TECHNOLOGY & PRODUCT REPORTS

tive species, including atomic oxygen, ozone, hydroxyls, and other charged molecules (Lowke and Morrow, 1994). Negative air ions (NAI), which have Technology been reported to have biological ef- fects, may also be produced (Krueger and Reed, 1976). Corona discharge has been reported to reduce the decay and extend the storage life of har- & Product vested fresh fruits and vegetables (Tanimura et al., 1997, 1998). Poten- tial mechanisms by which the corona could affect produce quality include Reports the destruction of ethylene and other volatiles, removal of airborne fungal spores, and the production of ozone and other reactive species that could charge when onions were stored for 2 Biological Effects or 4 weeks with or without an inhibit pathogenic microorganisms. additional 2 weeks of under To determine the effects of a com- of Corona high humidity. Corona discharge mercially available corona discharge treatment also reduced airborne mold system on the decay and quality of Discharge on spores in the storage room. No fresh onions, a commercial trial was significant changes in internal decay, conducted in onion storage rooms at Onions in a firmness, sprouting, or rooting, in Nova Agri Associates (Port Williams, Commercial treated onions were found. Nova Scotia). Nova Agri is one of the leading onion suppliers in Atlantic Storage Facility Canada, storing, packing, and market- nions can be stored for 6 ing onions throughout most of the months or longer at rec- year. Most of their onion storage uses Jun Song,1 Lihua Fan,2 O ommended storage condi- outside air for temperature and hu- tions of 32 °F (0° C) and 65% to 70% midity control. However, as onions Paul D. Hildebrand,3 and relatively humidity (RH) (Hardenburg are stored longer into the spring, re- et al., 1986). Proper curing and low Charles F. Forney4 frigerated storage is used and humidity humidity in storage is necessary to control becomes a problem. This study minimize postharvest diseases. How- explored the effects of corona dis- ever, under low humidity storage, on- charge on controlling decay and qual- ADDITIONAL INDEX WORDS. ozone, nega- ions can suffer substantial moisture ity of onions stored under high tive air ions, Allium cepa, onion loss resulting in reduced yield of sal- humidity during this time. storage, weight loss, mold able produce and softening and shriv- eling of the onions. In addition, Materials and methods SUMMARY . The biological effect of maintaining low RH in conventional corona discharge on onions (Allium EXPERIMENTAL SETUP. A corona dis- cepa L.) in a commercial storage was refrigerated storage rooms is difficult if charge system (model FE 40; Fletcher investigated. Surface discoloration not impossible. Under high humidity, Enterprises, Hudson, Quebec), was and mold were modestly but signifi- onions may develop blue and black installed in a 35,000-ft3 (991-m3) stor- cantly reduced by the corona dis- mold as well as soft rot caused by age room (treatment room) at Nova Penicillium (Link: Fr.), Aspergillus Agri which has high humidity and Agriculture and Agri-Food Canada, Atlantic Food and niger (van Tieghem), and Erwinia serious mold problems. The unit was Horticulture Research Centre, 32 Main Street, Kentville, carotovora pv. carotovora [(Jones) installed in the middle of the room, 20 N.S., B4N 1J5, Canada Bergey], respectively (Maude, 1983). ft (6.1 m) above the floor in the air- Contribution 2207 of the Atlantic Food and Horticul- Controlled atmosphere (CA) storage ture Research Centre, Agriculture & Agri-Food Canada. stream of the existing circulation fans. We thank Nova Agri Associates, Ltd (Port Williams, has been used with limited success to The treatment room was cooled by N.S.) and its employees for their support and coopera- reduce onion decay and maintain fresh- refrigeration at all times throughout tion in conducting this study. We thank Michael A. ness and eating quality (Adamicki and Jordan and Willy E. Renderos for their technical assis- the study. A second storage room was tance. The cost of publishing this was defrayed in Kepka, 1974), but it has not been used as the control room without a part by the payment of page charges. Under postal economical or practical for storage of corona discharge system. This room regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. pungent type onions. An effective was cooled with outside air briefly at method to inhibit decay and surface 1Postharvest physiologist, Fletcher Enterprises, P.O. the beginning of the study, but later 355, Hudson, Quebec J0P 1H0, Canada, c/o mold and maintain onion quality is was also cooled with refrigeration. AAFC, Atlantic Food and Horticulture Research Cen- needed by the industry as an alterna- The corona in the FE 40 pro- tre, 32 Main Street, Kentville, N.S., B4N 1J5, Canada. tive to CA or synthetic chemicals. duced ozone and NAI. Ozone con- 2 Food microbiologist. In corona discharge (nonthermal centration was monitored every 20 s 3Plant pathologist. plasmas), electrical energy is used to using an UV-ozone detector (model 4Postharvest physiologist. create large quantities of highly reac- I-2000; IN USA, Inc., Needham,

● July–September 2000 10(3) 608 Mass.). The ozone concentration was and-trap concentrator (LSC 2000; A rating scale of 0 to 5 was used to used to control the FE 40 via the system’s Tekmar, Cincinnati, Ohio). Quantiza- evaluate surface discoloration or inter- control software. When preset ozone tion was done using single ions of exter- nal decay where 0 = 0%, 1 = 1% to 5%, 2 levels were detected, the FE 40 began nal standards. All peaks were normalized = 6% to 10%, 3 = 11% to 30%, 4 = 31% either a startup or shutdown sequence. using the peak area of a 4 ng dodecane to 50% and 5 = 51% to 100% of the The system was operated at 350 Hz and standard run on the day of analysis. surface area affected. Surface discolora- 2.5 kV and the ozone concentration EVALUATION OF ONIONS. Each room tion included physiological staining and was controlled about a maximum set contained varying amounts of onions mold on the onion surface. Weight loss point of 50 ppb (nL·L–1) during the day throughout the study, ranging from was expressed as percent. Firmness was (0600 to 1800 HR) and 250 ppb during 30% to 60% of the room’s capacity. measured as force of penetration using the night (1800 to 0600 HR). During Twelve 5.0-lb (2.27-kg) of onions a digital force gauge (DFIS 50; GEO- the first week of the experiment only, (about 20 onions/), taken directly Met Instruments Inc., Kentville, Nova the ozone set point was 125 ppb during from the packing line were individually Scotia) equipped with a 6-mm (0.24- the night. weighed and placed on pallets in each inch) diameter wedge-shaped probe. Negative air ion concentrations storage room. Assessments of quality Number of roots formed per onion were measured with an ion meter (Al- were determined for three bags from were counted and the length of the pha Lab, Salt Lake City, Utah) and the each room after 2 and 4 weeks of stor- longest root was measured. Shoot for- ozone and NAI concentrations in the age. After each storage period, an addi- mation was noted and length of shoots treatment room were recorded con- tional three bags of onions from each measured. The experiment was repli- tinuously (every 5 s with 1-min aver- room were kept for another 2 weeks of cated three times from 15 Apr. through ages) on a multichannel data logger shelf life at 68 °F (20 °C) and 100% RH 27 May 1999. (Campbell Scientific, Logan, Utah). to observe the residual effect of the The experimental data were ana- Temperature and RH were also mea- corona treatment. Quality assessments lyzed with the ANOVA directive in sured with a probe (HMP 45C; Camp- included surface discoloration and in- Genstat 5 release 3 (Genstat 5 Commit- bell Scientific) and recorded ternal decay ratings, weight loss, firm- tee, 1993) according to a randomized continuously in both rooms in close ness, and formation of roots and sprouts. complete block design with three fac- proximity to the test onions. AIRBORNE MOLD SAMPLING. Airborne mold spores were collected using a cen- trifugal air sampler (RCS; Biotest Diag- nostics Corp., Denville, N.J.) with a sampling time of 1 min allowing 1.40 ft3 (39.6 L) of air to be sampled. Sterile, rose-bengal-agar strips that only permit the growth of mold were used. Two samples were taken in each of the con- trol and treatment rooms at weekly intervals. After 4 d of incubation at 75.2 °F (24 °C), microbial colonies on the strips were counted with the aid of a stereomicroscope. Data from the two strips were transformed to the logarith- mic scale and analyzed according to the Student’s paired t test. The number of mold colony forming units per volume of air were then back-transformed and expressed as cfu/m3. ETHYLENE AND VOLATILE ANALYSIS. Air samples for ethylene analysis were collected in the middle of each room using a 5.0-mL syringe and analyzed using a gas chromatograph (Carle AGE; series 400, Loveland, Colo.) equipped with a flame ionization detector. Other volatiles were collected using an air sam- pling pump (Supelco, Bellefonte, Pa.). One liter of air from each room was trapped onto 120 mg Tenax GR 20/35 (Alltech Associates, Inc., Deerfield, Ill.). Samples were analyzed on a gas chro- Fig. 1. Daily averages of relative humidity, temperature, and ozone concentra- matograph–mass spectrometer (GC– tion in the onion storage room treated with the corona discharge system and MS) system (Magnum; Finnigan MAT, the control room during the three replications of the experiment [°F = 1.8(°C) –1 San Jose, Calif.) equipped with an purge- + 32; ppb = nL·L ].

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average weekly airborne spore concen- tration of both rooms is presented in Fig. 3. Before the FE 40 system was installed (15 Apr.), there were higher concentrations of airborne spores in the treatment room than in the control room. However, after the FE 40 was installed, airborne spore concentrations were reduced in the treatment room compared to the control room through- out most of the duration of this study. SURFACE DISCOLORATION AND MOLD. The onion skins were colonized pre- dominantly by Penicillium sp. and to a lesser extent by Aspergillus sp. A pre- liminary analysis indicated that the first week of lower nighttime ozone levels in replicate one did not affect the results and so all three replicates were included for subsequent analysis. The surface dis- coloration plus mold rating (SDM) of onions increased as storage increased Fig. 2. Sample of temperature, relative humidity, ozone concentration, and from 2 to 4 weeks (Table 1), but there negative air ion concentration in the onion storage rooms over a 48-h period. was no difference between the treated Ozone was absent in the control room and the ambient negative air ion concen- and control onions, even though the tration ranged from 300 to 1000 per cm3 [°F = 1.8(°C) + 32; ppb = nL·L–1; treated onions were exposed to higher 1000/cm3 = 16,387/inch3]. humidity. The ANOVA indicated a sig- nificant interaction between the effects tors each with two levels: treatment the room. of treatment and shelf life. The averaged (corona, no corona), treatment time (2 A typical pattern of temperature, data for the 2- and 4-week storage or 4 weeks), and shelf life (0 or 2 weeks). RH, ozone, and NAI values over a 48- periods plus shelf life showed that the The percent discolored surface area of h period in the storage rooms is shown treated onions had a SDM rating that onions was also estimated from an equa- in Fig. 2. The defrost cycles in the was 27% (estimated affected surface tion obtained by regressing the mid- treatment room resulted in brief in- points of the severity categories of the creases in temperature and decreases in rating scale against the rating scale val- RH. Defrost cycles in the control room Fig. 3. Airborne spore concentrations ues. were less obvious. Increased levels of sampled at weekly intervals in the room treated with the corona NAI were detected whenever the FE 40 Results and discussion discharge system and in the control was on, but interestingly, they were room. Differences between the STORAGE CONDITIONS. Daily aver- quenched when the circulating fans treated and control rooms are noted ages of temperature, RH, and ozone in turned on after a defrost cycle and gradu- above the bars. NS,*,**Nonsignificant or the onion storage rooms are presented ally increased over a period of about 1 to significant at P < 0.1 or 0.05 accord- in Fig. 1. The RH in the treatment room 2 h. A possible explanation for this effect ing to Student’s paired t test (103 cfu/ was at or near 100% throughout the may be related to increased levels of m3 = 28.3 cfu/ft3). experiment. During the first 3 d, the airborne RH in the control room was less than microdroplets of wa- 90%, but increased to 90% to 95% when ter being blown from the refrigerated cooling system was the cooling coils fol- turned on. The temperature in the treat- lowing a defrost cycle. ment room and control rooms averaged The NAI may be at- about 45.7 °F (7.6 °C) and 46.4 °F (8.0 tracted to the water °C), respectively, but then dropped to droplets resulting in about 34.7 °F (1.5 °C) at the end of the lower free airborne experiment. The daily average ozone ions. Ozone concen- concentration was about 100 ppb dur- trations ranged from ing the first week of the experiment and 0 to 150 ppb during then increased to about 170 ppb in the day and about 150 response to an increase in the nighttime to 250 ppb during the ozone concentration set point. From 30 night, but were not Apr. to 15 May, the average ozone affected by defrost concentration was somewhat reduced cycles. which may have resulted from addi- AIRBORNE MOLD tional activity and loading of onions in CONCENTRATION. The

● July–September 2000 10(3) 610 Table 1. Effect of corona discharge treatment on the surface discoloration plus 1998). The decrease of dipropyl-disul- mold (SDM) ratingz and weight loss of onions. Onions were rated immediately fide could explain the reduced pungent following treatment times of 2 or 4 weeks or after an additional 2 weeks at 68 smell noted in the treatment room when ° ° F (20 C) under high humidity (shelf life). Values in parentheses are estimated the corona unit was turned off briefly. percentages of surface area affected. Corona discharge has been reported to be a promising energy efficient technol- SDMz Wt loss (%) ogy for the destruction of volatile gasses Treatment Control Treatment Control Treatment in pollution control (Ruan et al., 1997). 2 weeks 1.6 (5.6) 1.6 (5.6) 1.20 0.52 Similarly, it would be of considerable 2 weeks + shelf life 4.4 (53.2) 3.5 (25.7) 1.65 1.30 interest to further investigate the de- 4 weeks 2.0 (7.7) 1.9 (7.1) 1.55 1.03 struction of volatiles, not only from 4 weeks + shelf life 4.6 (62.6) 4.0 (38.6) 2.21 1.61 onions but other commodities, in re- Factors sponse to a corona discharge. Treatment (T) ** ** High humidity, often found in re- Treatment time (TT) ** ** Shelf life (S) ** ** frigerated storage, can reduce weight T × TT NS NS loss of produce, but may lead to in- T × S**NS creased levels of surface mold and decay. TT × S NS NS The corona discharge, as seen in this T × TT × S NS NS study, showed promise in overcoming this difficulty. Surface mold on the on- zRating scale: 0 = 0%, 1 = 1% to 5%, 2 = 6% to 10%, 3 = 11% to 30%, 4 = 31% to 50%, 5 = 51% to 100% of surface area affected. ions was reduced and airborne spores NS,**Nonsignificant or significant at P < 0.001. were also reduced thereby further limit- ing contamination of onions and patho- area) lower than the control onions. the control onions even after two weeks gen development. The ozone During the third replicate, it became of shelf life. These differences in weight presumably affected growth of fungal evident that the discoloration of the loss were likely a result of the higher RH colonies on the onion surfaces and may onion skins that was being rated along in the treatment room than in the con- have reduced airborne spores by reduc- with the surface mold did not change trol room. ing sporulation of mold colonies or by appreciably during storage. This dis- Throughout the experiment, there directly killing spores. The negative air coloration apparently occurred in the were no significant treatment effects on ions may also have contributed to cleans- field or during curing and was not internal decay or on numbers and lengths ing the air of airborne spores. Negative related to mold development. There- of roots and shoots. Although not sig- air ion generators have been shown to fore, surface mold only and SDM were nificant, firmness was slightly higher in effectively reduce airborne microbes by rated separately. This showed that the treated onions [8.82 lb (4.00 kg)] precipitation (Gabbay et al., 1990; treated onions had over two times less than in the control onions [8.69 lb Mitchell, 1998). Li et al. (1989) re- mold than the control onions upon (3.94 kg)], which likely was due to the ported that a combination of ozone removal from the storage rooms (Table higher humidity in the treatment room. with NAI was most beneficial to stor- 2). This observation, and the fact that Similar results have been reported by age of mandarin oranges. Tanimura et the SDM rating of treated onions was Van den Berg and Lentz (1973) for the al. (1997, 1998) also found that the reduced even after an additional pe- relationship between storage humidity combination of NAI and ozone pro- riod of harsh storage conditions on the and firmness. Also, there were no signs duced by corona discharge inhibited shelf, suggests that the fungal popula- of phytotoxicity on onions as a result of microbial growth more strongly than tion was substantially reduced during the corona treatment. ozone alone. However, the NAI con- the corona treatment. One compound identified as centration in the onion storage room WEIGHT LOSS. The weight loss of dipropyl-disulfide was consistently lower treated by the FE 40 was lower than in onions in the treatment room was only (70.4%) in the treatment room com- Tanimura’s reports and the degree to 0.52% after 2 weeks and 1.03% after 4 pared to the control room. There were which negative air ions played a role in weeks, which was significantly less than no substantial differences in other vola- limiting mold growth is not known. the weight loss that occurred in the tile concentrations between the two The role of NAI with respect to inhibit- control room (1.20% and 1.55%, re- rooms. Disulfides, trisulfides, and alkyl ing microbial growth during storage of spectively) (Table 1). Weight loss was thiosulfonates are considered to be the horticultural products requires further again less in the treated onions than in major onion volatiles (Schulz et al., investigation.

Table 2. Comparison between ratingsz of mold alone and surface discoloration plus mold (SDM). Data represent means of three sample bags of onions from Rep III. Values in parentheses are estimated percentage of surface area affected.

4 weeks 4 weeks + shelf life Parameter Control Treatment Control Treatment Mold 1.3 ± 0.24y (4.3) 0.2 ± 0.13 (1.8) 4.5 ± 0.06 (58.7) 3.9 ± 0.38 (34.1) SDM 2.3 ± 0.20 (9.9) 2.3 ± 0.22 (9.6) 4.8 ± 0.03 (71.2) 4.3 ± 0.24 (39.9) zRating scale: 0 = 0%, 1 = 1% to 5%, 2 = 6% to 10%, 3 = 11% to 30%, 4 = 31% to 50%, 5 = 51% to 100% of surface area affected. yStandard deviation.

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Mitchell, B.W. 1998. Effect of negative air Literature cited ionization on ambient particulates in a Adamicki, F. and A.K. Kepka. 1974. Stor- hatching cabinet. Appl. Eng. Agr. 14:551– age of onions in controlled atmospheres. 555. Acta Hort. 38:53–73. Ruan, R.R., W. Han, A. Ning, P. Chen, Gabbay, J., O. Bergerson, N. Levi, S. and P.R. Goodrich. 1997. Reduction of Brenner, and I. Eli. 1990. Effect of ioniza- harmful gases and odorous compounds by tion on microbial air pollution in the dental non-thermal plasma. Amer. Soc. Agr. Eng. clinic. Env. Res. 52:99–106. Mtg. Presentation 974038. 10–14 Aug., Minneapolis, Minn. Genstat 5 Committee. 1993. Genstat 5 release 3 reference manual. Clarendon Schulz, H., H. Krüger, J. Liebmann, and Press, Oxford, U.K. H. Peterka. 1998. Distribution of volatile sulfur compounds in an interspecific hy- Hardenburg, R.E., A.E. Watada, and C.Y. brid between onion (Allium cepa L.) and Wang. 1986. The commercial storage of leek (Allium porrum L.) J. Agr. Food fruits, vegetables, and florist and nursery Chem. 46:5220–5224. stocks. USDA Agr. Hdbk. 66. Tanimura, Y., J. Hirotsuji, and K. Tanaka. Krueger, A.P. and E.J. Reed. 1976. Bio- 1998. Food preservation technique using a logical impact of small air ions. Science mixed gas containing negative ions and 193:1209–1213. ozone. Shokuhin Kógyó (Food Ind.) Li, J., X.Y. Wang, H.L. Yao, Z.G. Yao, J.X. 41(10):71–77 (in Japanese). Wang, and Y.G. Luo. 1989. Influence of Tanimura, Y., N. Nakatsugawa, K. Ota, discharge products on postharvest physiol- and J. Hirotsuji. 1997. Inhibition of mi- ogy of fruit. 6th Intl. Symp. High Voltage crobial growth using negative air ions. J. Eng., 28 Aug–1 Sept., New Orleans, La. Antibact. Antifung. Agents 25(11):625– Lowke, J.J. and R. Morrow. 1994. Theory 631 (in Japanese). of electric corona including the role of Van Den Berg, L. and C.P. Lentz. 1973. plasma chemistry. Pure Appl. Chem. Effect of relative humidity, temperature 66(6):1287–1294. and length of storage on decay and quality Maude, R.B. 1983. Onions, p. 73–103. of potatoes and onions. J. Food Sci. 38:81– In: C. Dennis (ed.). Post-harvest pathol- 83. ogy of fruits and vegetables. Academic Press, New York.

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