Efficacy of Potassium Permanganate Impregnated into Alumina Beads to Reduce Atmospheric Ethylene

A systematic study was conducted on the ability of potassium permanganate absorbent to remove low levels of ethylene from the atmosphere. Absorption of potassium permanganate onto alumina beads by dipping in a saturated solution was maximal at 2 g/100 g after 2 hours at 20 °C and 4 g/100 g after 1 hour at 65 °C. Commercial alumina-based absorbents were found to contain potassium permanganate at 2.7 to 6.0 g/100 g suggesting many are prepared at elevated temperature. Trials in a closed system at 20 °C and 60% to 70% RH with alumina beads containing potassium permanganate at 4 g/100 g showed a logarithmic decrease in ethylene concentration with 90% of the ethylene removed after 2.5 to 3.0 hours. Relative humidity (RH) had a marked inverse effect on ethylene absorption with reactivity at 100% RH calculated to be 15% of that at 0% RH. Performance of potassium permanganate where ethylene was continually generated by a continuous fl ow of ethylene at 14 μL·h-1 through the container showed a steady state was attained within 1 hour and maintained for 24 hours. Ethylene removal increased linearly with bead weight and ranged from 30% with 1 g to 90% with 50 g. Examination over 20 days showed a continuing decrease in rate of ethylene removal which after 14 days had declined to 10% of incoming ethylene although 44% of the original level of potassium permanganate still remained in the beads. Calculations based on known endogenous ethylene production rates suggest that at 20 °C and 90% RH, use of a potassium permanganate-alumina absorbent would be benefi cial with produce having a low level of ethylene generation. Suitability for larger packages of produce generating higher ethylene levels is questionable as >1 kg of absorbent may be required. Reducing the level of ethylene in the atmosphere around harvested horticultural commodities has been well recognized to delay the development of senescence of non-climacteric produce and the onset of ripening in climacteric fruits. A concentration of ethylene in air of 0.1 μL·L–1 is often quoted as the threshold level for physiological activity (Kader, 1985). However, from studies on bananas by Peacock (1972) and recently on a wide range of fruit and vegetables by Wills et al. (1999; 2001) any level of ethylene is considered to have a deleterious effect on produce with decrease in postharvest life linearly related to increasing log10 ethylene concentration. Many intervention strategies have been developed for protecting postharvest commodities from the detrimental effects of ethylene. Potassium permanganate is a stable purple solid that is a strong oxidizing agent and readily oxidizes ethylene. The ability of potassium permanganate to reduce ethylene concentration in the atmosphere around horticultural produce was fi rst demonstrated by Forsyth et al. (1967) on apples. The reduction in ethylene affected by addition of potassium permanganate was subsequently found to delay the ripening of many climacteric fruits such as banana (Scott et al., 1970), kiwifruit (Scott et al., 1984), mango (Esguerra et al., 1978) and avocado (Hatton and Reeder, 1972). Fewer studies have been conducted with non-climacteric produce but potassium permanganate has been found to extend postharvest life by retarding loss of green color and microbial wastage of lemon (Wild et al., 1976) and lettuce (Kim and Wills, 1995) and inhibiting rotting of strawberry (Wills and Kim, 1995). For potassium permanganate to be effective in oxidizing quite small concentrations of ethylene from the atmosphere around produce where natural convection and diffusion are the only driving forces giving contact between ethylene and the oxidant, the potassium permanganate needs to have a high surface area exposed to the atmosphere. This has been achieved by the absorption of potassium permanganate onto porous inert minerals such Received for publication 29 July 2003. Accepted for publication 12 Dec. 2003. as celite (Forsyth et al., 1967), vermiculite (Scott et al., 1970), alumina (Jayamaran and Raju, 1992), zeolite (Oh et al., 1996) and clay (Picon et al., 1993). A range of commercial potassium permanganate products is now available with a common carrier being alumina beads (e.g., Purafi l (Purafi l, Doraville Ga.), Circul-Aire (Circul-Aire, Montreal), Ethysorb (Molecular Products, Thaxted, Essex) and Bloomfresh (Ausdel, Cheltenham, Victoria)). However, these products are mainly used to remove organic contaminants from the atmosphere in ducted air conditioning systems and there has not been any meaningful commercial use with horticultural produce. Most published studies using a laboratory prepared or a commercial potassium permanganate product have evaluated the effect of single weight of product on a single produce with most papers not reporting the concentration of potassium permanganate in the product used and often not determining the effect on ethylene concentration. The most comprehensive studies on potassium permanganate were fi rstly by Lidster et al. (1985) who found that alumina pellets retained a much greater amount of potassium permanganate than expanded glass beads, the effi ciency of each product to reduce ethylene increased with temperature but contrasting effects were obtained with increasing relative humidity (RH). Secondly, Kavanagh and Wade (1987) studied 21 carrier materials and found large variation among carriers with those of lower bulk density and a higher capacity to absorb potassium permanganate being more effi cient in reducing ethylene levels. It is considered that the lack of systematic quantitative studies on the ability of potassium permanganate to absorb ethylene has hindered commercial use with horticultural commodities. The lack of such studies has meant that no recommendations have been published for the amount of potassium permanganate needed to achieve either a specifi ed decrease in ethylene concentration or an increase in postharvest life of a commodity. Further, it is noted that the labels of commercial products do not indicate the concentration of potassium permanganate although Abeles et al. (1992) report that it is typically 4 to 6 g/100 g. This paper quantitatively examined 129-Post 433 3/13/04, 12:25:16 PM 434 J. AMER. SOC. HORT. SCI. 129(3):433–438. 2004. the ability of alumina beads to absorb potassium permanganate and the effect of impregnated beads on the absorption of ethylene under a range of environmental conditions. Materials and Methods PREPARATION AND ANALYSIS OF POTASSIUM PERMANGANATE PRODUCTS. A range of laboratory prepared products was generated by dipping activated alumina beads (0.5 cm in diameter) (Sigma, St Louis Mo.) in a saturated solution of potassium permanganate (Grade 2 FSE, Homebush, Sydney) (6.4 g/100 mL (Lide, 1994) at 20 °C for 1) varying times between 3 min to 4 h, and 2) for 3 min, allowing to dry in air for 30 min and repeating the dipping and drying process for up to four times. The fi nal drying of the impregnated beads was achieved by allowing beads to dry in ambient air at 20 °C for 16 h. The effect of dipping temperature was examined by dipping in a solution containing potassium permanganate at 10 g/100 mL at 20, 65, and 90 °C for 1, 3, and 5 h at each temperature. A constant concentration of potassium permanganate was used even though at 20 °C some of the potassium permanganate was only in suspension while at the higher temperatures it was all dissolved (Lide, 1994). The color of potassium permanganate impregnated beads was assessed visually and with a color meter (Minolta Chroma Meter CR-300, Osaka) using Hunterlab L (lightness), a (green-red) and b (blue-yellow) color coordinate values. Whole or powdered beads were placed onto a small dish and the color values were recorded as the mean values of 10 measurements. From the L, a and b values, the color characteristics, chroma (C) = √(a2 + b2); and total color difference (∆E) = √[(L – Lo) + (a – ao) + (b – bo)], were calculated. The concentration of potassium permanganate was determined by grinding a sample of beads (2 g) with a mortar and pestle. A sample of powder (0.2 g) was placed in water made up to 100 mL in a foil covered volumetric fl ask, which was shaken for 2 min then centrifuged (J2-MC; Beckman, Palo Alto Calif.) for 45 min at 2,000 gn. The solution was placed into a cuvette and the absorbance at 528 nm determined with a spectrophotometer (Cary Win UV; Varian, Mulgrave, Victoria). The absorbance was quantifi ed by comparison with standard solutions of potassium permanganate. The concentration of potassium permanganate in some commercial products was also determined. EFFICIENCY OF ETHYLENE ABSORPTION. The ability of alumina beads impregnated with potassium permanganate at 4 g/100 g to remove ethylene from the surrounding atmosphere was determined using a closed system with an initial ethylene loading and a fl ow system with a constant ethylene concentration in the passing air stream. In the closed system, impregnated alumina beads (1 g) were placed as a single layer in a sealed 4-L container at ambient relative humidity (60% to 70% RH) at 20 °C and 80 or 150 μL of a gas containing 98% ethylene (BOC Gases, Sydney) was added. At regular intervals, a sample (1 mL) of the atmosphere was collected in a syringe and the ethylene concentration was determined by fl ame ionization gas chromatography (series 580; Gow-Mac, Bridgewater, N.J.) fi tted with a stainless steel column (180 × 0.3 cm) packed with activated alumina (80 to 100 mesh) (Alltech, Sydney) and operating conditions of column temperature 110 °C, injector and detector temperature 150 °C, nitrogen carrier gas fl ow velocity 30 mL·min–1, hydrogen fl ow rate 30 mL·min–1 and air fl ow rate 300 mL·min–1. Ethylene was quantifi ed by comparison of the peak height with that obtained for a standard gas mixture containing 0.16 ± 0.05 μL·L–1 (BOC Gases, Sydney). Ethylene was monitored until the concentration had declined to 0.005 μL·L–1, the limit of detection of the analytical method. The effect of RH on absorption effi ciency at 20 °C was determined by placing in the sealed container with potassium permanganate impregnated alumina beads, a dish containing a saturated solution of potassium hydroxide, sodium dichromate, barium chloride, potassium acetate and potassium carbonate to generate air of about 25%, 35%, 50%, 70%, and 90% RH, respectively (Dean 1995). The system was left to equilibrate for 16 h when the RH was measured using an RH probe (MultiMeterMate; Vaisala, Helsinki). Ethylene (150 μL) was then injected into the container and its concentration monitored every 10 min until 90% of the ethylene had been removed. In the fl ow-through system, air at 70% RH containing 0.7 μL·L–1 ethylene was passed through the 4-L container at 20 L·h–1. The weight of beads in the container was varied from 1 to 50 g, while maintaining a single layer of beads to maximise exposure to the atmosphere. The level of ethylene entering and leaving the container was monitored over 24 h. The longevity of the impregnated beads to absorb ethylene was determined on 1 g of beads, which were ventilated until the rate of ethylene absorption was greatly reduced. The total amount of ethylene absorbed and the proportion of potassium permanganate oxidized were calculated. The change in potassium permanganate content of alumina beads and bead color during exposure to ethylene was determined. Freshly prepared beads were placed in the fl ow-through system and the visual change in bead color was monitored on a 5 to 1 scale where 5 = 100% purple, 4 = 75% purple, 3 = 50% purple, 2 = 25% purple, and 1 = 100% brown. A sample of beads was removed at various visual color stages and Hunterlab color measurements were made on the intact beads and ground bead powder. The amount of potassium permanganate remaining in each bead sample was also determined. Table 1. Potassium permanganate uptake and color of alumina beads dipped in saturated potassium permanganate solution at 20 °C for various times. Values are the mean of three replications.