USING THE SHIRA AEROPONICS SYSTEM TO MAINTAIN WEIGHT
AND FIRMNESS IN STORED POTATO TUBERS
U. Afek, Janeta Orenstein and Ephraim Nuriel
ABSTRACT
A new fogger ?the Shira Aeroponics System ?features unique ultrasonic nebulizers. The droplets produced have a very small mass (less than 10 m ,) and behave like a dry cloud, producing 96-98% relative humidity (RH) without depositing free water on the tubers. After 6 months of storage at 10C, with 96-98% RH applied with the Shira Aeroponics System (room A), 92-94% applied with a spin-disk humidifier (room B), and 82-86% with no added humidity (room C), the weight losses for stored potatoes were 2, 7 and 12%, respectively. The firmness of the tubers expressed as newtons (N) required to penetrate the tubers before storage, and after 6 months in room A, B, or C was 76, 74, 68 or 63 N, respectively.
Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 425-98.
ADDITIONAL KEYWORDS: Solanum tuberosum L., weight loss, storage, quality, humidity.
INTRODUCTION
Storage losses of potatoes are often specified as weight and quality losses and are caused mainly by respiration, sprouting, spread of diseases, dehydration, changes in chemical composition of the tuber or damage by extreme temperatures (Brook, et al., 1995; Es and Hartmans, 1987a,b; Graee and Aarseth, 1993; Meijers, 1987; Rastovski, 1987; Rees, et al., 1981).
Moisture transport occurring during potato storage depends on three main factors: the permability of the periderm (Villa and Bakker-Arkema, 1974), the extent to which water inside the tuber is free for transport to the outside, and the extent to which the outside - in this case, the storage atmosphere - can absorb this water. This difference between inside and outside is expressed by the difference in vapor pressure, or water potential difference (Es and Hartmans, 1987c; Kleinkopf, 1995). During storage, water in the tuber is in constant flux with the storage atmosphere. Air with a high relative humidity (RH) can take up little more moisture, whereas air with a low RH can take up a great deal by evaporation (Es and Hartmans, 1987c), reducing the tuber weight. Tubers which have lost a lot of water are more susceptible to mechanical damage such as bruising and blue discoloration, and have greater peeling losses (Rastovski, 1987).
The study compared the Shira Aeroponics System and a spinning disk humidifier for their respective effects on potato tuber weight and firmness during storage.
MATERIALS AND METHODS
Plant material and storage of potatoes
The humidifier comparison was conducted between June and December during the years 1995-1997. Potato (var. ‘Desiree? tubers, four months after planting, were harvested from the northern Negev, Israel. Two storage rooms measuring 15 X 15 X 7 m3, and one storage room measuring 5 X 5 X 5 m3, each with independent air circulation, were used for the experiments. The larger rooms, which were used for the humidification treatments, were fitted with five perforated ducts of 70 cm diameter, laid on the floor from the plenum to the opposite wall. The potatoes were stored in a 750-ton pile, 5 m high, which covered the ducts in each room. The small storage room was used for the control (no added humidity). It was fitted with two perforated ducts and contained a 30-ton pile, 3 m high, that covered the ducts. Sacks containing 50 kg of potatoes were weighed before storage, and 54 sacks were buried in each of the three piles at three depths (30, 70 and 120 cm), 18 sacks randomly distributed at each depth. The sacks were tied with ropes of different colors (one color for each depth). Each month, 9 sacks - 3 from each depth - were removed from the piles in order to evaluate weight loss. The potatoes were stored for 6 months at 10C ± 1C.
Application of humidity
In both of the large rooms, three fans (1 m diameter) were used; in the control (no humidity) room one fan was used. The air, driven into the perforated ducts by the high pressure produced by the fans in the plenum, passed upwards through the potato pile into the lower-pressure space above the pile (Afek and Warshavsky 1998). The air flow rate measured in the space above the pile in each room was 0.3± 0.1 m per sec.
Humidity was established in the plenum of room A by fogging with the Shira Aeroponics System. which was located 2 m downstream of the fans and 2 m in front of the ducts. For the plenum of room B there was one spinning disk humidifier with a capacity of 40 l/h, which was located as described above. Room C, the control, received no added humidity. RH was monitored by fiber optic humidity sensors, rotronics or Testo, and controlled by an optical wetness controller based on dew point (Optiguide Ltd., Science Park Technion Nesher, P.O. Box 212 Nesher 36601, Israel). The RH sensors in each room were located at 5 points on the top of the pile: one m from each corner, and in the center of the pile. The RH was the average calculation of the 5 points and the same RH was adjusted in each room.
The firmness of the potatoes, expressed as the force in newtons required to penetrate the tubers, was gauged with a ‘Chatillon Force Measurement?(Force Measurement Division, P. O. Box 35668, Greensboro, NC 27425-5668, USA) pressure tester equipped with a conical probe, 6 mm in diameter. Weight loss and firmness of potatoes were evaluated monthly during 6 months of storage.
Statistical analysis
All weight loss comparisons were conducted with three randomized replicates for each treatment and each evaluation. A replicate was comprised of three sacks of 50 kg of potatoes, accounting for the total of 54 sacks per room (nine sacks were pulled up randomly from different zones and from 3 different depths between 30 - 120 cm of the pile every month for a total of 6 months). Comparisons were conducted for three years: 1995 through 1997. Data were analyzed by ANOVA procedures using the software package from Statistical Analysis System (SAS, Cary, NC, USA).
RESULTS
After 6 months of storage at 10C ± 1C, the weight losses from potatoes stored using the Shira Aeroponics, the spinning disk humidifier, and no added humidity were 2, 7 and 12%, respectively (Table 1). The firmness of the tubers expressed in newtons (N) required to penetrate them before storage was 76 N before storage, and after six months in rooms A, B and C was 74, 68 and 63 N, respectively (Table 1). After six months of storage, the firmness of the tubers before storage was not significantly different from that of the tubers which were stored using the Shira Aeroponics, but was significantly higher than in the other two storages, according to Fisher's protected least significant difference test (P=0.05). During the three years of comparison, similar results were obtained each year (no significant differences in the data were found between years).
DISCUSSION
It is necessary to add moisture to potato storage rooms since storage at RH of 92-94% or less causes dehydration, shrinkage and weight loss (Meijers, 1987; Rastovski,1987). A common humidifying technique utilizes a spinning disk that sprays water through the ventilation system (plenum) (Afek and Warshavsky, 1998). However, the droplets produced are large. The droplet size was evaluated according to a method developed by Spraying System Co. (North Ave. at Schmale Rd. P.O. Box 7900, Wheaton, Illinois 60189-7900, USA). They established 10 categories of droplet size according to the humidity produced by these droplets. The droplets produced by a spinning disk were Number 6: “wet fog? having sizes of 10 - 50 m . When the RH increases to 94%, free water (condensation) starts to accumulate on the tubers, making them susceptible to dry and soft rot diseases (Afek and Warshavshky, 1988; Meijers, 1987). Furthermore, because of the heavy weight of the droplets, much of the water sprayed by this humidifier is deposited on the floor.
The Shira Aeroponics System produces droplets of very small mass: less than 10 m diameter. Such small dr, behaving like a dry cloud, generate 96-98% RH without depositing free water on the tubers. In such high RH, the atmosphere in the storage room can absorb little more moisture from the tubers whereas air with a low RH can take up a great deal of moisture (Es and Hartmans, 1987c) thus reducing weight loss to a minimum.
LITERATURE CITED
Afek, U. and S. Warshavsky, 1998. Problems in storage of potatoes in Israel. In: Levy, D. (ed.). Potato in Hot Climate. Israel Agresearch, Journal of the Agricultural Research Organization 9: 97-114. (in Hebrew, English abstract) Agricultural Research Organization (ARO), The Volcani Center, Israel.
Brook, R.C., R.J. Fick and T.D. Forbush. 1995. Potato storage design and management. Am Potato J 72:463-480.
Es, A.van and K.J. Hartmans., 1987a. Starch and sugar during tuberization, storage and sprouting. In: Rastovski, A. and A. van Es (Eds), Storage of Potatoes: Post-harvest Behavior, Store Design, Storage Practice, Handling. Pudoc, Wageningen, The Netherlands, pp. 79-113.
Es, A.van and K.J. Hartmans. 1987b. Dormancy, sprouting and sprout inhibition. In: Rastovski, A. and A. van Es (Eds), Storage of Potatoes: Post-harvest Behavior, Store Design, Storage Practice, Handling. Pudoc, Wageningen, The Netherlands, pp. 114-132.
Es, A. van and K.J. Hartmans. 1987c. Water balance of potato tuber. In: Rastovski, A. and A. van Es (Eds), Storage of Potatoes: Post-harvest Behavior, Store Design, Storage Practice, Handling. Pudoc, Wageningen, The Netherlands, pp.141-147.
Graee T. and L.I. Aarseth. 1993. Universal stores for fresh plant products. Melding Inst. Tekniske-Fag Norges Landbrukshogskole. 20-21, 28 pp. (in Norwegian). Kleinkopf, G.E. 1995. Early season storage. Am Potato J 72:449-462.
Kleinkopf, G.F. 1995. Early season storage. Am. Potato J. 72:449-462.
Meijers, C.P. 1987. Diseases and defects liable to affect potatoes during storage. In: Rastovski, A. and A. van Es (Eds.), Storage of Potatoes: Post-Harvest Behavior, Store Design, Storage Practice, Handling. Pudoc, Wageningen, The Netherlands. pp. 148-174.
Rastovski, A. 1987. Storage losses. In: Rastovski, A. and A. van Es (Eds.), Storage of Potatoes: Post-Harvest Behavior, Store Design, Storage Practice, Handling. Pudoc, Wageningen, The Netherlands. pp.177-180.
Rastovski, A. and A. van Es (Eds.) 1987. Storage of Potatoes: Post-Harvest Behavior, Store Design, Storage Practice, Handling. Pudoc, Wageningen, The Netherlands. pp. 141-147.
Rees, T., W.L. Dixon, C.J. Pollock and F. Franks. 1981. Low temperature sweetening of higher plants. In: J. Friend and M.J.C. Rhodes (Eds.), Recent Advances in the Biochemistry of Fruits and Vegetables. Academic Press, London. pp. 41-61.
Villa, L.G. and F.W. Bakker-Arkema. 1974. Moisture losses from potatoes during storage. ASAF Paper No. 74-6510.
Table 1. Weight loss (WL) and firmness (FIR) of potatoes, expressed as newtons (N) required to penetrate the tubers before storage1 and during 6 months of storage at 10C ± 1C and humidification technologies.
Weight loss (%) and Firmness (N) of potatoes during storage2
Humid Tech. |
1 month WL FIR |
2 months WL FIR |
3 months WL FIR |
4 months WL FIR |
5 months WL FIR |
6 months WL FIR |
| Tabor | 0.7 a 76 a3 | 1.0 a 76 a | 1.2 a 75 a | 1.5 a 75 a | 1.7 a 74 a | 2.0 a 74 a |
| Spin | 1.5 b 75 a | 3.0 b 73 b | 4.0 b 72 b | 5.0 b 70 b | 6.0 b 69 b | 7.0 b 68 b |
| None | 3.0 c 71 b | 5.0 c 68 c | 7.0 c 66 c | 9.5 c 65 c | 10.5 c 64 c | 12.0 c 63 c |
1The firmness before storage was 76 N.
2 Numbers are means of three replicates. Three sacks of 50 kg in each replicate from different depths between 30-120 cm.
3Different letters within a column indicate significant differences among the numbers, according to Fisher's protected least significant difference test (P=0.05).
Note: Published with author's permission