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Spray evaporation of liquid foods
LWT - Food Science and Technology 42 (2009) 119–124
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Spray evaporation of liquid foods Shobhana Garg a, b,1, Pankaj Sharma a, 2, S.G. Jayaprakashan a, R. Subramanian a, * a b
Department of Food Engineering, Central Food Technological Research Institute, Mysore 570020, Karnataka, India Department of Human Resource Development, Central Food Technological Research Institute, Mysore 570020, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 January 2008 Received in revised form 30 March 2008 Accepted 7 April 2008
‘Spray evaporation’ based on the principle of adiabatic humidification was examined as an evaporation technique for the concentration of two types of representative liquid foods, namely, fruit juice and milk. The concentration of apple juice could be increased from 10.0 to 13.0 Brix by this technique without application of heat utilizing the humidity potential to an extent of 89%. Employing heated process air during processing increased the amount of water evaporation owing to increased saturation humidity level, enabling higher concentrations to be achieved in apple juice (48 Brix), reconstituted milk (29 Brix) and single toned milk (22 Brix). Feed flow rate had an inverse effect on the final concentration under otherwise similar conditions. Evaporative cooling associated with spray evaporation actually delivered the concentrate at a relatively lower temperature. By manipulating the operating conditions, humidity potential could be utilized to the extent of 55% with heated process air. The spray evaporation technique seemed to have a good potential for the concentration of liquid foods. Ó 2008 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
Keywords: Adiabatic humidification Apple juice Evaporative cooling Milk Spray evaporation
1. Introduction Concentration is a process of partial removal of water from aqueous solutions. Fruit juice concentrates, because of their low water activity, have a higher stability than single-strength juices. In addition, package, storage and shipping costs are remarkably reduced. Milk is another important liquid food and its processing is a forerunner to all the other liquid foods. During the last 60 years, several concentration methods have been developed. Among these, evaporative concentration, freeze concentration and reverse osmosis (RO) have found commercial application (Ramteke, Singh, Rekha, & Eipeson, 1993). Generally concentration of liquid foods is done by thermal evaporation although it is known that it results in a loss of most of the volatile aromatic compounds with a consequent remarkable qualitative decline (Maccarone, Campisi, Lupo, Fallico, & Asmundo, 1996). In response, the industry has developed complex essence recovery, careful process control and blending techniques to produce a good quality concentrate that is acceptable to consumers, but still easily distinguishable from fresh juice owing to ‘cooked’ notes. Commercial freeze concentration systems require remarkably high energy consumption and also result in high loss of total soluble solids. Another limitation is that the achievable * Corresponding author. Tel.: þ91 821 251 3910; fax: þ91 821 251 7233. E-mail address: [email protected] (R. Subramanian). 1 M/s Britannia Industries limited, Britannia Garden, Vimanapura, Airport Road, Bangalore 560017, India. 2 M/s Hindustan Unilever Research Centre, 64, Main Road, Whitefield, Bangalore 560066, India.
concentration (w50 Brix) is lower than the values obtained by thermal evaporation (60–65 Brix). Introduction of membrane concentration processes in industrial processing of fruit juices represents one of the technological answers to the problem of production of high quality juices (Jiao, Cassano, & Drioli, 2004). Although the use of RO presents potential advantages, the final concentration of juices is limited to w25– 30 Brix owing to high osmotic pressure. In comparison membrane distillation (MD) permits higher concentrations (60–65 Brix) to be reached. However, the requirement to heat the feed stream to maintain water vapour pressure gradient can be a cause for significant loss of organic volatiles (Cassano, Conidi, Timpone, D’Avella, & Drioli, 2007). Concentration by osmotic distillation (OD) has the potential advantage in overcoming the drawbacks of RO and MD since OD is not limited by the osmotic pressure and can be carried out at room temperature, but involves high production cost compared to thermal evaporation (Jiao et al., 2004). The technically established concentration techniques described above have specific advantages as well as limitations and the search for newer techniques is in progress. Spray evaporation had been synonymously discussed along with spray drying in an invention on an improved apparatus suitable for both the operations (Damgaard–Iversen, Hansen, & Lund, 1974). Thermodynamics of spray evaporation have been studied to predict the second law efficiency for the process in terms of free stream temperature, Reynolds number and drop size distribution of the spray (Som & Dash, 1993). However, spray evaporation technique has not been explored further critically to exploit its potential commercially. Although it is believed to have been employed by some of the food
0023-6438/$34.00 Ó 2008 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2008.04.001
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Nomenclature Cpa: Cpf: CVLPG: Evap: H1: H2: H3: H4: ma: me: mf: mLPG: Tf: T i: Tp:
Specific heat of air at mean temperature (kJ/kg/ C) Specific heat of feed (kJ/kg/ C) Calorific value of LPG (kJ/kg) Latent heat of vaporization (corresponding to WBT of process air) (kJ/kg) Heat supplied by LPG (kJ) Heat gained by process air (kJ) Heat lost by process air (kJ) Heat gained by feed (kJ) Mass of process air used in each run (kg) Mass of water evaporated from the feed (kg) Mass of feed (kg) Mass of LPG consumed (kg) Temperature of feed ( C) Temperature of inlet air ( C) Temperature of product ( C)
Greek symbols DT1: Temperature difference between atmospheric and heated (evaporation chamber inlet) air ( C) DT2: Temperature difference between inlet and outlet air of evaporation chamber ( C) DT3: Difference between feed temperature and WBT of process air ( C) hBurner: Burner efficiency (%) hOverall: Overall efficiency (%) hThermal: Thermal efficiency (%)
processors, there are no reports in literature on this subject except for a few stray reports on its applications in the field of effluent treatment. In the present study, spray evaporation has been investigated in a spray drier in detail under a wide range of operating conditions that could be manipulated for experimentation with two major representative liquid foods, milk and fruit (apple) juice.
2.2.2. Apparatus A co-current flow spray drier (model: BE 1216; M/s Bowen Engineering Inc., Somerville, USA) fitted with a two-stream nozzle atomizer employing compressed air (3 kg/cm2) as the secondary fluid stream was used in the study. Liquefied petroleum gas (LPG) was the source of heating in the system and flue gas was used for direct heating of the process air. A metering (peristaltic) pump fitted with a variable speed drive was employed for uniform feeding at various desired feed flow rates. 2.2.3. Experimental scheme Small scale trials were conducted with all the three liquid samples. Subsequently, large scale trials were conducted with apple juice and reconstituted milk at chosen operating conditions. The effect of operating conditions, namely, feed flow rate and process air temperature and humidity on the rate of evaporation was studied. Temperatures of process streams and air, relative humidity (RH) of air and Brix values of feed and concentrates were measured using infrared thermometer (model: Center 350 series; M/s HTA Instrumentation, Bangalore, India), dial thermometers fitted in the spray drier, humidity meter (model: Testo 625, M/s Vaiseshika Electric Devices, Ambala Cantt, India) and hand refractometer (M/s Erma Inc., Tokyo, Japan), respectively. The corresponding uncertainties in measurements were 0.5 C, 1 C, 0.1% and 0.2 Brix, respectively. Feed flow rate and quantity of concentrate collected in each run were also recorded. Weight of LPG consumed for each run was calculated based on the duration of the run and the consumption rate obtained from a longer duration run at corresponding process air temperatures. 2.3. Thermal and process calculations Engineering calculations for energy supplied by LPG, heat gained by process air, heat lost by process air and heat gained by feed were carried out in relation to various process efficiency parameters as described below: Heat supplied by LPG: Heat released by combustion of LPG is given by:
H1 ¼ mLPG CVLPG 2. Materials and methods 2.1. Raw material Commercially available ready-to-serve (RTS) apple juice (with no added sugar) (brand: Tropicana), skimmed milk powder (brand: Sagar) and single toned milk (brand: Nandini) were procured from the local market of Mysore.
where CVLPG ¼ 49,400 kJ/kg (www.lpgforyou.com). Heat gained by process air: Heat gained by process air by mixing with the products of combustion of LPG is given by:
H2 ¼ ma Cpa DT1
2.2.1. Feed preparation Apple juice: RTS apple juice (w10 Brix) was directly used in the experiments. Milk: Two distinct types of milk, fat free (reconstituted) and toned milk (3–3.5% fat) were used in the investigation. Reconstituted milk was prepared by dissolving skimmed milk powder in lukewarm water with continuous stirring to obtain a well dispersed sample having a concentration of w10 Brix. The milk thus obtained was filtered through a muslin cloth to remove any undissolved lumps before use. Single toned milk was diluted with distilled water to adjust the concentration to w10 Brix before its use.
(2)
Heat lost by process air: Amount of heat lost by process air in the evaporation chamber is partially used for heating the product while the balance heat is lost due to various other losses. Heat lost by process air in the chamber is given by:
H3 ¼ ma Cpa DT2 2.2. Spray evaporation
(1)
(3)
Heat gained by feed: Heat gained by feed from the heated process air for evaporation is given by:
H4 ¼ Sensible heat þ Latent heat ¼ mf Cpf DT3 þ me Evap
(4)
Thermal efficiency: It is defined as the ratio of energy used for evaporation of water from the feed to the amount of energy lost by process air.
hThermal ¼
Heat utilized for evaporation H ¼ 4 H3 Heat lost by process air
(5)
S. Garg et al. / LWT - Food Science and Technology 42 (2009) 119–124
Overall efficiency: It is defined as the ratio of energy used for evaporation of water from the feed to the amount of energy supplied by LPG.
hOverall ¼
Heat utilized for evaporation H ¼ 4 H1 Heat supplied by LPG
3. Results and discussion 3.1. Spray evaporation – concentration without application of heat Concentration of apple juice as well as milk was attempted without heating the process air. Atmospheric air is not fully saturated and therefore has the ability to pick up additional moisture during an adiabatic humidification process. In such a process, the heat necessary for vaporization is actually drawn from the circulating air. However, if the rate of mass transfer exceeds the heat transfer needed to supply the heat of vaporization, the temperature of the material will drop leading to evaporative cooling (Toledo, 1999). A similar phenomenon was observed when apple juice was sprayed in to a co-current stream of unheated process air (atmospheric air, RH 40%). The concentration of apple juice increased from 10.0 to 13.0 Brix without application of any heat but only by utilizing the humidity potential to the extent of 89% (Table 1). During this evaporation process, the latent heat of evaporation was drawn from the circulating air as well as from the product, leading to drop in their temperatures. Since the heat required for evaporation was supplied both by process air and the product, the thermal efficiency was greater than 100% calculated based only on the heat drawn from the process air. As a result, the resultant concentrate was cooled from 20.0 to 17.5 C (Table 1). In the case of milk, the concentration increased marginally from 10.0 to 11.5 and 11.0 Brix for reconstituted (atmospheric air, RH 54%) and toned (atmospheric air, RH 64%) milk, respectively, but the humidity potential was utilized to a much greater extent of 97% in both the cases (Table 1). These results demonstrated that it is possible to concentrate liquid foods employing this ‘spray evaporation’ technique. Further investigation was carried out by varying the operating conditions to assess the potential of process in terms of achievable concentration and water evaporation capacity in the experimental apparatus.
(6)
Burner efficiency: It is defined as the ratio of heat gained by the process air to the amount of energy supplied by LPG (95–97%).
hBurner ¼
Heat gained by process air H ¼ 2 H1 Heat supplied by LPG
(7)
2.4. Vacuum concentration Vacuum concentration of apple juice was carried out in a laboratory rotary vacuum evaporator (model: Rotavapor RE–111, M/s Buchi, Switzerland). Concentration was monitored by periodical measurement of Brix value and terminated when the desired concentration (w30 Brix) had reached. 2.5. Quality evaluation Quality of spray (large scale run) and vacuum concentrated apple juice samples were comparatively evaluated along with a control (commercial RTS) sample. 2.5.1. pH Measurement pH was measured with a digital glass electrode pH meter (M/s Control Dynamics, Bangalore, India). It was calibrated using buffer solutions of pH 4.0 and 7.0 at room temperature and the uncertainty in measurement was 0.1. 2.5.2. Titratable acidity Total acid content in feed, concentrate and control juice samples was analyzed by titratable acidity method (Ranganna, 2003). Using a pH meter, 20 g of each sample was titrated to pH 8.3 with 0.1 mol/L NaOH. Titratable acidity is expressed as percent malic acid by weight using the following formula:
Malic acid ð%Þ ¼
3.2. Spray evaporation of apple juice with application of heat Concentration of apple juice was carried out at different feed flow rates and by employing heated process air at different temperatures. The impact of natural conditions such as variations in RH of atmospheric air would have a large influence on the amount of water evaporation. This could be overcome by heating the process air to maintain a certain specific humidity potential (difference between the absolute humidity of atmospheric air and its saturation limit) which in turn would maintain the water evaporation capacity in the process equipment. The feed flow rate, another important variable was also assessed considering its likely influence on the performance.
NaOH used ðmLÞ Normality of NaOH 0:067 Weight of sampleðgÞ 100
121
ð8Þ
2.5.3. Colour measurement Colour of juice was measured using Hunter Lab Colour Measuring System (model: LabScan XE, M/s Hunter Associates Laboratory Inc., Reston, USA) at 2 /C angle of illumination. Values were measured in terms of colour difference (DE), lightness (L) and colour (þa: red, a: green, þb: yellow, b: blue). All the experimental runs were carried out in duplicate and the mean values are reported. The values were within 3%.
3.2.1. Effect of process air temperature The performance of spray evaporation at different process air temperatures from 80 to 100 C and at various feed flow rates (110– 140 mL/min) is presented in Table 2. At a constant feed flow rate (140 mL/min), employing heated process air increased the evaporation rate from 32 (unheated air) to 58 mL/min (at 80 C)
Table 1 Concentration without heating process air Sample description
Apple juice Reconstituted milk Toned milk
Feed
Ambient
Flow rate (mL/min)
140 150 180
10.0 10.0 10.0
Brix
Product
Tf ( C)
RH (%)
Ti ( C)
20.0 29.5 19.0
40 54 64
35 28 30
13.0 11.5 11.0
Brix
Tp ( C) 17.5 24.5 17.0
Evaporation rate (mL/min) 32 20 16
Efficiency
Humidity
hThermal (%)
Potential (g/kg)
Utilization (%)
>100 >100 >100
4.5 2.5 2.1
89 97 97
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S. Garg et al. / LWT - Food Science and Technology 42 (2009) 119–124
Table 2 Effect of process conditions on concentration of apple juicea Hot air Product Evaporation Efficiency Humidity rate Ti ( C) Brix hThermal hOverall Potential Utilization Flow rate Brix (mL/min) (%) (%) (g/kg) (%) (mL/min)
Feed
Small scale runs 140 10.0 80 140 10.0 90 140 10.0 100 130 9.6 80 130 10.0 90 120 10.0 80 120 9.8 90 110 10.0 80 Large scale run 130 10.0 a
90
17.0 28.0 47.5 18.0 35.0 23.2 41.0 46.0 39.0
58 90 111 61 93 68 91 86 97
57 77 82 60 79 68 78 85 72
39 50 52 41 51 46 51 58 54
20.2 23.5 27.0 20.2 23.5 20.2 23.5 20.2 23.5
36 48 51 37 49 42 48 53 51
Tf: 30.0 C; Ambient conditions: 32 C, RH 40%.
indicating that higher concentration of the product as well as higher capacity utilization of the equipment could be achieved by heating the process air (Tables 1 and 2). Since spray evaporation involves adiabatic humidification, higher the temperature of process air, higher the level of saturation humidity or in other words humidity potential and so also the amount of water evaporation at a constant RH in the atmospheric air. As the temperature of process air was increased from 80 to 90 C, its humidity potential increased from 20.2 to 23.5 g/kg. Correspondingly, the evaporation rate increased from 58 to 90 mL/min, resulting in an increase in the final concentration of the juice from 17.0 to 28.0 Brix and higher utilization of humidity potential from 36% to 48%. Further increasing the process air temperature to 100 C increased the evaporation rate to a much higher level of 111 mL/min, resulting in a final concentration of as high as 47.5 Brix. Similar trend in the performance was also observed with the increase in process air temperature at two other feed flow rates (120 and 130 mL/min) employed (Table 2). Saturation humidity potential of process air had a direct bearing on the amount of water evaporation rate. Lewis relation applies reasonably well for the air–water system (Treybal, 1981) implying that thermal and mass diffusivities are identical. Further, spray droplets are not converted to powder form during spray evaporation and therefore the process conditions could be compared to constant rate period drying wherein mass transfer rates match with the heat transfer rates. Under such conditions, rate of heat transfer is proportional to the temperature gradient and mass transfer is proportional to the gradient of humidity ratios, between the process air and product surface (Paul Singh & Heldman, 2001). In the present study, higher temperature and saturation humidity of heated process air resulted in higher rate of evaporation. Further, higher process air temperature also resulted in better utilization of energy added in the form of heat as was evident from the greater thermal and overall energy efficiencies, besides better utilization of its humidity potential (Table 2). 3.2.2. Effect of feed flow rate The performance of spray concentration was assessed over a range of feed flow rates from 110 to 140 mL/min at two different process air temperatures, 80 and 90 C. The effect of feed flow rate on the water evaporation rate is shown in Fig. 1. At a constant process air temperature of 80 C, as the feed flow rate was increased from 110 to 140 mL/min, the final concentration of the juice decreased from 46.0 to 17.0 Brix (Table 2). Feed flow rate had an inverse influence on the concentration, which was rather expected, since the amount of water evaporated per unit weight of feed would be lower at higher flow rates and vice versa. However, the evaporation rate did not remain constant and decreased from 86 to 58 mL/min (Fig. 1) despite maintaining the process air conditions at
Evaporation rate (mL/min)
110
90
70 Apple juice - 80°C Reconstituted milk- 80°C Apple juice - 90°C Reconstituted milk- 90°C
50 100
120
140
160
180
200
Feed flow rate (mL/min) Fig. 1. Effect of feed flow rate and process air temperature on the evaporation rate of apple juice and reconstituted milk (ambient: 30 C, RH 40%).
a humidity potential of 20.2 g/kg. As a consequence, the utilization of humidity potential decreased from 53% to 36% and so also the thermal (85–57%) and overall (58–39%) energy efficiencies. There are many variables affecting the spray mean droplet size during pneumatic atomization, namely, air–liquid mass ratio, relative velocity, liquid viscosity and air density. The mass ratio of air–liquid is one of the important variables having a greater influence on the droplet size and an increase in the ratio decreases the droplet size (Masters, 1979). Elversson, Millqvist-Fureby, Alderborn, and Elofsson (2003) demonstrated that nozzle orifice diameter and airflow controlled the droplet size during spray drying of lactose solutions by examining the droplet size using laser diffraction technique. Increasing the atomization airflow rate from 20 to 32 L/min substantially decreased the droplet size from 12 to 6 mm in a linear manner under otherwise similar conditions. In the present study, the variation in evaporation rate at a constant humidity potential revealed the possible role of feed flow rate on the spray nozzle performance. At higher feed flow rates (lower mass ratios of atomization air to feed liquid), much larger droplets are formed leading to lower evaporation rates owing to lower overall surface area of droplets as well as shorter residence time in the chamber. While at lower feed flow rates, fine droplet formation could have led to better overall performance. At a higher process air temperature of 90 C, final concentration of the juice decreased with increase in feed flow rate similar to earlier observations made at 80 C (Table 2). However, the evaporation rate remained nearly constant (90–93 mL/min) when the humidity potential of process air was maintained constant at 23.5 g/kg. The utilization of humidity potential (48–49%) and so also the thermal (77–79%) and overall (50–51%) energy efficiencies remained nearly constant. The results revealed that the performance at 90 C was more stable in terms of evaporation capacity, capable of handling the variations in the droplet formation in the range of feed flow rates studied in the system. 3.2.3. Large scale run Spray evaporation of apple juice on a large scale run (1.8 L/ batch) was carried out after analyzing the results obtained with small scale runs (500 mL/batch). Accordingly, a process air temperature of 90 C was employed for achieving better performance in the process. Although, lower feed flow rate (120 mL/min) resulted in higher concentration of juice (41.0 Brix), 130 mL/min feed flow rate was employed to avoid handling problems in the experimental apparatus posed at higher concentrations owing to stickiness. Under similar operating conditions of feed flow rate (130 mL/min) and humidity potential (23.5 g/kg), the performance in large scale operation showed slight improvement in terms of rate
S. Garg et al. / LWT - Food Science and Technology 42 (2009) 119–124
of evaporation resulting in comparatively higher concentration of juice (39.0 Brix). The overall efficiency showed an increase from 51% to 54% and so also the utilization of humidity potential that increased from 49% to 51% in the large scale run (Table 2). 3.2.4. Quality evaluation of spray concentrated apple juice Increase in pH and decrease in acidity due to processing has been observed in various foods by earlier researchers (Yu & Chiang, 1986). In the present investigation, the acidity of apple juice reduced from 0.27% to 0.23% in reconstituted (with distilled water) spray concentrated juice and 0.21% in vacuum concentrated juice with a corresponding increase in pH from 3.9 to 4.0 and 4.2, respectively. The greater reduction in acidity in vacuum concentration could be attributed to the longer processing time at a comparatively higher temperature leading to greater degradation of malic acid present in the juice. Hunter colour values (L, a, b) of spray and vacuum concentrated apple juice were measured after adjusting the concentration of spray concentrate to w30 Brix. The colour difference (DE) values were nearly equal (82.36 and 82.25) adequately compensating the variations in ‘a’ and ‘b’ values. The spray evaporated sample (L ¼ 8.42) was lighter compared to the vacuum concentrated sample (L ¼ 7.96) which was in fair agreement with the visual appearance (brighter). The spray evaporation process was rather mild owing to shorter residence time (w3.6 s) while duration of the batch vacuum evaporation process was quite longer. Besides, wet bulb temperature (WBT) of heated process air in spray evaporation was only w35 C while the evaporation temperature in vacuum concentration was >60 C. 3.3. Spray evaporation of milk with application of heat Concentration of milk was carried out employing spray evaporation technique with reconstituted milk as well as with single toned milk at different process air temperatures for assessing the performance under different humidity potential and also at various feed flow rates. 3.3.1. Reconstituted milk Effect of process air temperature: The performance of spray evaporation of reconstituted milk at different process air temperatures (80 and 90 C) and feed flow rates (150–180 mL/min) is presented in Table 3. Increase in process air temperature at a constant feed flow rate (180 mL/min), increased the water evaporation rate from 74 to 94 mL/min owing to increase in the saturation humidity potential from 20.2 to 23.5 g/kg, resulting in an increase in the final concentration of milk from 17.0 to 21.0 Brix as well as higher utilization of humidity potential from 46% to 50%. Similar trend was noticed at three other feed flow rates employed in the Table 3 Effect of process conditions on concentration of reconstituted milka Humidity Hot air Product Evaporation Efficiency rate Ti ( C) Brix Flow rate Brix (mL/min) hThermal hOverall Potential Utilization (mL/min) (%) (%) (g/kg) (%) Feed
Small scale runs 180 10.0 80 180 10.0 90 170 10.0 80 170 9.6 90 160 9.8 90 150 10.0 80 150 10.0 90
17.0 21.0 19.0 23.0 25.5 25.0 29.0
74 94 81 99 99 90 98
74 80 80 84 84 89 84
50 52 54 55 55 60 55
20.2 23.5 20.2 23.5 23.5 20.2 23.5
46 50 50 52 52 55 52
Large scale run 150 10.0 90
28.0
96
72
53
23.5
51
a
Tf: 30.0 C; Ambient conditions: 32 C, RH 40%.
123
study. When the process air temperature was increased beyond 90 C, powder formation was noticed indicating transition of operating conditions favourable for drying. Under identical conditions of process air (at constant humidity potential), rate of water evaporation was higher for milk compared to apple juice despite employing higher feed flow rates in the case of milk (Fig. 1). Besides, utilization of humidity potential, and thermal and overall energy efficiencies were higher for milk indicating better utilization of the process air conditions. This could be attributed to the differences in the material characteristics of milk and apple juice. In fact, it is well known that sugary materials are rather difficult to dry, especially in spray driers (Adhikari, Howes, Bhandari, & Truong, 2000). Effect of feed flow rate: The performance of spray evaporation over a range of feed flow rates from 150 to 180 mL/min at two different process air temperatures, 80 and 90 C is presented in Table 3 and Fig. 1. As expected and observed in the case of apple juice, feed flow rate had an inverse influence on the final concentration of milk. Water evaporation rate and other performance parameters (utilization of humidity potential and thermal and overall energy efficiencies) remained nearly constant at 90 C process air temperature. These values reduced with increase in feed flow rate at 80 C despite maintaining the humidity potential of process air constant, similar to the behaviour observed with apple juice. Under identical conditions, the rate of evaporation was higher at higher process air temperature at any corresponding feed flow rate with milk as well as apple juice (Fig. 1). As mentioned earlier, rate of evaporation of milk was higher compared to apple juice at any corresponding process air temperature. Typically, the rate of evaporation during processing milk and apple juice at a constant humidity potential of 23.5 g/kg were in the range of 94–99 and 90– 93 g/kg, respectively (Tables 2 and 3). Large scale run: Spray evaporation of milk on a large scale run (4 L/batch) was carried out after analyzing the results obtained with small scale runs (500 mL/batch) (Table 3). Accordingly, a process air temperature of 90 C and lower feed flow rate (150 mL/ min) were employed for achieving better performance and higher concentration. Under similar operating conditions, the performance in the large scale operation was comparable with that of the small scale run except that the thermal efficiency showed a drop of 12% (Table 3). The final concentration of milk (28.0 Brix) was lower than that of the apple juice (39.0 Brix) which could be directly attributed to the higher feed flow rate employed for reconstituted milk (150 mL/min) compared to apple juice (130 mL/min). However, the other performance factors such as the rate of evaporation, humidity potential utilization, and thermal and overall efficiencies were almost equal to that of apple juice (Tables 2 and 3). 3.3.2. Single toned milk The performance of spray evaporation of single toned milk was assessed under similar conditions of process air temperatures (80– 100 C) and feed flow rates (150–180 mL/min) used for reconstituted milk. The independent effect of process air temperature as well as the feed flow rate under otherwise similar operating conditions showed more or less a similar trend on various performance parameters as was observed with processing apple juice and reconstituted milk (Tables 2–4). The water evaporation rate increased with increase in process air temperature owing to increase in the humidity potential at all the three feed flow rates. Although the process air conditions were maintained, the evaporation rate decreased when the feed flow rate was increased from 150 to 165 mL/min (Fig. 2) which could be attributed to the spray nozzle performance as described earlier. However, further increase in the feed flow rate to 180 mL/min, actually increased the evaporation rate (Fig. 2) owing to low RH of atmospheric air that led to increase in the humidity potential of process air (Table 4). The evaporation rate
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Table 4 Effect of process conditions on concentration of single toned milka Feed
Ambient
Hot air
Product
Flow rate (mL/min)
Tf ( C)
RH (%)
Ti ( C)
180 180 180 180 165 165 165 150 150 150
16.0 17.5 18.0 19.0 21.5 21.0 22.5 23.0 20.0 22.0
64 64 64 64 95 95 95 95 95 95
80 85 90 100 80 90 100 80 85 90
14.0 17.0 19.0 22.0 13.0 17.0 20.0 14.0 16.5 19.5
a
Evaporation rate (mL/min)
Brix
51 74 85 98 38 68 83 43 59 73
Efficiency
Humidity
hThermal
hOverall
(%) 52 68 74 74 38 59 62 43 56 63
(%)
Potential (g/kg)
Utilization (%)
35 46 48 47 26 38 40 29 36 41
19.8 21.3 23.1 26.8 18.0 22.0 25.8 18.0 20.0 22.0
32 43 46 46 26 38 40 30 37 41
Feed concentration: 10.0 Brix; Ambient temperature: 30 C.
during processing reconstituted milk was higher (74 g/kg) compared to toned milk (51 g/kg) under similar conditions of humidity potential (w20 g/kg), process air temperature (80 C) and feed flow rate (180 mL/min). The difference in evaporation rates between the two types of milk reduced at a higher humidity potential (w23 g/kg) (Tables 3 and 4). Higher evaporation rate observed with reconstituted milk could be attributed to the fact that it was completely fat free while the single toned milk contained w3.5% fat. 4. Conclusions The present investigation could be the first scientific report revealing the potential of spray evaporation technique for concentration of liquid foods. By employing this technique, the concentration of apple juice could be increased from 10.0 to 13.0 Brix without the application of heat but utilizing only the humidity potential to a greater extent of 89% (Table 1). Application of heat to process air increased the humidity potential that enabled achieving a concentration as high as 48.0 Brix in apple juice. This approach would be useful in maintaining the water evaporation capacity in the process equipment as the process is sensitive to ambient conditions, especially the RH. The performance assessment under various operating conditions (feed flow rate, and temperature and humidity of drying medium) in a spray drier (existing apparatus close to the requirements) with milk and apple juice provided the nuances of the process and characteristics of feed. Spray evaporation that involves adiabatic saturation and evaporative cooling seemed to have a great potential in the concentration of liquid foods as it could be effectively used for conserving thermal energy requirements and producing superior quality products. The low temperature evaporation corresponding to the WBT of process air would enable preserving the freshness, aroma and nutrition of
Evaporation rate (mL/min)
110
90
70
150 mL/min - Ambient: 30°C, RH 95% 165 mL/min - Ambient: 30°C, RH 95%
50
180 mL/min - Ambient: 30°C, RH 64%
30
75
80
85
90
95
100
105
Process air temperature (°C) Fig. 2. Effect of process air conditions and feed flow rate on the evaporation rate of single toned milk.
the fresh liquid foods. Further, the concentrate is actually delivered at a lower temperature (below WBT) owing to evaporative cooling. Besides, the residence time in the evaporation chamber is in the order of few seconds (w3.6 s) comparable to the best of the thermal evaporators leading to superior quality final product. Higher thermal efficiency with minimal heating and maximal utilization of humidity potential of process air could be achieved by selecting appropriate operating conditions as well as suitable design of spray evaporation system. The design shall also take care of humidity variations in atmospheric air by employing dehumidifiers (adsorbent type) and flexible heating arrangement (including systems exploiting solar radiations), utilizing equipment space effectively with multiple spray arrangements and providing shortest residence time for the product owing to its exposure to oxygen during processing. Acknowledgements Authors thank G. Bammigatti for his help in conducting the experiments, L. Jaganmohan Rao and B. Manohar for their suggestions in analyses and H. Umesh Hebbar for his help in humidity measurements. References Adhikari, B., Howes, T., Bhandari, B. R., & Truong, V. (2000). Experimental studies and kinetics of single drop drying and their relevance in drying of sugar-rich foods. International Journal of Food Properties, 3, 323–351. Cassano, A., Conidi, C., Timpone, R., D’Avella, M., & Drioli, E. (2007). A membranebased process for the clarification and the concentration of cactus pear juice. Journal of Food Engineering, 80, 914–921. Damgaard–Iversen, J., Hansen, O. E., & Lund, B. (1974). Apparatus for evaporating liquid from a solution or suspension. US Patent 3, 828,837. Elversson, J., Millqvist-Fureby, A., Alderborn, G., & Elofsson, U. (2003). Droplet and particle size relationship and shell thickness of inhalable lactose particles during spray drying. Journal of Pharmaceutical Sciences, 92, 900–910. Jiao, B., Cassano, A., & Drioli, E. (2004). Recent advances on membrane processes for the concentration of fruit juices: a review. Journal of Food Engineering, 63, 303–324. Maccarone, E., Campisi, S., Lupo, M. C. C., Fallico, B., & Asmundo, C. N. (1996). Thermal treatments effects on the red orange juice constituents. Industria Bevande, 25, 335–341. Masters, K. (1979). Spray drying handbook (3rd ed.). Newyork: John Wiley & Sons. pp. 208–250. Paul Singh, R., & Heldman, D. R. (2001). Introduction to food engineering (3rd ed.). London: Academic Press. pp. 557–590. Ramteke, R. S., Singh, N. I., Rekha, M. N., & Eipeson, W. E. (1993). Methods for concentration of fruit juices: a critical evaluation. Journal of Food Science and Technology, 30, 391–402. Ranganna, S. (2003). Handbook of analysis and quality control for fruit and vegetable products. New Delhi: Tata McGraw-Hill Publishers. pp. 9–10. Som, S. K., & Dash, S. K. (1993). Thermodynamics of spray evaporation. Journal of Physics D: Applied Physics, 26, 574–584. Toledo, R. T. (1999). Fundamentals of food process engineering (2nd ed.). Gaithersburg, MD: Aspen Publishers. pp. 456–506. Treybal, R. E. (1981). Mass transfer operations (3rd ed.). Singapore: McGraw-Hill. pp. 220–274. Yu, Z. R., & Chiang, B. H. (1986). Passion fruit juice concentration by ultrafiltration and evaporation. Journal of Food Science, 51, 1501–1505.
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LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Spray evaporation of liquid foods Shobhana Garg a, b,1, Pankaj Sharma a, 2, S.G. Jayaprakashan a, R. Subramanian a, * a b
Department of Food Engineering, Central Food Technological Research Institute, Mysore 570020, Karnataka, India Department of Human Resource Development, Central Food Technological Research Institute, Mysore 570020, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 January 2008 Received in revised form 30 March 2008 Accepted 7 April 2008
‘Spray evaporation’ based on the principle of adiabatic humidification was examined as an evaporation technique for the concentration of two types of representative liquid foods, namely, fruit juice and milk. The concentration of apple juice could be increased from 10.0 to 13.0 Brix by this technique without application of heat utilizing the humidity potential to an extent of 89%. Employing heated process air during processing increased the amount of water evaporation owing to increased saturation humidity level, enabling higher concentrations to be achieved in apple juice (48 Brix), reconstituted milk (29 Brix) and single toned milk (22 Brix). Feed flow rate had an inverse effect on the final concentration under otherwise similar conditions. Evaporative cooling associated with spray evaporation actually delivered the concentrate at a relatively lower temperature. By manipulating the operating conditions, humidity potential could be utilized to the extent of 55% with heated process air. The spray evaporation technique seemed to have a good potential for the concentration of liquid foods. Ó 2008 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
Keywords: Adiabatic humidification Apple juice Evaporative cooling Milk Spray evaporation
1. Introduction Concentration is a process of partial removal of water from aqueous solutions. Fruit juice concentrates, because of their low water activity, have a higher stability than single-strength juices. In addition, package, storage and shipping costs are remarkably reduced. Milk is another important liquid food and its processing is a forerunner to all the other liquid foods. During the last 60 years, several concentration methods have been developed. Among these, evaporative concentration, freeze concentration and reverse osmosis (RO) have found commercial application (Ramteke, Singh, Rekha, & Eipeson, 1993). Generally concentration of liquid foods is done by thermal evaporation although it is known that it results in a loss of most of the volatile aromatic compounds with a consequent remarkable qualitative decline (Maccarone, Campisi, Lupo, Fallico, & Asmundo, 1996). In response, the industry has developed complex essence recovery, careful process control and blending techniques to produce a good quality concentrate that is acceptable to consumers, but still easily distinguishable from fresh juice owing to ‘cooked’ notes. Commercial freeze concentration systems require remarkably high energy consumption and also result in high loss of total soluble solids. Another limitation is that the achievable * Corresponding author. Tel.: þ91 821 251 3910; fax: þ91 821 251 7233. E-mail address: [email protected] (R. Subramanian). 1 M/s Britannia Industries limited, Britannia Garden, Vimanapura, Airport Road, Bangalore 560017, India. 2 M/s Hindustan Unilever Research Centre, 64, Main Road, Whitefield, Bangalore 560066, India.
concentration (w50 Brix) is lower than the values obtained by thermal evaporation (60–65 Brix). Introduction of membrane concentration processes in industrial processing of fruit juices represents one of the technological answers to the problem of production of high quality juices (Jiao, Cassano, & Drioli, 2004). Although the use of RO presents potential advantages, the final concentration of juices is limited to w25– 30 Brix owing to high osmotic pressure. In comparison membrane distillation (MD) permits higher concentrations (60–65 Brix) to be reached. However, the requirement to heat the feed stream to maintain water vapour pressure gradient can be a cause for significant loss of organic volatiles (Cassano, Conidi, Timpone, D’Avella, & Drioli, 2007). Concentration by osmotic distillation (OD) has the potential advantage in overcoming the drawbacks of RO and MD since OD is not limited by the osmotic pressure and can be carried out at room temperature, but involves high production cost compared to thermal evaporation (Jiao et al., 2004). The technically established concentration techniques described above have specific advantages as well as limitations and the search for newer techniques is in progress. Spray evaporation had been synonymously discussed along with spray drying in an invention on an improved apparatus suitable for both the operations (Damgaard–Iversen, Hansen, & Lund, 1974). Thermodynamics of spray evaporation have been studied to predict the second law efficiency for the process in terms of free stream temperature, Reynolds number and drop size distribution of the spray (Som & Dash, 1993). However, spray evaporation technique has not been explored further critically to exploit its potential commercially. Although it is believed to have been employed by some of the food
0023-6438/$34.00 Ó 2008 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2008.04.001
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Nomenclature Cpa: Cpf: CVLPG: Evap: H1: H2: H3: H4: ma: me: mf: mLPG: Tf: T i: Tp:
Specific heat of air at mean temperature (kJ/kg/ C) Specific heat of feed (kJ/kg/ C) Calorific value of LPG (kJ/kg) Latent heat of vaporization (corresponding to WBT of process air) (kJ/kg) Heat supplied by LPG (kJ) Heat gained by process air (kJ) Heat lost by process air (kJ) Heat gained by feed (kJ) Mass of process air used in each run (kg) Mass of water evaporated from the feed (kg) Mass of feed (kg) Mass of LPG consumed (kg) Temperature of feed ( C) Temperature of inlet air ( C) Temperature of product ( C)
Greek symbols DT1: Temperature difference between atmospheric and heated (evaporation chamber inlet) air ( C) DT2: Temperature difference between inlet and outlet air of evaporation chamber ( C) DT3: Difference between feed temperature and WBT of process air ( C) hBurner: Burner efficiency (%) hOverall: Overall efficiency (%) hThermal: Thermal efficiency (%)
processors, there are no reports in literature on this subject except for a few stray reports on its applications in the field of effluent treatment. In the present study, spray evaporation has been investigated in a spray drier in detail under a wide range of operating conditions that could be manipulated for experimentation with two major representative liquid foods, milk and fruit (apple) juice.
2.2.2. Apparatus A co-current flow spray drier (model: BE 1216; M/s Bowen Engineering Inc., Somerville, USA) fitted with a two-stream nozzle atomizer employing compressed air (3 kg/cm2) as the secondary fluid stream was used in the study. Liquefied petroleum gas (LPG) was the source of heating in the system and flue gas was used for direct heating of the process air. A metering (peristaltic) pump fitted with a variable speed drive was employed for uniform feeding at various desired feed flow rates. 2.2.3. Experimental scheme Small scale trials were conducted with all the three liquid samples. Subsequently, large scale trials were conducted with apple juice and reconstituted milk at chosen operating conditions. The effect of operating conditions, namely, feed flow rate and process air temperature and humidity on the rate of evaporation was studied. Temperatures of process streams and air, relative humidity (RH) of air and Brix values of feed and concentrates were measured using infrared thermometer (model: Center 350 series; M/s HTA Instrumentation, Bangalore, India), dial thermometers fitted in the spray drier, humidity meter (model: Testo 625, M/s Vaiseshika Electric Devices, Ambala Cantt, India) and hand refractometer (M/s Erma Inc., Tokyo, Japan), respectively. The corresponding uncertainties in measurements were 0.5 C, 1 C, 0.1% and 0.2 Brix, respectively. Feed flow rate and quantity of concentrate collected in each run were also recorded. Weight of LPG consumed for each run was calculated based on the duration of the run and the consumption rate obtained from a longer duration run at corresponding process air temperatures. 2.3. Thermal and process calculations Engineering calculations for energy supplied by LPG, heat gained by process air, heat lost by process air and heat gained by feed were carried out in relation to various process efficiency parameters as described below: Heat supplied by LPG: Heat released by combustion of LPG is given by:
H1 ¼ mLPG CVLPG 2. Materials and methods 2.1. Raw material Commercially available ready-to-serve (RTS) apple juice (with no added sugar) (brand: Tropicana), skimmed milk powder (brand: Sagar) and single toned milk (brand: Nandini) were procured from the local market of Mysore.
where CVLPG ¼ 49,400 kJ/kg (www.lpgforyou.com). Heat gained by process air: Heat gained by process air by mixing with the products of combustion of LPG is given by:
H2 ¼ ma Cpa DT1
2.2.1. Feed preparation Apple juice: RTS apple juice (w10 Brix) was directly used in the experiments. Milk: Two distinct types of milk, fat free (reconstituted) and toned milk (3–3.5% fat) were used in the investigation. Reconstituted milk was prepared by dissolving skimmed milk powder in lukewarm water with continuous stirring to obtain a well dispersed sample having a concentration of w10 Brix. The milk thus obtained was filtered through a muslin cloth to remove any undissolved lumps before use. Single toned milk was diluted with distilled water to adjust the concentration to w10 Brix before its use.
(2)
Heat lost by process air: Amount of heat lost by process air in the evaporation chamber is partially used for heating the product while the balance heat is lost due to various other losses. Heat lost by process air in the chamber is given by:
H3 ¼ ma Cpa DT2 2.2. Spray evaporation
(1)
(3)
Heat gained by feed: Heat gained by feed from the heated process air for evaporation is given by:
H4 ¼ Sensible heat þ Latent heat ¼ mf Cpf DT3 þ me Evap
(4)
Thermal efficiency: It is defined as the ratio of energy used for evaporation of water from the feed to the amount of energy lost by process air.
hThermal ¼
Heat utilized for evaporation H ¼ 4 H3 Heat lost by process air
(5)
S. Garg et al. / LWT - Food Science and Technology 42 (2009) 119–124
Overall efficiency: It is defined as the ratio of energy used for evaporation of water from the feed to the amount of energy supplied by LPG.
hOverall ¼
Heat utilized for evaporation H ¼ 4 H1 Heat supplied by LPG
3. Results and discussion 3.1. Spray evaporation – concentration without application of heat Concentration of apple juice as well as milk was attempted without heating the process air. Atmospheric air is not fully saturated and therefore has the ability to pick up additional moisture during an adiabatic humidification process. In such a process, the heat necessary for vaporization is actually drawn from the circulating air. However, if the rate of mass transfer exceeds the heat transfer needed to supply the heat of vaporization, the temperature of the material will drop leading to evaporative cooling (Toledo, 1999). A similar phenomenon was observed when apple juice was sprayed in to a co-current stream of unheated process air (atmospheric air, RH 40%). The concentration of apple juice increased from 10.0 to 13.0 Brix without application of any heat but only by utilizing the humidity potential to the extent of 89% (Table 1). During this evaporation process, the latent heat of evaporation was drawn from the circulating air as well as from the product, leading to drop in their temperatures. Since the heat required for evaporation was supplied both by process air and the product, the thermal efficiency was greater than 100% calculated based only on the heat drawn from the process air. As a result, the resultant concentrate was cooled from 20.0 to 17.5 C (Table 1). In the case of milk, the concentration increased marginally from 10.0 to 11.5 and 11.0 Brix for reconstituted (atmospheric air, RH 54%) and toned (atmospheric air, RH 64%) milk, respectively, but the humidity potential was utilized to a much greater extent of 97% in both the cases (Table 1). These results demonstrated that it is possible to concentrate liquid foods employing this ‘spray evaporation’ technique. Further investigation was carried out by varying the operating conditions to assess the potential of process in terms of achievable concentration and water evaporation capacity in the experimental apparatus.
(6)
Burner efficiency: It is defined as the ratio of heat gained by the process air to the amount of energy supplied by LPG (95–97%).
hBurner ¼
Heat gained by process air H ¼ 2 H1 Heat supplied by LPG
(7)
2.4. Vacuum concentration Vacuum concentration of apple juice was carried out in a laboratory rotary vacuum evaporator (model: Rotavapor RE–111, M/s Buchi, Switzerland). Concentration was monitored by periodical measurement of Brix value and terminated when the desired concentration (w30 Brix) had reached. 2.5. Quality evaluation Quality of spray (large scale run) and vacuum concentrated apple juice samples were comparatively evaluated along with a control (commercial RTS) sample. 2.5.1. pH Measurement pH was measured with a digital glass electrode pH meter (M/s Control Dynamics, Bangalore, India). It was calibrated using buffer solutions of pH 4.0 and 7.0 at room temperature and the uncertainty in measurement was 0.1. 2.5.2. Titratable acidity Total acid content in feed, concentrate and control juice samples was analyzed by titratable acidity method (Ranganna, 2003). Using a pH meter, 20 g of each sample was titrated to pH 8.3 with 0.1 mol/L NaOH. Titratable acidity is expressed as percent malic acid by weight using the following formula:
Malic acid ð%Þ ¼
3.2. Spray evaporation of apple juice with application of heat Concentration of apple juice was carried out at different feed flow rates and by employing heated process air at different temperatures. The impact of natural conditions such as variations in RH of atmospheric air would have a large influence on the amount of water evaporation. This could be overcome by heating the process air to maintain a certain specific humidity potential (difference between the absolute humidity of atmospheric air and its saturation limit) which in turn would maintain the water evaporation capacity in the process equipment. The feed flow rate, another important variable was also assessed considering its likely influence on the performance.
NaOH used ðmLÞ Normality of NaOH 0:067 Weight of sampleðgÞ 100
121
ð8Þ
2.5.3. Colour measurement Colour of juice was measured using Hunter Lab Colour Measuring System (model: LabScan XE, M/s Hunter Associates Laboratory Inc., Reston, USA) at 2 /C angle of illumination. Values were measured in terms of colour difference (DE), lightness (L) and colour (þa: red, a: green, þb: yellow, b: blue). All the experimental runs were carried out in duplicate and the mean values are reported. The values were within 3%.
3.2.1. Effect of process air temperature The performance of spray evaporation at different process air temperatures from 80 to 100 C and at various feed flow rates (110– 140 mL/min) is presented in Table 2. At a constant feed flow rate (140 mL/min), employing heated process air increased the evaporation rate from 32 (unheated air) to 58 mL/min (at 80 C)
Table 1 Concentration without heating process air Sample description
Apple juice Reconstituted milk Toned milk
Feed
Ambient
Flow rate (mL/min)
140 150 180
10.0 10.0 10.0
Brix
Product
Tf ( C)
RH (%)
Ti ( C)
20.0 29.5 19.0
40 54 64
35 28 30
13.0 11.5 11.0
Brix
Tp ( C) 17.5 24.5 17.0
Evaporation rate (mL/min) 32 20 16
Efficiency
Humidity
hThermal (%)
Potential (g/kg)
Utilization (%)
>100 >100 >100
4.5 2.5 2.1
89 97 97
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S. Garg et al. / LWT - Food Science and Technology 42 (2009) 119–124
Table 2 Effect of process conditions on concentration of apple juicea Hot air Product Evaporation Efficiency Humidity rate Ti ( C) Brix hThermal hOverall Potential Utilization Flow rate Brix (mL/min) (%) (%) (g/kg) (%) (mL/min)
Feed
Small scale runs 140 10.0 80 140 10.0 90 140 10.0 100 130 9.6 80 130 10.0 90 120 10.0 80 120 9.8 90 110 10.0 80 Large scale run 130 10.0 a
90
17.0 28.0 47.5 18.0 35.0 23.2 41.0 46.0 39.0
58 90 111 61 93 68 91 86 97
57 77 82 60 79 68 78 85 72
39 50 52 41 51 46 51 58 54
20.2 23.5 27.0 20.2 23.5 20.2 23.5 20.2 23.5
36 48 51 37 49 42 48 53 51
Tf: 30.0 C; Ambient conditions: 32 C, RH 40%.
indicating that higher concentration of the product as well as higher capacity utilization of the equipment could be achieved by heating the process air (Tables 1 and 2). Since spray evaporation involves adiabatic humidification, higher the temperature of process air, higher the level of saturation humidity or in other words humidity potential and so also the amount of water evaporation at a constant RH in the atmospheric air. As the temperature of process air was increased from 80 to 90 C, its humidity potential increased from 20.2 to 23.5 g/kg. Correspondingly, the evaporation rate increased from 58 to 90 mL/min, resulting in an increase in the final concentration of the juice from 17.0 to 28.0 Brix and higher utilization of humidity potential from 36% to 48%. Further increasing the process air temperature to 100 C increased the evaporation rate to a much higher level of 111 mL/min, resulting in a final concentration of as high as 47.5 Brix. Similar trend in the performance was also observed with the increase in process air temperature at two other feed flow rates (120 and 130 mL/min) employed (Table 2). Saturation humidity potential of process air had a direct bearing on the amount of water evaporation rate. Lewis relation applies reasonably well for the air–water system (Treybal, 1981) implying that thermal and mass diffusivities are identical. Further, spray droplets are not converted to powder form during spray evaporation and therefore the process conditions could be compared to constant rate period drying wherein mass transfer rates match with the heat transfer rates. Under such conditions, rate of heat transfer is proportional to the temperature gradient and mass transfer is proportional to the gradient of humidity ratios, between the process air and product surface (Paul Singh & Heldman, 2001). In the present study, higher temperature and saturation humidity of heated process air resulted in higher rate of evaporation. Further, higher process air temperature also resulted in better utilization of energy added in the form of heat as was evident from the greater thermal and overall energy efficiencies, besides better utilization of its humidity potential (Table 2). 3.2.2. Effect of feed flow rate The performance of spray concentration was assessed over a range of feed flow rates from 110 to 140 mL/min at two different process air temperatures, 80 and 90 C. The effect of feed flow rate on the water evaporation rate is shown in Fig. 1. At a constant process air temperature of 80 C, as the feed flow rate was increased from 110 to 140 mL/min, the final concentration of the juice decreased from 46.0 to 17.0 Brix (Table 2). Feed flow rate had an inverse influence on the concentration, which was rather expected, since the amount of water evaporated per unit weight of feed would be lower at higher flow rates and vice versa. However, the evaporation rate did not remain constant and decreased from 86 to 58 mL/min (Fig. 1) despite maintaining the process air conditions at
Evaporation rate (mL/min)
110
90
70 Apple juice - 80°C Reconstituted milk- 80°C Apple juice - 90°C Reconstituted milk- 90°C
50 100
120
140
160
180
200
Feed flow rate (mL/min) Fig. 1. Effect of feed flow rate and process air temperature on the evaporation rate of apple juice and reconstituted milk (ambient: 30 C, RH 40%).
a humidity potential of 20.2 g/kg. As a consequence, the utilization of humidity potential decreased from 53% to 36% and so also the thermal (85–57%) and overall (58–39%) energy efficiencies. There are many variables affecting the spray mean droplet size during pneumatic atomization, namely, air–liquid mass ratio, relative velocity, liquid viscosity and air density. The mass ratio of air–liquid is one of the important variables having a greater influence on the droplet size and an increase in the ratio decreases the droplet size (Masters, 1979). Elversson, Millqvist-Fureby, Alderborn, and Elofsson (2003) demonstrated that nozzle orifice diameter and airflow controlled the droplet size during spray drying of lactose solutions by examining the droplet size using laser diffraction technique. Increasing the atomization airflow rate from 20 to 32 L/min substantially decreased the droplet size from 12 to 6 mm in a linear manner under otherwise similar conditions. In the present study, the variation in evaporation rate at a constant humidity potential revealed the possible role of feed flow rate on the spray nozzle performance. At higher feed flow rates (lower mass ratios of atomization air to feed liquid), much larger droplets are formed leading to lower evaporation rates owing to lower overall surface area of droplets as well as shorter residence time in the chamber. While at lower feed flow rates, fine droplet formation could have led to better overall performance. At a higher process air temperature of 90 C, final concentration of the juice decreased with increase in feed flow rate similar to earlier observations made at 80 C (Table 2). However, the evaporation rate remained nearly constant (90–93 mL/min) when the humidity potential of process air was maintained constant at 23.5 g/kg. The utilization of humidity potential (48–49%) and so also the thermal (77–79%) and overall (50–51%) energy efficiencies remained nearly constant. The results revealed that the performance at 90 C was more stable in terms of evaporation capacity, capable of handling the variations in the droplet formation in the range of feed flow rates studied in the system. 3.2.3. Large scale run Spray evaporation of apple juice on a large scale run (1.8 L/ batch) was carried out after analyzing the results obtained with small scale runs (500 mL/batch). Accordingly, a process air temperature of 90 C was employed for achieving better performance in the process. Although, lower feed flow rate (120 mL/min) resulted in higher concentration of juice (41.0 Brix), 130 mL/min feed flow rate was employed to avoid handling problems in the experimental apparatus posed at higher concentrations owing to stickiness. Under similar operating conditions of feed flow rate (130 mL/min) and humidity potential (23.5 g/kg), the performance in large scale operation showed slight improvement in terms of rate
S. Garg et al. / LWT - Food Science and Technology 42 (2009) 119–124
of evaporation resulting in comparatively higher concentration of juice (39.0 Brix). The overall efficiency showed an increase from 51% to 54% and so also the utilization of humidity potential that increased from 49% to 51% in the large scale run (Table 2). 3.2.4. Quality evaluation of spray concentrated apple juice Increase in pH and decrease in acidity due to processing has been observed in various foods by earlier researchers (Yu & Chiang, 1986). In the present investigation, the acidity of apple juice reduced from 0.27% to 0.23% in reconstituted (with distilled water) spray concentrated juice and 0.21% in vacuum concentrated juice with a corresponding increase in pH from 3.9 to 4.0 and 4.2, respectively. The greater reduction in acidity in vacuum concentration could be attributed to the longer processing time at a comparatively higher temperature leading to greater degradation of malic acid present in the juice. Hunter colour values (L, a, b) of spray and vacuum concentrated apple juice were measured after adjusting the concentration of spray concentrate to w30 Brix. The colour difference (DE) values were nearly equal (82.36 and 82.25) adequately compensating the variations in ‘a’ and ‘b’ values. The spray evaporated sample (L ¼ 8.42) was lighter compared to the vacuum concentrated sample (L ¼ 7.96) which was in fair agreement with the visual appearance (brighter). The spray evaporation process was rather mild owing to shorter residence time (w3.6 s) while duration of the batch vacuum evaporation process was quite longer. Besides, wet bulb temperature (WBT) of heated process air in spray evaporation was only w35 C while the evaporation temperature in vacuum concentration was >60 C. 3.3. Spray evaporation of milk with application of heat Concentration of milk was carried out employing spray evaporation technique with reconstituted milk as well as with single toned milk at different process air temperatures for assessing the performance under different humidity potential and also at various feed flow rates. 3.3.1. Reconstituted milk Effect of process air temperature: The performance of spray evaporation of reconstituted milk at different process air temperatures (80 and 90 C) and feed flow rates (150–180 mL/min) is presented in Table 3. Increase in process air temperature at a constant feed flow rate (180 mL/min), increased the water evaporation rate from 74 to 94 mL/min owing to increase in the saturation humidity potential from 20.2 to 23.5 g/kg, resulting in an increase in the final concentration of milk from 17.0 to 21.0 Brix as well as higher utilization of humidity potential from 46% to 50%. Similar trend was noticed at three other feed flow rates employed in the Table 3 Effect of process conditions on concentration of reconstituted milka Humidity Hot air Product Evaporation Efficiency rate Ti ( C) Brix Flow rate Brix (mL/min) hThermal hOverall Potential Utilization (mL/min) (%) (%) (g/kg) (%) Feed
Small scale runs 180 10.0 80 180 10.0 90 170 10.0 80 170 9.6 90 160 9.8 90 150 10.0 80 150 10.0 90
17.0 21.0 19.0 23.0 25.5 25.0 29.0
74 94 81 99 99 90 98
74 80 80 84 84 89 84
50 52 54 55 55 60 55
20.2 23.5 20.2 23.5 23.5 20.2 23.5
46 50 50 52 52 55 52
Large scale run 150 10.0 90
28.0
96
72
53
23.5
51
a
Tf: 30.0 C; Ambient conditions: 32 C, RH 40%.
123
study. When the process air temperature was increased beyond 90 C, powder formation was noticed indicating transition of operating conditions favourable for drying. Under identical conditions of process air (at constant humidity potential), rate of water evaporation was higher for milk compared to apple juice despite employing higher feed flow rates in the case of milk (Fig. 1). Besides, utilization of humidity potential, and thermal and overall energy efficiencies were higher for milk indicating better utilization of the process air conditions. This could be attributed to the differences in the material characteristics of milk and apple juice. In fact, it is well known that sugary materials are rather difficult to dry, especially in spray driers (Adhikari, Howes, Bhandari, & Truong, 2000). Effect of feed flow rate: The performance of spray evaporation over a range of feed flow rates from 150 to 180 mL/min at two different process air temperatures, 80 and 90 C is presented in Table 3 and Fig. 1. As expected and observed in the case of apple juice, feed flow rate had an inverse influence on the final concentration of milk. Water evaporation rate and other performance parameters (utilization of humidity potential and thermal and overall energy efficiencies) remained nearly constant at 90 C process air temperature. These values reduced with increase in feed flow rate at 80 C despite maintaining the humidity potential of process air constant, similar to the behaviour observed with apple juice. Under identical conditions, the rate of evaporation was higher at higher process air temperature at any corresponding feed flow rate with milk as well as apple juice (Fig. 1). As mentioned earlier, rate of evaporation of milk was higher compared to apple juice at any corresponding process air temperature. Typically, the rate of evaporation during processing milk and apple juice at a constant humidity potential of 23.5 g/kg were in the range of 94–99 and 90– 93 g/kg, respectively (Tables 2 and 3). Large scale run: Spray evaporation of milk on a large scale run (4 L/batch) was carried out after analyzing the results obtained with small scale runs (500 mL/batch) (Table 3). Accordingly, a process air temperature of 90 C and lower feed flow rate (150 mL/ min) were employed for achieving better performance and higher concentration. Under similar operating conditions, the performance in the large scale operation was comparable with that of the small scale run except that the thermal efficiency showed a drop of 12% (Table 3). The final concentration of milk (28.0 Brix) was lower than that of the apple juice (39.0 Brix) which could be directly attributed to the higher feed flow rate employed for reconstituted milk (150 mL/min) compared to apple juice (130 mL/min). However, the other performance factors such as the rate of evaporation, humidity potential utilization, and thermal and overall efficiencies were almost equal to that of apple juice (Tables 2 and 3). 3.3.2. Single toned milk The performance of spray evaporation of single toned milk was assessed under similar conditions of process air temperatures (80– 100 C) and feed flow rates (150–180 mL/min) used for reconstituted milk. The independent effect of process air temperature as well as the feed flow rate under otherwise similar operating conditions showed more or less a similar trend on various performance parameters as was observed with processing apple juice and reconstituted milk (Tables 2–4). The water evaporation rate increased with increase in process air temperature owing to increase in the humidity potential at all the three feed flow rates. Although the process air conditions were maintained, the evaporation rate decreased when the feed flow rate was increased from 150 to 165 mL/min (Fig. 2) which could be attributed to the spray nozzle performance as described earlier. However, further increase in the feed flow rate to 180 mL/min, actually increased the evaporation rate (Fig. 2) owing to low RH of atmospheric air that led to increase in the humidity potential of process air (Table 4). The evaporation rate
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Table 4 Effect of process conditions on concentration of single toned milka Feed
Ambient
Hot air
Product
Flow rate (mL/min)
Tf ( C)
RH (%)
Ti ( C)
180 180 180 180 165 165 165 150 150 150
16.0 17.5 18.0 19.0 21.5 21.0 22.5 23.0 20.0 22.0
64 64 64 64 95 95 95 95 95 95
80 85 90 100 80 90 100 80 85 90
14.0 17.0 19.0 22.0 13.0 17.0 20.0 14.0 16.5 19.5
a
Evaporation rate (mL/min)
Brix
51 74 85 98 38 68 83 43 59 73
Efficiency
Humidity
hThermal
hOverall
(%) 52 68 74 74 38 59 62 43 56 63
(%)
Potential (g/kg)
Utilization (%)
35 46 48 47 26 38 40 29 36 41
19.8 21.3 23.1 26.8 18.0 22.0 25.8 18.0 20.0 22.0
32 43 46 46 26 38 40 30 37 41
Feed concentration: 10.0 Brix; Ambient temperature: 30 C.
during processing reconstituted milk was higher (74 g/kg) compared to toned milk (51 g/kg) under similar conditions of humidity potential (w20 g/kg), process air temperature (80 C) and feed flow rate (180 mL/min). The difference in evaporation rates between the two types of milk reduced at a higher humidity potential (w23 g/kg) (Tables 3 and 4). Higher evaporation rate observed with reconstituted milk could be attributed to the fact that it was completely fat free while the single toned milk contained w3.5% fat. 4. Conclusions The present investigation could be the first scientific report revealing the potential of spray evaporation technique for concentration of liquid foods. By employing this technique, the concentration of apple juice could be increased from 10.0 to 13.0 Brix without the application of heat but utilizing only the humidity potential to a greater extent of 89% (Table 1). Application of heat to process air increased the humidity potential that enabled achieving a concentration as high as 48.0 Brix in apple juice. This approach would be useful in maintaining the water evaporation capacity in the process equipment as the process is sensitive to ambient conditions, especially the RH. The performance assessment under various operating conditions (feed flow rate, and temperature and humidity of drying medium) in a spray drier (existing apparatus close to the requirements) with milk and apple juice provided the nuances of the process and characteristics of feed. Spray evaporation that involves adiabatic saturation and evaporative cooling seemed to have a great potential in the concentration of liquid foods as it could be effectively used for conserving thermal energy requirements and producing superior quality products. The low temperature evaporation corresponding to the WBT of process air would enable preserving the freshness, aroma and nutrition of
Evaporation rate (mL/min)
110
90
70
150 mL/min - Ambient: 30°C, RH 95% 165 mL/min - Ambient: 30°C, RH 95%
50
180 mL/min - Ambient: 30°C, RH 64%
30
75
80
85
90
95
100
105
Process air temperature (°C) Fig. 2. Effect of process air conditions and feed flow rate on the evaporation rate of single toned milk.
the fresh liquid foods. Further, the concentrate is actually delivered at a lower temperature (below WBT) owing to evaporative cooling. Besides, the residence time in the evaporation chamber is in the order of few seconds (w3.6 s) comparable to the best of the thermal evaporators leading to superior quality final product. Higher thermal efficiency with minimal heating and maximal utilization of humidity potential of process air could be achieved by selecting appropriate operating conditions as well as suitable design of spray evaporation system. The design shall also take care of humidity variations in atmospheric air by employing dehumidifiers (adsorbent type) and flexible heating arrangement (including systems exploiting solar radiations), utilizing equipment space effectively with multiple spray arrangements and providing shortest residence time for the product owing to its exposure to oxygen during processing. Acknowledgements Authors thank G. Bammigatti for his help in conducting the experiments, L. Jaganmohan Rao and B. Manohar for their suggestions in analyses and H. Umesh Hebbar for his help in humidity measurements. References Adhikari, B., Howes, T., Bhandari, B. R., & Truong, V. (2000). Experimental studies and kinetics of single drop drying and their relevance in drying of sugar-rich foods. International Journal of Food Properties, 3, 323–351. Cassano, A., Conidi, C., Timpone, R., D’Avella, M., & Drioli, E. (2007). A membranebased process for the clarification and the concentration of cactus pear juice. Journal of Food Engineering, 80, 914–921. Damgaard–Iversen, J., Hansen, O. E., & Lund, B. (1974). Apparatus for evaporating liquid from a solution or suspension. US Patent 3, 828,837. Elversson, J., Millqvist-Fureby, A., Alderborn, G., & Elofsson, U. (2003). Droplet and particle size relationship and shell thickness of inhalable lactose particles during spray drying. Journal of Pharmaceutical Sciences, 92, 900–910. Jiao, B., Cassano, A., & Drioli, E. (2004). Recent advances on membrane processes for the concentration of fruit juices: a review. Journal of Food Engineering, 63, 303–324. Maccarone, E., Campisi, S., Lupo, M. C. C., Fallico, B., & Asmundo, C. N. (1996). Thermal treatments effects on the red orange juice constituents. Industria Bevande, 25, 335–341. Masters, K. (1979). Spray drying handbook (3rd ed.). Newyork: John Wiley & Sons. pp. 208–250. Paul Singh, R., & Heldman, D. R. (2001). Introduction to food engineering (3rd ed.). London: Academic Press. pp. 557–590. Ramteke, R. S., Singh, N. I., Rekha, M. N., & Eipeson, W. E. (1993). Methods for concentration of fruit juices: a critical evaluation. Journal of Food Science and Technology, 30, 391–402. Ranganna, S. (2003). Handbook of analysis and quality control for fruit and vegetable products. New Delhi: Tata McGraw-Hill Publishers. pp. 9–10. Som, S. K., & Dash, S. K. (1993). Thermodynamics of spray evaporation. Journal of Physics D: Applied Physics, 26, 574–584. Toledo, R. T. (1999). Fundamentals of food process engineering (2nd ed.). Gaithersburg, MD: Aspen Publishers. pp. 456–506. Treybal, R. E. (1981). Mass transfer operations (3rd ed.). Singapore: McGraw-Hill. pp. 220–274. Yu, Z. R., & Chiang, B. H. (1986). Passion fruit juice concentration by ultrafiltration and evaporation. Journal of Food Science, 51, 1501–1505.