-350 F) and high (1200 F)ranges. Tests using moderately high temperature drying

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1 _--- HGH TEMPERATURE WALNUT DRYNG Tom Rumsey and Zhimin Lu ABSTRACT On farm walnut drying is often a major bottle neck during the start of walnut harvest The recommended maximum drying air temperature for English walnuts is 110 of This limit is based on tests by university personnel during the introduction of heated air drying (Batchelor, 1924) which demonstrated that long term exposure to temperatures higher that 110 F caused walnuts to turn rancid This project investigated the use of high temperature (200 of of) drying air temperatures for short time periods (on the order of minutes) Several reports have appeared in the literature that indicate high temperature-short time drying might be feasible for drying English walnuts Tang, et al (1982)used air temperatures up to 1000 of to flash dry macadamia nuts form 18% to 6% in under 3 minutes Lowe, et a1(1961) used moderately high temperatures (250 of)to dry low moisture (8%) inshell English walnuts without apparent damage We conducted over 50 experiments (consuming some 340 pounds of walnuts) to investigate drying of English walnuts using air temperatures in the moderate (200 F -350 F) and high (1200 F)ranges Tests using moderately high temperature drying air usually cracked the shells of the walnuts before any significant moisture could be removed Several experiments were performed using moderately high temperature air for a short time (less that 4 minutes) followed by drying at 110 of This predrying reduced the initial moisture by about 4%; however, the total drying time to 8% moisture was the same ~ a control group without predrying A series of high temperature (1200 of) flash drying experiments demonstrated that this is not a feasible technique for English walnuts Flash drying for times greater than 5 minutes proved to be destructive as the the kernels and shells turned offcolor and exhibited an off-flavor For times less than five minutes, shell cracking was a significant problem and the amount of moisture removal was only on the order of 2% to 3% OBJECTVES The purpose of this project was to investigate use of high temperature drying techniques in an effort to reduce on-farm drying times Two temperature ranges were tried: moderately high air temperatures (300 of) and flash drying temperature (1200 of) Exposure ~mes for both temperature ranges were on the order of several minutes 335

2 PROCEDURES Two drying units were used to conduct the tests The dryer used for the moderately high temperature (300-SOO F) -testswas built in the Agricultural Engineering shop at VC Davis Drying air was generated using two heat guns purchased oommerdally They were connected via an insulated duct to a 12inch diameter, 12inch high, insulated steel plenum chamber A single layer of walnuts was placed on top of the chamber in a steel wire bottomed basket Hot air was blown up through the basket, and the walnuts were rotated by hand (using gloves) as they dried The second dryer unit could produce drying air temperatures from 500 to 12000F The unit was designed by personnel in the Food Science Department at UC Davis to test high temperature, short time heating of canned Products The cans were rotated on two parallel steel rollers over a rectangular gas burner The rotation rate of the rollers was controlled by a variable speed electric motor For our tests, the can was replaced by a 6-inch diameter, 8-inch long cylindrical container made of expanded metal The inner dimension of the container was such that its entire length was exposed to the burner flame Walnuts were p]~ced inside the container during the drying tests, and the speed of rotation was adjusted so that the nuts were well mixed during the drying test The air temperature was regulated manually by adjusting the gas flow rate to the burner Air temperature measurements were made using hand-held thermocouple probes connected to a digital read out device, and were taken at least once during each test at several locations within the walnut holding device Moisture contents were determined using laboratory convection ovens set at 2000F The whole walnuts were weighed with a laboratory balance and then reweighed after hours in the oven n some cases, individual nuts were weighed to determine the range of ~oistures for a given test Samples sizes varied from 20 to 30 nuts, depending on the type of test Most of the tests were performed using thawed, whole -Englishwalnuts that had been frozen during the previous (1989) harvest from a local orchard This was necessary because of the short harvest season and the large number of tests to be done The final tests with the heat gun dryer were done using freshly harvested nuts from the 1990 season RESULTS The results will be reported for each of the drying units The flame drying unit was used first, and was followed by a series of tests with the heat guns The flame drying unit gave the highest air temperatures for drying - up-to 1200 of The first series of tests were conducted with drying times limited to 2, 3, 4, and 5 336

3 minutes and a drying air temperature of 1200 of Twenty-one walnuts 'were used in each test, and the moisture of each nut was determined before and after drying A summary of results for tests 12 to 17 is given in Table Key findings were: 1) only a small amount of moisture could be removed (2% to 3%) in this time period, and 2) if the nuts were dried for longer than 5 minutes, most ended up cracked Usually one single large crack appeared in the shell, running the length of the walnut The cracks did not tend to develop along the suture connecting the two halves of the shell; they propagated lengthwise on one side of the halves A plot of moisture loss versus initial moisture is shown in Figure 1 for tests 12 and 15 The large scatter of the initial moistures from the same sample is not unusual for English walnuts From the plot, it can be seen initial moisture did not seem to affect moisture loss, as it was nearly constant over the range of moisture from 15 to 37% An investigation of the relation between initial walnut moisture and walnut size (volume) was conducted using walnuts from these tests H initial moisture is correlated to size, then it might be possible to separate out those that are nearly dry The approximate volume was determined by measuring the height and width of individual nuts with a micrometer and then using the formula for the volume of an ellipsoid A plot of the initial moisture versus this approximate volume is given in Figure 2 As can be seen, there was no apparent relation between volume and initial moisture The danger of using high temperature flame drying became apparent while conducting these tests During one run, the rotating drive was accidentally turned off, and the walnuts in the drying cylinder caught fire and burned This series of test results was extremely disappointing, as it was at these conditions that Tang, et al (1982) ~ad such remarkable success with flash drying macadamia nuts They found moisture losses on the order of 4% per minute over a 25-minute exposure time, and did not report any cracking Our tests did not show similar results for English walnuts Two series of tests were conducted with the flame drying unit at lower drying temperatures and shorter exposure times The lower temperatures and times were used in an effort to prevent cracking of the shells Twenty walnuts were used in each test, and the initial and final moistures were determined for each walnut The average drying air temperatures were 520 of for the first series and 630 OFfor the second Drying times of 1, 15, 2, and 25 minutes were replicated three times at each temperature t was difficult to maintain constant temperatures with the flame unit at the lower ranges; measured temperatures at the drying basket ranged from 480 to 560 OFalmost continuously over the course of a test run Results for the first series (Tests 19-30) are summarized in Table The average moisture loss increased with exposure 337

4 time, ranging from 070% removed at 1 minute, to 14% removed after 25 minutes A plot of average moisture loss versus exposure time is shown in Figure 3 From 15 minutes through 25 minutes, the relation between exposure time and moisture loss is nearly linear Shell cracking was also related to exposure time, increasing as the exposure time increased No nuts were cracked for up to 2 minutes of exposure at the 520 F temperature, while 5 nuts out of 60 were cracked after 25 minutes in the uni t ncreasing the average temperature to 630 F increased the moisture loss at a given exposure time, as shown in Table n The highest moisture removal was 19% at the 25 minute exposure time Walnuts were cracked at all levels of exposure at this higher temperature, with the highest number being 4 out of 60 at the 25 minute time t was apparent that only a small amount of moisture could safely be removed by this unit before cracking occurred This prompted a series of tests using the heat guns, which could develop moderately high temperature drying air more easily than the flame unit Tests 44 through 51 were conducted Uil1gtnpheat gun drying unit Table ill contains the results for Tests 44 through 49, wluch were used for a preliminary shakedown of the unit ndividual nut moist~es were recorded at the start and finish of drying of Tests 44 and 45, and the average moisture loss for each,test was 30% and 47% for 4 and 6 minutes of drying, respectively About two thirds of the walnuts were cracked in the two tests Tests 46 tluough 49 were conducted in an effort to investigate changes in the average shell and kernel moistures during drying For each test, an initial sample of 25 walnuts was cracked by hand, and shell and kernel moistures were determined After drying, the nuts were cracked by hand, and the shell and kernel moistures were determined Unfortunately, there was such a large variability between samples that it was difficult to detect the changes in shell and kernel moistures from start to the end of drying We were only able to combine the shell and k!"t"ldv"ck"htsto calculate the overall net moisture loss of 36% for these four tests S~mil~Tto Tests 44 and 45, almost two thirds of the nuts were cracked by the drying pr~ss for Tests Tests 50 Land 50 H were conducted to show the effect of drying time on moisture removed These were the first tests done ",rithwalnuts fresh from the field Results are shown in Tables V and V along with Figure 4 Tne amount of moisture lost increased with drying time for both te~t~- in a Jinecu fashion for the lower temperature test and at an increasing rate for the high temperature test (Figure 4) The number of cracked nuts was relatively low for both tests All tests were done with nuts from the same large sample, with 25 t\uts selected at random from a 10- pound bag for each test The variation in average initial moisture between 25 nut samples is large, making it difficult to conduct repetitive drying experiments that start from the same initial moisture This is illustrated in Figure 5 in which the shell and kernel moisture for 25 nut samples are plotted versus drying time The large difference behveen samples is due to differing initial moisture contents of the 338

5 individual samples When the differen~e between initial and final moistures is plotted as shown in Figure 4, a smooth curve results Three tests were conducted using the heat gun unit (using 300 of air) to predry approximately 200 walnuts which were then finish dried in a thin layer drying unit using air at 110 F and 20% relative humidity A similarly sized lot of nuts from the same batch was dried in a separate chamber of the thin layer unit using the same drying air The results for one of the tests (Aminco test 5) are shown in Figure 6 The average moisture content of the initial fresh sample was 278%, and 150 nuts were predried down to 221% using the heat gun unit A total of 13 of the 150 nuts in the sample cracked during pre drying Both samples reached the 8% level at approximately the same time, indicating that there was no advantage to predrying the walnuts Similar results were found for the other two tests in this series CONCLUSONS High temperature drying of English walnuts does not appear to be feasible because of low moisture removal rates and significant cracking of shells We were unable to measure elevated drying rates similar to those found by Tang, et al (1982) for macadamia nuts (4% per minute) Using air temperatures of 1200 of, we were able to remove approximately 25% moisture in 3 minutes Exposure for longer periods of time cracked nearly all of the nuts When the air temperature was reduced to 600 of, less moisture was removed in the same drying period, and cracking was still significant Predrying walnuts for 300of for 4 minutes reduced the moisture about 5% without significant cracking; however, exposure for longer drying times resulted in more cracking When nuts were predried for 4 minutes at these conditions and then finish dryed at 110 of, the total drying time to an 8% final moisture was the same as a control group which had not been predried ACKNOWLEDGEMENTS This project was sponsored by the California Committee on the Relation of Energy to Agriculture

6 REFERENCES Batchelor, LD, AW Christie, EHGuthier and RG LaRue 1924 Sun-drying and dehydration of walnuts University of California Agricultural Experiment Station Bulletin Number 376 University of California Printing Office, Berkeley, CA Lowe, E, LB Rockland and K Yanase 1961 Studies on English (Persian) Walnuts, Juglans regia V Belt-Trough Drying of n-shell Walnuts Food Technology March, pp: Tang, CP, B Snobar and T liang 1982 Flash drying macadamia nuts for improved kernel extraction Transactions of the ASAE 25(6): Table FlameDrying Tests, 1200of Average Moistures Test Time nitial Final Loss Cracked! (min) Flame TQ medium / high low low low low

7 - Table n FlameDryingTests,52 of Average Moistures Test Time Temp nitial Final Loss Cracked! &!minl CbbJ CbbJ CbbJ gw /& /& /& /& /& /& '1J& /&1 Tablem Heat Gun Drying Tests AverageMoistures Test Time Temp nitial Final Loss Cracked! (min) eel CbbJ CbbJ CbbJ g1l /

8 TableV Heat Gun Drying Test 50L, 375of Average Moistures Time nitial Final Loss Cracked! (min) gw OflS flS Tabley Heat Gun Drying Test 50 H, 480of AverageMoistures Time nitial Final Loss Cracked! (min) gw

9 50 :i = 40 - J j!! TestS,3minutes Test 17,5 minutes r/j i' nitial moisture (%wet basis) 400 Figure 1 Flame drying tests, 1200 F,low flame, 3-5 minutes

10 30 'i = 25 Q ; 20 tjt - " 15 0! 10 j 1 05 Tests Tests İ Time (see) Figure 3 Low temperature flame, Tests 19-30, ī 10-8ī - 5 Ī it rft Test50L Test50H c o o Time (Min) Figure 4 Heat gun tests 50 L and 50 H

11 ';' ] 1 - ::s 40 i bm 20 o Time (Min) Figure 5 Shell and kernel moistures for test SOL & J! J _m~ :;1 : i : ~ i j t t t---- '\: i i, : i i "'"! i \ i i i,! : i i : '!! : ~ i!!- r t,---- i' i i i i : :! : 00 o Tima ~ Fisure 6 Thjn Jayr drying ()f pre-driedmpje - "345