is important to mention that the freezing points of fruit juices were found in the 0

) 0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 100,00 C º

( freezing curves of sucrose and fructose. e r u t -5 a r e p apple pear peach orange m e T -10 fructose sucrose Sucrose Fructose

0 10 20 30 40 50 -15 0

-20 -2 Time (min) ) C

º -4 (

Fig. 3 Example of cooling curve of apple juice at 24,93ºBrix r u

t -6 a r e p -8 m e t

n -10 i

The freezing point values of apple, pear, orange and z e e r -12 peach juices at different concentrations of soluble solids F are shown in Table 2. -14

-16 Table 2 Freezing points of fruit juices Soluble solids (ºBrix)

Figure 4. Freezing point values of juices and sugars at different ºBrix Apple ºBrix Pear ºBrix Peach ºBrix Orange concentrations 11 9,4 -1,1 10,3 -1 10,1 -1,2 16,1 -2,2 16,1 -2,1 14,8 -1,7 15 -2,1 20,28 -2,9 20,2 -2,9 19,1 -2,2 19,2 -2,8 These results were compared with the results obtained 24,93 -3,7 24,8 -4,4 24,7 -3,3 24,6 -3,7 by other researchers. Riedel [12] found the following 28,87 -4,8 29,27 -5,1 29,9 -4,3 29,8 -5,1 empirical expression to determine the freezing point 35,26 -6,9 34,7 -7 36,1 -6 35,4 -7 depression (FPD) of fruit and vegetal fruits with massic 39,87 -8,6 42 -10 39,1 -7,6 40,5 -9,1 fractions between 0 to 0.50 (w/w): Fig. 6 Pear juice

3 Riedel Chen Chen Riedel T f  10  X s  50  X s

Xs = soluble solids (kg/kg solution) -14 -12 -10 -8 -6 -4 -2 0 Chen [9] proposed the following mathematical 0 equation to determine the freezing point depression -2 (FPD ) of liquid foods: -4 -6 T f  K  X s /(1 X s  bX s )  M s -8

Xs = soluble solids (kg/kg solution) -10 -12 K = constant -14 b = bound water coefficient Exp. Freezing temperature (ºC) Ms = effective molecular weight (g/mol) Using the results obtained with experiments performed, it was possible to calculate b and Ms values Fig. 7 Peach juice for each fruit juice tested: Riedel Chen Chen Riedel

Apple Pear Orang Peach -15 -10 -5 0 b 0,28 0,37 0,48 0,45 0 Ms 175,5 175,7 186,5 226 -2

Correlation graphics of the fruit juices studied are -4 shown in Figure 5, 6, 7 and 8. The freezing temperatures -6 obtained in this research at different concentrations are situated along the abscissa axe and the freezing points -8 obtained using Riedel and Chen models at the same -10 concentrations are located along the ordinate axe. -12 Exp. Freezing temp. (ºC)

Fig. 5 Apple juice

Fig. 8 Orange juice Riedel Chen Chen Riedel

Riedel Chen Chen Riedel

-16 -14 -12 -10 -8 -6 -4 -2 0 -12 -10 -8 -6 -4 -2 0 0 0 -2 -2 -4 -4 -6 -6 -8 -8 -10 -10 -12 -12 -14 -14 -16 Experimental freezing temperature (ºC) Exp. freezing temp. (ºC)

The results obtained showed an acceptable correlation with the Chen model in all the concentrations tested and of soluble solids, the pear juice showed a lower specific a high correlation with the Riedel model but only using heat value than the apple juice. This behaviour could be concentrations up to 20ºBrix. Moreover, it is important explained by the higher content of sugars that the pear to note that the peach juice also showed a high juice had. Therefore, in the case of pear juice, a lesser correlation with the Riedel model in concentrations up to amount of heat was needed to decrease the temperature 40ºBrix. of the sample. This also means that a higher amount of pear juice could be treated using the freeze concentration D. Density equipment. Density values at 24±0,5ºC of fruit juices at different concentrations of soluble solids are shown in Table 3: ice layer 2 mm ice layer 4 mm ice layer 6 mm ice layer 8 mm ice layer 10 mm compressor

Table 3 Density of fruit juices at different concentrations (g/l) 0 0 2000 4000 6000 8000 Juice/ (ºBrix) Apple Pear Orange Peach -2 10 1040 1038 1038 1040

20 1082 1078 1082 1082 -4 T. E. -5ºC 30 1128 1125 1127 1128

-6 40 1191 1177 1189 1194

Density is another variable that influences the process -8 T. E. -7,5ºC of cryoconcentration, since the higher the density of a solution, the more difficult it is to accomplish the process. As the process of cryoconcentration advances, -10 the soluble solids content increases, density also increases and, in addition, the freezing point will be lower. The density values of juice showed an appropriate -12 correlation with the results obtained by Alvarado and Heat transfer through ice layer(W) Romero [2], Ibarz and Miguelsanz [7], and Ramos and Ibarz [3], giving an equation of the following type: 2   a  bT  cC  d C Figure 9. Heat elimination capacity of ice and compressor where  is density in kg/m3, T is temperature in ºC and C is soluble solid content in ºBrix. The optimal thickness of ice layers formed on the freeze concentration plates needs to be determined because it influences the heat transmission. If the ice E. Specific heat and thermal conductivity layers are thicker than necessary they limit the heat Water, carbohydrates, proteins and ash have been the transmission and reduce effectiveness of the equipment. sample compounds considered to calculate the specific Ice heat transmission capacity also depends on other heat and the thermal conductivity. The specific heat factors such as its thermal conductivity. Figure 9 shows values of apple and pear juices obtained at different compressor performance of our semi-industrial temperatures are shown in Table 4. equipment versus heat transmission values at different ice layer thicknesses. In this case, the optimal ice layer Table 4 thicknesses were between 6 and 10 mm when the Specific heat at 20ºBrix (kJ/kgºC) equipment worked with evaporation temperatures from – T (ºC) Apple juice Pear juice 5 to –7.5ºC. 20 3,808 3,697 10 3,804 3,693 5 3,803 3,692 0 3,802 3,69 -3 3,741 3,632 IV. CONCLUSIONS ▪ The fruit juices studied showed consistent physical- At the same values of temperature and concentration chemical properties with similar results obtained by other researchers ▪ The temperature and concentration showed a clear influence on the properties studied ▪ The determination of the freezing points allowed us to optimize the processing conditions of the freeze concentration equipment to improve ice separation ▪ The heat properties studied (specific heat and thermal conductivity) are important to estimate the chilling requirements and the optimal thickness of the ice layers formed on the freeze concentration plates.

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