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Decanter centrifugation of apple mash: effect of centrifuge parameters, apple variety and apple storage
Food Research International 25 (1992) 125-l 30
Decanter centrifugation of apple mash: effect of centrifuge parameters, apple variety and apple storage T. Beveridge, J. E. Harrison & R. R. Gayton Agriculture Canada, Research Station, Summer/and, British Columbia, Canada VOH IZO
Enzyme treated (0.02% Pectinex Ultra SP, 40°C. 334 h) apple mash gave juice yields of 80-90% with suspended solids ~3% when decanter centrifuged. Centrifugal conditions were a 230 mm bowl, 5696 rev/min, pool depth of 39 mm and mash temperature of 40°C. Capacities were 0.33-0.39 kg/s (2600-3100 lb/h) under appropriate conditions for Red Delicious, Spartan, McIntosh and Golden Delicious apples. Storage in air at 0-1°C for 8 months did not affect machine capacity with Red Delicious and McIntosh varieties but storage of Spartan and Golden Delicious varieties beyond 34 months resulted in markedly reduced (half or less) machine capacities. The effects of varying decanter centrifuge settings and problems associated with decanter separation of apple juice are documented and discussed. Kqvwords: apple juice extraction,
centrifuge, decanter.
yields (Rao et al., 1975a). Adding press-aid may offer some improvement in yield (Rao et al., 1975b) but this procedure would defeat at least one potential advantage (i.e. cost savings) of the decanter and should be avoided if possible. Different apple varieties behave differently in the machine and enzyme digestion of fruit is a necessary prerequisite to juice separation by decanter (Beveridge et al., 1988). This paper is an attempt to further define the conditions necessary for successful juice extraction from apples by the decanter centrifuge.
INTRODUCTION
Decanter centrifuges offer an alternative juice extraction method to presses and have the potential to produce apple juice with yields of 8590% (w/w) without press-aid (Beveridge et al., 1988). Furthermore, the machine is capable of processing a wide variety of fruit and vegetable feedstocks including fruit mashes such as ripe and/or heated pears which are very hard to press using conventional methods. This makes the production of specialty juices from fruit mashes thermally treated to control polyphenol oxidase possible (Beveridge et al., 1986). These properties make the decanter potentially valuable to producers utilizing numerous feedstocks to produce a wide range of high quality juice products. However, published information on the use of the decanter centrifuge is limited, as indicated by a recent review (Beveridge et al., 1988), and little of this literature represents a systematic examination of the machine’s capabilities. Decanter capacities for non-enzyme treated apple mash is increased by a deep fluid pool in the centrifuge and by increased differential scroll speeds. But increased differential scroll speeds also reduce
MATERIALS
AND METHODS
Apples were harvested from Summerland Research Station orchards at commercial harvest dates in 1987 and 1988, respectively, and stored in air at 0-1°C. Apples were removed from storage immediately prior to processing and a random sample was obtained for determination of starch distribution indices (Lau, 1985) and pressure (Looney et al., 1981). Apples were hammer-milled to pass 9 mm holes then pumped (Waukesha MO. 134 Sanitary Pump, Waukesha Div., Abex Corp., Waukesha, WI, USA) through a scraped surface
0 1992 Canadian Federal Government 125
126
T. Beveridge, J. E. Harrison, R. R. Gayton
heat exchanger (Cherry-Burrell MO. 324 SWB Thermutator, Cherry-Burrell Corp., Cedar Rapids, IA, USA) for either temperature adjustment to 40°C or blanching at 90°C (Beveridge et al., 1986). Enzyme digestion of apple mash was accomplished at 40°C in 50 kg batches. Pectinex Ultra SP (0.02% (w/v); Nova Industries AS, Copenhagen, Denmark) was thoroughly incorporated into the mash by mixing 3 min at speed 2 in a Hobart mixer (Hobart, MO. H600, Hobart Canada Inc., Don Mills, ON). The enzyme-treated mash was incubated at 40°C for 3 h then pumped (Waukesha) through a decanter centrifuge (AlfaLava1 MO. NX309$31G, Alfa-Lava1 Ltd, Scarborough, ON) at flow rates between 0.066 and 0833 kg/s for the production of apple juice. Mash feed rate and juice yields were calculated as: feed rate = input mash (kg)/run time (s) gross yield = (output juice (kg)/ input mash (kg)) X 100 net yield = gross yield - suspended solids
(1) (2) (3)
Suspended solids were measured in triplicate by centrifuging 40 ml samples of juice 10 min at 3000g in a Sorval RC-5 centrifuge (DuPont Instruments, Newton, CT, USA). Suspended solids were expressed as weight percent of the total juice weight. Centrifuge parameters were adjusted in stepwise fashion to maximize yield and throughput with minimum suspended solids in Red Delicious output juice. Scroll differential was maintained at 14 rev/min throughout all experiments. Centrifuge rotational speeds were adjusted between 4675 and 5696 rev/ min while maintaining the pool depth at 39 mm. The pool depth was then adjusted between 27 and 45 mm while maintaining the bowl rotational speed at 5696 rev/min. Temperature effects were analyzed by pasteurizing the mash (Beveridge et al., 1986), immediately after enzyme incubation, to destroy enzyme activity. The mashes were then stored in 40, 30 and 5°C rooms overnight, mixed, weighed, temperature measured, then passed through the centrifuge (5696 rev/min; 39 mm pool depth). All varietal runs utilized a centrifuge bowl speed of 5696 rev/min, a pool depth of 39 mm and a mash feed temperature of 40°C. The data obtained in the 1988/89 season were analyzed by the SAS GLM procedure (SAS Institute Inc., Cary, NC, Version 6.0), and multiple t-tests to determine the effect of apple variety and storage time on machine capacity.
RESULTS AND DISCUSSION The slowest scroll differential speeds available on this decanter were 14 rev/mm and 28 rev/min. Since scroll differential speeds beyond 20 rev/min markedly degrade juice yield (Rao et al., 1975a) and no evidence for scroll overloading was reported at 10 rev/min, it was decided to set scroll differentials at 14 rev/min. The important parameter for most purposes is machine capacity and a measure of this parameter is obtained by increasingly loading the machine (increasing mash flow rate) until a marked increase or break-through of suspended solids in the juice stream is noted (Beveridge et al., 1988). Increasing rotational speed markedly increased capacity (Fig. 1) as would be expected. If 3% suspended solids is arbitrarily chosen as a reasonable figure for comparison with other juice extraction systems then the capacities were about 0.16, 0.23 and 0.36 kg/s at 4675, 5356 and 5696 rev/min respectively with a pool depth of 39 mm. Small increases in drum rotational speed resulted in large capacity increases. Increasing pool depth also increased machine capacity (Fig. 2). Again taking 3% suspended solids for comparison purposes machine capacities at 5696 rev/min are 0.18, 0.30 and 0.36 kg/s for pool depths of 27, 36 and 39 mm, respectively. Runs utilizing a 45 mm pool depth were abandoned as liquid spilled out the solids ejection port, posing an obvious overflow problem. These results compare well with those of Rao et al. (1975a). Using the highest attainable rotational speed and a pool depth of 39 mm, the centrifuge capacity for juice with less than 3% suspended solids is about 0.36 kg/s (2800 lb/h), about three times the capacity (900 lb/h) reported by Rao et al. (1975a) but still lower than reported capacities
Fig. 1. Effect of decanter drum rotational speed on machine capacity and yield utilizing Red Delicious apples milled and enzyme treated at 40°C. Centrifuge parameters were a 230 mm drum, a 39 mm pool depth and a scroll differential of 14 rev/min.
Decanter centrljiigation of apple mash
T3
25
_
POOL DEPTH
(mm)
127
0f :: 19
;*o-
FEED RATE &a’s)
Fig. 2. Effect of decanter pool depth on machine capacity and yield using 40°C enzyme treated Red Delicious apple mash. Centrifuge parameters were a drum rotational speed of 5696 rev/min and a scroll differential of 14 rev/min.
for most small presses (Beveridge et al., 1988). However, most net yields range from 80 to 90% which were comparable to or better than most presses and were obtained without the need of press-aid. On occasion, during normal operation of the decanter, dry, sticky ejecta clung to the centrifuge bowl shroud (Fig. 3(A)) accumulating to the point where the solids exit ports were totally blocked for most of the diameter of the rotating bowl. This caused a complete blockage of the centrifuge scroll (Fig. 3(B)) and spillage of large quantities of suspended solids into the juice stream. The onset of this condition was preceded by a high pitched whine which occurred presumably as the solids build-up contacted the rotating bowl. This may explain the intermittent appearance of suspended solids in the juice stream reported by Moyer (1984). This condition was occasionally self-clearing but could be avoided by machine design since the condition was not experienced during preliminary work with a (230 mm) 9-in diameter Mercobowl decanter (Dorr-Oliver Mercobowl, Hazelton, PA, USA) equipped with fin-like structures in the solids ejection region of the bowl. In the present experiment this problem was partially overcome after the 1987 season by mounting water jets under the shroud to continuously wash away the collecting ejecta. Effect of temperature Centrifugation temperature had a marked effect on suspended solids (Table l), which decreased notably as the temperature increased to 40°C. Gross yield values remained above 86% over a
(b) Fig. 3. Solids build-up in the solids ejection region of the decanter. (A) bowl shroud rotated upward and away to show extent of ejection port blockage; (B) centrifuge scroll completely blocked by retained ejecta. Rope was used to lift scroll from centrifuge bowl.
Table 1. Effect of masha temperature during centrifugation
(“C)
Feed rate (kg/s)
Grass yield (‘% w/w)
Net yield (% w/w)
16 30 40 40 42 42
0.378 0.363 0.355 0.354 0.358 0.353
90.7 91.1 91.4 89.9 89.8 86.3
74.9 84.9 88.8 85.7 87.3 84.5
Temperature
Suspended solids (‘%!w/w) 15.8 6.15 2.57 2.83 2.46 1.83
a Red Delicious apples milled through 9 mm screens, treated with 0.2% Pectinex ultra SP 4 h at 40°C. pasteurized, then temperature equilibrated overnight.
T. Beveridge, J. E. Harrison, R. R. Gayton
128
temperature range of 1642°C. While gross yield remained constant with temperature, net yields improved with temperature increases due to the decrease in suspended solids. Effect of variety and storage Gross juice yields (Fig. 4(A)) for all varieties were between 80 and 90% (w/w). Net yields (Fig. 4(B)) of Red Delicious and McIntosh varieties were consistently above 80%. Data representing net yields below 80% were predominantly from Golden Delicious and Spartan apples processed after 4 months storage. This drop in machine capacity was primarily due to an increase in suspended solids in the juices. The natural logarithm of suspended solids was linearly related to feed rate (Fig. 5) for all varieties and run dates (r2 = 0.74). The log regression slopes of the process runs of Golden Delicious apples in
(4
April and of the Spartan apples in February and April were significantly different (p < 0.05) from those of all Red Delicious, McIntosh and November runs of Golden Delicious and Spartan, but were not significantly different from each other. The regression of the February runs of Golden Delicious mash was significantly higher than those of the Red Delicious runs. The Red Delicious, McIntosh and November Golden Delicious and Spartan data were pooled to obtain the curve drawn on Fig. 5 0, = e@w7+2.91~;r2 = 0.64). This line allows one to compare the data in the four graphs more easily. With both Red Delicious and McIntosh apples, decanter capacity was approximately 0.36 kg/s (3100 lb/h) (Fig. 5(A and B)) as was the initial capacity determination (Figs 1 and 2). For both cultivars, refrigerated storage in air (O-l°C) had no effect on juice extraction over a 9 month storage period (Ott-June). Spartan apples (Fig. 5(C)) gave good centrifuge capacity early in the season (-0.33 kg/s in November), however, with storage the weight of suspended solids increased, decreasing capacity to approximately 0.17 kg/s by February. Golden Delicious apples (Fig. 5(D)) gave results somewhat similar to Spartans. Centrifuge capacity was 0.36 kg/s until February, after which the suspended solids in the juice rose at feed rates exceeding 0.2 kg/s (1600 lb/h) and juice consistencies similar to apple sauce were occasionally encountered. With both Spartan and Golden Delicious varieties, the decrease in machine capacity was associated with over maturity (Table 2); however, machine capacities with Red Delicious and McIntosh varieties were much less sensitive to the overmature conditions. Differences in the behaviour of McIntosh and Spartan apples with respect to enzyme digestion requirements prior to centrifugation have been seen before (Beveridge et al., 1989).
CONCLUSIONS
(b) Fig. 4. Effect of feed rate on (A) gross yield and (B) net yield of apple juice produced from 40°C enzyme treated Golden Delicious, McIntosh, Spartan and Red Delicious apple mashes. Decanter Centrifuge parameters were a drum rotational speed of 5696 rev/min, a scroll differential of 14 rev/min and a pool depth of 39 mm.
The decanter centrifuge can produce apple juice efficiently with yields of 80-90% and less than 2-3% suspended solids under appropriate conditions. Capacity depends upon such factors as centrifuge parameters, apple variety and storage period. Important centrifuge parameters include drum rotational speed (as fast as possible), fluid pools (as deep as possible without causing overflow), slow scroll differentials, centrifugation temperatures (near 40°C) and absence of block-
Decanter centrifugation of’ apple mash
129
Fig. 5. Effect of feed rate on suspended solids in apple juice produced from (A) Red Delicious, (B) McIntosh, (C) Spartan, (D) Golden Delicious apples stored for various time periods after harvest. Apple mash was enzyme treated at 40°C and centrifuged at 5696 rev/min, with a scroll differential of 14 rev/min, and pool depth of 39 mm. The regression curve of log suspended solids versus flow (kg/s) was computed from and fitted to all 1988/89 Red Delicious and McIntosh varietal data and the November Spartan and Golden Delicious Data.
age of solids ejection ports. Increasing rotational speed is an excellent method of increasing capacity since an increase from 5356 to 5696 rev/min (340 rev/min) allowed an increase in feed rate of approximately 60%. Storage in air at &l”C did not affect the processing of Red Delicious or McIntosh apples but storage of Spartan and Golden Delicious apples beyond 3-4 months made processing diffi-
cult, requiring much reduced feed rates to sustain high yields with low suspended solids in the juice. REFERENCES Beveridge, T., Harrison, J. E. & Bains, D. (1986). Pilot scale production and composition of juice from heated pear mashes. Lebensm. Wiss. Technol., 19, 432-6.
Table 2. Average pressure tests’ and maturity (starch)b levels
Variety1 Date processed November February April June
Golden Delicious
McIntosh
Red Delicious
Spartan
Pressure
Starch
Pressure
Starch
Pressure
Starch
Pressure
Starch
6.71 4.71 5.39
59 8.4 8.9
4.27 4.74 4.02 3.31
7.7 9.0 9.0 9.0
7.18 5.90 4.86 5.14
2.9 5.9 7.4 9.0
7.63 5.38 4.61
4.3 8.9 9.0
a Kilogram force, N = 10. h 1, immature; 9, overmature; average for N = 10. (’Apples picked at commercial maturity dates in 1988.
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Beveridge, T., Harrison, J. E. & McKenzie, D-L. (1988). Juice extraction with the decanter centrifuge - A review. Can. Inst. Food Sci. Technol. J., 21, 43-9. Lau, 0. L. (1985). Harvest indices for British Columbia apples. Brit. Columb. Orchard., July, 7-13. Looney, N. E., Hikichi, M. & Scheltgen, J. (1981). Automated recording of fruit pressure tests. Can. J. Plant Sci., 61, 751-5. Moyer, J. C. (1984). Apple juice extraction. In Apple Juice Workshop, ed. D. L. Downing. Special Report No. 54,
New York State Agricultural Experiment Station, Cornell Cooperative Extension, Institute of Food Science, Cornell University, Ithaca, New York. Rao, M. A., Moyer, J. C., Wooster, G. D. & Piontek, E. A. (1975a). Extraction of apple juice with a solid bowl decanter centrifuge. Food Technol., 29, 32. Rao, M. A., Moyer, T. C., Wooster, G. D. & Piontek, E. A. (19756). Extraction of apple juice with a solid bowl decanter centrifuge. Effects of adding pomace, press aid and mixture of enzyme and PVP to feed. Confructa, 20, 262-7.
Decanter centrifugation of apple mash: effect of centrifuge parameters, apple variety and apple storage T. Beveridge, J. E. Harrison & R. R. Gayton Agriculture Canada, Research Station, Summer/and, British Columbia, Canada VOH IZO
Enzyme treated (0.02% Pectinex Ultra SP, 40°C. 334 h) apple mash gave juice yields of 80-90% with suspended solids ~3% when decanter centrifuged. Centrifugal conditions were a 230 mm bowl, 5696 rev/min, pool depth of 39 mm and mash temperature of 40°C. Capacities were 0.33-0.39 kg/s (2600-3100 lb/h) under appropriate conditions for Red Delicious, Spartan, McIntosh and Golden Delicious apples. Storage in air at 0-1°C for 8 months did not affect machine capacity with Red Delicious and McIntosh varieties but storage of Spartan and Golden Delicious varieties beyond 34 months resulted in markedly reduced (half or less) machine capacities. The effects of varying decanter centrifuge settings and problems associated with decanter separation of apple juice are documented and discussed. Kqvwords: apple juice extraction,
centrifuge, decanter.
yields (Rao et al., 1975a). Adding press-aid may offer some improvement in yield (Rao et al., 1975b) but this procedure would defeat at least one potential advantage (i.e. cost savings) of the decanter and should be avoided if possible. Different apple varieties behave differently in the machine and enzyme digestion of fruit is a necessary prerequisite to juice separation by decanter (Beveridge et al., 1988). This paper is an attempt to further define the conditions necessary for successful juice extraction from apples by the decanter centrifuge.
INTRODUCTION
Decanter centrifuges offer an alternative juice extraction method to presses and have the potential to produce apple juice with yields of 8590% (w/w) without press-aid (Beveridge et al., 1988). Furthermore, the machine is capable of processing a wide variety of fruit and vegetable feedstocks including fruit mashes such as ripe and/or heated pears which are very hard to press using conventional methods. This makes the production of specialty juices from fruit mashes thermally treated to control polyphenol oxidase possible (Beveridge et al., 1986). These properties make the decanter potentially valuable to producers utilizing numerous feedstocks to produce a wide range of high quality juice products. However, published information on the use of the decanter centrifuge is limited, as indicated by a recent review (Beveridge et al., 1988), and little of this literature represents a systematic examination of the machine’s capabilities. Decanter capacities for non-enzyme treated apple mash is increased by a deep fluid pool in the centrifuge and by increased differential scroll speeds. But increased differential scroll speeds also reduce
MATERIALS
AND METHODS
Apples were harvested from Summerland Research Station orchards at commercial harvest dates in 1987 and 1988, respectively, and stored in air at 0-1°C. Apples were removed from storage immediately prior to processing and a random sample was obtained for determination of starch distribution indices (Lau, 1985) and pressure (Looney et al., 1981). Apples were hammer-milled to pass 9 mm holes then pumped (Waukesha MO. 134 Sanitary Pump, Waukesha Div., Abex Corp., Waukesha, WI, USA) through a scraped surface
0 1992 Canadian Federal Government 125
126
T. Beveridge, J. E. Harrison, R. R. Gayton
heat exchanger (Cherry-Burrell MO. 324 SWB Thermutator, Cherry-Burrell Corp., Cedar Rapids, IA, USA) for either temperature adjustment to 40°C or blanching at 90°C (Beveridge et al., 1986). Enzyme digestion of apple mash was accomplished at 40°C in 50 kg batches. Pectinex Ultra SP (0.02% (w/v); Nova Industries AS, Copenhagen, Denmark) was thoroughly incorporated into the mash by mixing 3 min at speed 2 in a Hobart mixer (Hobart, MO. H600, Hobart Canada Inc., Don Mills, ON). The enzyme-treated mash was incubated at 40°C for 3 h then pumped (Waukesha) through a decanter centrifuge (AlfaLava1 MO. NX309$31G, Alfa-Lava1 Ltd, Scarborough, ON) at flow rates between 0.066 and 0833 kg/s for the production of apple juice. Mash feed rate and juice yields were calculated as: feed rate = input mash (kg)/run time (s) gross yield = (output juice (kg)/ input mash (kg)) X 100 net yield = gross yield - suspended solids
(1) (2) (3)
Suspended solids were measured in triplicate by centrifuging 40 ml samples of juice 10 min at 3000g in a Sorval RC-5 centrifuge (DuPont Instruments, Newton, CT, USA). Suspended solids were expressed as weight percent of the total juice weight. Centrifuge parameters were adjusted in stepwise fashion to maximize yield and throughput with minimum suspended solids in Red Delicious output juice. Scroll differential was maintained at 14 rev/min throughout all experiments. Centrifuge rotational speeds were adjusted between 4675 and 5696 rev/ min while maintaining the pool depth at 39 mm. The pool depth was then adjusted between 27 and 45 mm while maintaining the bowl rotational speed at 5696 rev/min. Temperature effects were analyzed by pasteurizing the mash (Beveridge et al., 1986), immediately after enzyme incubation, to destroy enzyme activity. The mashes were then stored in 40, 30 and 5°C rooms overnight, mixed, weighed, temperature measured, then passed through the centrifuge (5696 rev/min; 39 mm pool depth). All varietal runs utilized a centrifuge bowl speed of 5696 rev/min, a pool depth of 39 mm and a mash feed temperature of 40°C. The data obtained in the 1988/89 season were analyzed by the SAS GLM procedure (SAS Institute Inc., Cary, NC, Version 6.0), and multiple t-tests to determine the effect of apple variety and storage time on machine capacity.
RESULTS AND DISCUSSION The slowest scroll differential speeds available on this decanter were 14 rev/mm and 28 rev/min. Since scroll differential speeds beyond 20 rev/min markedly degrade juice yield (Rao et al., 1975a) and no evidence for scroll overloading was reported at 10 rev/min, it was decided to set scroll differentials at 14 rev/min. The important parameter for most purposes is machine capacity and a measure of this parameter is obtained by increasingly loading the machine (increasing mash flow rate) until a marked increase or break-through of suspended solids in the juice stream is noted (Beveridge et al., 1988). Increasing rotational speed markedly increased capacity (Fig. 1) as would be expected. If 3% suspended solids is arbitrarily chosen as a reasonable figure for comparison with other juice extraction systems then the capacities were about 0.16, 0.23 and 0.36 kg/s at 4675, 5356 and 5696 rev/min respectively with a pool depth of 39 mm. Small increases in drum rotational speed resulted in large capacity increases. Increasing pool depth also increased machine capacity (Fig. 2). Again taking 3% suspended solids for comparison purposes machine capacities at 5696 rev/min are 0.18, 0.30 and 0.36 kg/s for pool depths of 27, 36 and 39 mm, respectively. Runs utilizing a 45 mm pool depth were abandoned as liquid spilled out the solids ejection port, posing an obvious overflow problem. These results compare well with those of Rao et al. (1975a). Using the highest attainable rotational speed and a pool depth of 39 mm, the centrifuge capacity for juice with less than 3% suspended solids is about 0.36 kg/s (2800 lb/h), about three times the capacity (900 lb/h) reported by Rao et al. (1975a) but still lower than reported capacities
Fig. 1. Effect of decanter drum rotational speed on machine capacity and yield utilizing Red Delicious apples milled and enzyme treated at 40°C. Centrifuge parameters were a 230 mm drum, a 39 mm pool depth and a scroll differential of 14 rev/min.
Decanter centrljiigation of apple mash
T3
25
_
POOL DEPTH
(mm)
127
0f :: 19
;*o-
FEED RATE &a’s)
Fig. 2. Effect of decanter pool depth on machine capacity and yield using 40°C enzyme treated Red Delicious apple mash. Centrifuge parameters were a drum rotational speed of 5696 rev/min and a scroll differential of 14 rev/min.
for most small presses (Beveridge et al., 1988). However, most net yields range from 80 to 90% which were comparable to or better than most presses and were obtained without the need of press-aid. On occasion, during normal operation of the decanter, dry, sticky ejecta clung to the centrifuge bowl shroud (Fig. 3(A)) accumulating to the point where the solids exit ports were totally blocked for most of the diameter of the rotating bowl. This caused a complete blockage of the centrifuge scroll (Fig. 3(B)) and spillage of large quantities of suspended solids into the juice stream. The onset of this condition was preceded by a high pitched whine which occurred presumably as the solids build-up contacted the rotating bowl. This may explain the intermittent appearance of suspended solids in the juice stream reported by Moyer (1984). This condition was occasionally self-clearing but could be avoided by machine design since the condition was not experienced during preliminary work with a (230 mm) 9-in diameter Mercobowl decanter (Dorr-Oliver Mercobowl, Hazelton, PA, USA) equipped with fin-like structures in the solids ejection region of the bowl. In the present experiment this problem was partially overcome after the 1987 season by mounting water jets under the shroud to continuously wash away the collecting ejecta. Effect of temperature Centrifugation temperature had a marked effect on suspended solids (Table l), which decreased notably as the temperature increased to 40°C. Gross yield values remained above 86% over a
(b) Fig. 3. Solids build-up in the solids ejection region of the decanter. (A) bowl shroud rotated upward and away to show extent of ejection port blockage; (B) centrifuge scroll completely blocked by retained ejecta. Rope was used to lift scroll from centrifuge bowl.
Table 1. Effect of masha temperature during centrifugation
(“C)
Feed rate (kg/s)
Grass yield (‘% w/w)
Net yield (% w/w)
16 30 40 40 42 42
0.378 0.363 0.355 0.354 0.358 0.353
90.7 91.1 91.4 89.9 89.8 86.3
74.9 84.9 88.8 85.7 87.3 84.5
Temperature
Suspended solids (‘%!w/w) 15.8 6.15 2.57 2.83 2.46 1.83
a Red Delicious apples milled through 9 mm screens, treated with 0.2% Pectinex ultra SP 4 h at 40°C. pasteurized, then temperature equilibrated overnight.
T. Beveridge, J. E. Harrison, R. R. Gayton
128
temperature range of 1642°C. While gross yield remained constant with temperature, net yields improved with temperature increases due to the decrease in suspended solids. Effect of variety and storage Gross juice yields (Fig. 4(A)) for all varieties were between 80 and 90% (w/w). Net yields (Fig. 4(B)) of Red Delicious and McIntosh varieties were consistently above 80%. Data representing net yields below 80% were predominantly from Golden Delicious and Spartan apples processed after 4 months storage. This drop in machine capacity was primarily due to an increase in suspended solids in the juices. The natural logarithm of suspended solids was linearly related to feed rate (Fig. 5) for all varieties and run dates (r2 = 0.74). The log regression slopes of the process runs of Golden Delicious apples in
(4
April and of the Spartan apples in February and April were significantly different (p < 0.05) from those of all Red Delicious, McIntosh and November runs of Golden Delicious and Spartan, but were not significantly different from each other. The regression of the February runs of Golden Delicious mash was significantly higher than those of the Red Delicious runs. The Red Delicious, McIntosh and November Golden Delicious and Spartan data were pooled to obtain the curve drawn on Fig. 5 0, = e@w7+2.91~;r2 = 0.64). This line allows one to compare the data in the four graphs more easily. With both Red Delicious and McIntosh apples, decanter capacity was approximately 0.36 kg/s (3100 lb/h) (Fig. 5(A and B)) as was the initial capacity determination (Figs 1 and 2). For both cultivars, refrigerated storage in air (O-l°C) had no effect on juice extraction over a 9 month storage period (Ott-June). Spartan apples (Fig. 5(C)) gave good centrifuge capacity early in the season (-0.33 kg/s in November), however, with storage the weight of suspended solids increased, decreasing capacity to approximately 0.17 kg/s by February. Golden Delicious apples (Fig. 5(D)) gave results somewhat similar to Spartans. Centrifuge capacity was 0.36 kg/s until February, after which the suspended solids in the juice rose at feed rates exceeding 0.2 kg/s (1600 lb/h) and juice consistencies similar to apple sauce were occasionally encountered. With both Spartan and Golden Delicious varieties, the decrease in machine capacity was associated with over maturity (Table 2); however, machine capacities with Red Delicious and McIntosh varieties were much less sensitive to the overmature conditions. Differences in the behaviour of McIntosh and Spartan apples with respect to enzyme digestion requirements prior to centrifugation have been seen before (Beveridge et al., 1989).
CONCLUSIONS
(b) Fig. 4. Effect of feed rate on (A) gross yield and (B) net yield of apple juice produced from 40°C enzyme treated Golden Delicious, McIntosh, Spartan and Red Delicious apple mashes. Decanter Centrifuge parameters were a drum rotational speed of 5696 rev/min, a scroll differential of 14 rev/min and a pool depth of 39 mm.
The decanter centrifuge can produce apple juice efficiently with yields of 80-90% and less than 2-3% suspended solids under appropriate conditions. Capacity depends upon such factors as centrifuge parameters, apple variety and storage period. Important centrifuge parameters include drum rotational speed (as fast as possible), fluid pools (as deep as possible without causing overflow), slow scroll differentials, centrifugation temperatures (near 40°C) and absence of block-
Decanter centrifugation of’ apple mash
129
Fig. 5. Effect of feed rate on suspended solids in apple juice produced from (A) Red Delicious, (B) McIntosh, (C) Spartan, (D) Golden Delicious apples stored for various time periods after harvest. Apple mash was enzyme treated at 40°C and centrifuged at 5696 rev/min, with a scroll differential of 14 rev/min, and pool depth of 39 mm. The regression curve of log suspended solids versus flow (kg/s) was computed from and fitted to all 1988/89 Red Delicious and McIntosh varietal data and the November Spartan and Golden Delicious Data.
age of solids ejection ports. Increasing rotational speed is an excellent method of increasing capacity since an increase from 5356 to 5696 rev/min (340 rev/min) allowed an increase in feed rate of approximately 60%. Storage in air at &l”C did not affect the processing of Red Delicious or McIntosh apples but storage of Spartan and Golden Delicious apples beyond 3-4 months made processing diffi-
cult, requiring much reduced feed rates to sustain high yields with low suspended solids in the juice. REFERENCES Beveridge, T., Harrison, J. E. & Bains, D. (1986). Pilot scale production and composition of juice from heated pear mashes. Lebensm. Wiss. Technol., 19, 432-6.
Table 2. Average pressure tests’ and maturity (starch)b levels
Variety1 Date processed November February April June
Golden Delicious
McIntosh
Red Delicious
Spartan
Pressure
Starch
Pressure
Starch
Pressure
Starch
Pressure
Starch
6.71 4.71 5.39
59 8.4 8.9
4.27 4.74 4.02 3.31
7.7 9.0 9.0 9.0
7.18 5.90 4.86 5.14
2.9 5.9 7.4 9.0
7.63 5.38 4.61
4.3 8.9 9.0
a Kilogram force, N = 10. h 1, immature; 9, overmature; average for N = 10. (’Apples picked at commercial maturity dates in 1988.
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