Postharvest heat treatments: introduction and workshop
summary
I.B. Ferguson
a,*, S. Ben-Yehoshua
b, E.J. Mitcham
c, R.E. McDonald
d,
S. Lurie
baHortResearch,Pri6ate Bag921609,Auckland, New Zealand
bPosthar6est Science of Fresh Produce,Volcani Centre,P.O.Box6,Bet Dagan50250,Israel cDepartment of Pomology,Uni6ersity of California,Da6is,CA965616-8683,USA
dUSDA,ARS,2101South Rock Rd.,Ft Pierce,FL34945,USA
www.elsevier.com/locate/postharvbio
1. Introduction
Wide international interest in heat treatments for maintenance of postharvest quality, disease control, and as a quarantine technology, was reflected in the range of papers presented at this BARD Workshop on Postharvest Heat Treat-ments. Heat treatments are currently used com-mercially in several countries, for example as hot water dips, hot water brushing techniques, and hot air treatments. Research is continuing on these methods, on new techniques, and on the responses to high temperature treatments of fruit and vegetables, fungal pathogens, and insects. It was therefore timely that a workshop devoted to heat treatments should be held, and appropriate that it should be held in Israel, where much of the recent research has been conducted.
This Special Issue of Posthar6est Biology &
Technology comprises both review and research
articles, based on papers given at the Workshop.
Since substantial time was set aside for discus-sions, we have assembled this summary paper, covering some of the highlights, some of the gaps, and some speculation regarding the future.
2. Plant responses to heat and development of heat treatments
The heat shock response is manifested in most living organisms as induction or enhanced synthe-sis of heat shock proteins (HSPs). These are be-lieved to confer tolerance to heat by protecting proteins from irreversible denaturation and break-down. Although this specific role has not been well demonstrated in plants, there are many in-stances where both HSP gene expression and protein synthesis are associated with high temper-ature exposure of various plant parts, including fruits. With postharvest heat treatments, and ex-posure of fruit to high temperatures on the tree, HSP transcripts and protein levels in such fruits have been shown to increase. These responses have been reviewed in the past (e.g. Lurie, 1998) and were highlighted at the meeting by Paull (US), McCollum (US), and Woolf (New Zealand) (see also Paull and Chen, 2000; Woolf and Fergu-son, 2000; both in this issue).
* Corresponding author. Present address: The Horticulture and Food Research Institute of New Zealand, Mt Albert Research Centre, Private Bag 92169, Aukland, New Zealand. Tel.: +64-9-8154200; fax:+64-9-8154202.
E-mail address:iferguson@hort.cri.nz (I.B. Ferguson).
Our assumptions that HSPs are critical to fruit postharvest heat responses should, however, be qualified. For a start, we only have correla-tive evidence from gene expression and protein synthesis studies. Nover (Germany) highlighted the importance of the dynamics of heat shock granules, containing protective HSPs together with housekeeping genes, as a major response. This has not been investigated in terms of fruit responses. Until we can modify gene expression, we will not know the true extent of HSP in-volvement.
We sometimes forget the wider consequences of HSP induction, such as diversion of protein synthesis; the heat response involves a substan-tial shift in protein synthesis away from normal turnover to that of HSP synthesis. In cut lettuce, browning is inhibited by prior heat treatment. Saltveit (US; Saltveit, 2000 in this issue) sug-gested that when a wounding response is re-quired, the diversion of protein synthesis is such as to prevent increases in PAL and PPO enzyme protein levels and activity. Similarly, an accumu-lation of heat shock transcription factors may persist and allow this protein diversion to occur over extended times. Thus the dynamics of protein synthesis as a response to stress may determine some physiological responses to heat.
Concentrating on HSPs may also deflect our attention away from other metabolic heat re-sponses which may determine tolerance. A wide range of fruit ripening processes are affected by heat, such as ethylene synthesis, respiration, fruit softening and cell wall metabolism, pigment metabolism, and volatile production (Paull, US; Paull and Chen 2000 in this issue). Heat effects on carbohydrate metabolism can be included in this list; heat treatments can induce the riciness syndrome in mango fruit, associated with the persistence of starch granules (Jacobi, Australia). The involvement of changes in membrane per-meability and function, and of other stress proteins, were raised in discussion, but remain unresolved issues in determining the mechanisms of the heat response.
One of the most interesting aspects of postharvest heat treatments is the beneficial ef-fects in reducing chilling injury in a range of
fruits during subsequent low temperature stor-age. A role for HSPs has been implicated (e.g. Lurie, 1998), arising from persistence of both HSP transcripts and protein at low temperatures after heat treatments, and in avocado and apple fruit which had experienced high temperatures in the field (Woolf, New Zealand; Woolf and Fer-guson 2000 in this issue). A direct role for HSPs in low temperature tolerance still needs to be demonstrated. Nover (Germany) suggested that HSPs induced by heat are likely to be different genes from those induced by low temperature, although there is increasing evidence for low temperature induction of recognized HSPs (e.g. Li et al., 1999). Although untested, it is also possible that heat shock granules may persist at low temperatures, perhaps providing a basis to reduced chilling injury.
The interaction of heat and low temperature occurs in many different ways; there may be no single mechanism involved. For example, ex-posed avocado fruit, or sides of fruit, show less postharvest chilling injury (Woolf, New Zealand; Woolf and Ferguson, 2000). By contrast, both citrus and persimmon fruit show chilling injury (skin pitting and flesh gelling respectively) on the exposed sides of the fruit. Postharvest heat treat-ments show benefits against chilling injuries with a variety of symptoms such as skin pitting, skin and flesh browning, and water-soaking.
3. Heat treatments and disease development
Postharvest heat treatments can have positive effects in reducing pathogen levels and disease development. One way this may occur is through the physical effects of heat on the fruit surface. Heat treatments, particularly those using addi-tional physical treatments such as hot water brushing (Fallik, Israel), can result in occlusion of cuticular fractures and microwounds, thus pro-tecting the fruit from the prevailing wound patho-gens (Schirra, Italy; Schirra et al., 2000 in this issue). This effect seems to involve melting of the cuticular waxes, evident in both hot water and hot water brushing treatments.
Direct effects of heat on fungal pathogens have not been studied very extensively. This was high-lighted by Fallik (Israel; Schirra et al., 2000 in this issue). The main effects are on inhibiting or re-tarding germ tube elongation, or on killing spores, thus effectively reducing inoculum size and subse-quent lesion development. More research on di-rect effects on pathogenic organisms may produce better treatments, and help resolve issues of acqui-sition of thermotolerance by such organisms, a major issue in heat-related control of insects on fruit.
Apart from physical effects, and direct effects on the pathogenic organisms, the other major way in which heat may be effective in reducing disease is in inducing defense mechanisms. It is likely that there is a multi-layered suite of mechanisms that the fruit has developed against pathogen and sect attack (Ben-Yehoshua et al., 1997). For in-stance, in citrus, constitutive antifungal materials act as a first line of defence against invading pathogens, followed by induction of several addi-tional mechanisms such as the building of a pas-sive barrier to the pathogen by production of lignin-like polymers, synthesis of phytoalexins, and biogenesis of several pathogen-related proteins such as chitinase.
Contradictory evidence was obtained in two laboratories regarding the direct effect of heat in inducing the production of phytoalexins in apples. Fallik (Israel) suggested that hot air treatment for 3 days induced such production in ‘Golden Deli-cious’ apples. Conway (US), however, did not find
phytoalexin induction in ‘Gala’ apples after simi-lar treatment. Simisimi-larly, results on citrus fruits also did not show that heat per se induced the production of phytoalexins; Ben-Yehoshua et al. (1998) reported that hot air or hot water heat treatments did not by themselves induce the pro-duction of the phytoalexins scoparone and scopo-letin, unless fruit were previously inoculated byP.
digitatum. It was interesting that UV illumination
itself, unlike heat, could mimic the pathogen and induce higher scoparone and scopoletin production.
These heat treatments did not by themselves induce the production of lignin-like materials un-less in combination with inoculation or wounding. This combined treatment was effective in inducing all the previously cited mechanisms of defence and prevented development of the pathogen P.
digitatum in various citrus fruits. Alternatively,
Porat (Israel; Porat et al., 2000) suggested that hot water brushing by itself induced the biogene-sis of the pathogen-related proteins chitinase and
b-1,3 glucanase in ‘Star Ruby’ grapefruit and also conferred greater resistance against Penicillium
digitatum. It is also likely that heat treatments
may favour the development of one pathogen over another in the treated commodity (Rodov, Israel).
Just as heat treatments are being used in combi-nation with other treatments such as controlled atmospheres for insect control (Nevins, 2000; Shellie and Mangan, 2000 both in this issue), so combined treatments are being investigated for pathogens. Conway (US; Leverentz et al., 2000 in this issue) showed that combining a curing treat-ment of 38°C for 4 days with a pathogen antago-nist such as Pseudomonas syringae provided optimal control ofPenicillium expansumon ‘Gala’ apples.
4. Heat treatments for insect control
change in temperature under the Ramped Func-tion will likely be different, and the rate of heating must be considered in predicting mortality (Neven, US; Neven, 2000 in this issue). The change in the thermal tolerance of insects during various heating profiles has not been considered in the development of quarantine heat treatments. Exposure of insects to intermediate temperatures (30 – 42°C) can condition the insects to better withstand exposure to lethal temperatures (\
42°C).
While determining the mortality of insects fol-lowing exposure to a constant temperature will not predict mortality following a ramped heating in the commodity, data on response to Step Func-tions can be used to model insect mortality for any type of Ramp Function (Waddell (NZ), and Laidlaw (US); Waddell et al., 2000 in this issue). (Waddell et al. (2000) in this issue) clearly demon-strated that it does matter how a target heat dose is reached. The longer the insects were in the 32 – 42°C range, the more time at 46°C was re-quired to reach LT99 for Queensland fruit fly
(Bactrocera tryoni). Heating treatments should be
defined as the combination of heating rate fol-lowed by lethal stress as opposed to the current specifications of time at a target fruit centre tem-perature. Modelling insect response to heating using Step Functions would allow the mortality to be predicted for heating in any commodity and regardless of the load factor (heating rate). The effects of both conditioning and lethal tempera-tures are included in the model. If extensive Step Function data were available for the important quarantine pests, this could eliminate a consider-able amount of empirical testing. Similar mod-elling might be used to determine the tolerance of fruit to heat treatments (Raham et al., 2000).
While it has been known for many years that insects produce HSPs in response to heat stress, this does not explain the total impact of high temperatures on insect physiology (Neven, 2000 in this issue). The response of the insect to heat involves metabolic responses, nervous system sponses, endocrine effects, developmental re-sponses, and respiratory changes. While plants can be conditioned to tolerate cold temperatures by exposure to heat shock, studies indicate that
insects may not have this cross-resistance to mul-tiple stresses. It is clear that more work is needed on the physiological response of insects to heat.
The effects of conditioning become lost with time, and this rate of loss may also be different between insects and plants. Following condition-ing at moderately high temperatures to protect against heat stress, Queensland fruit flies lost 80% of the conditioning effect after 10 h at 25°C (Waddell, New Zealand). For avocado fruit, simi-lar conditioning protected the fruit for up to 5 days at 15°C (Woolf, New Zealand). While the temperatures following conditioning were not the same in the above examples, the magnitude of difference in decay rate indicates potential for developing protocols to enhance the tolerance of the commodity without enhancing the tolerance of the insect pest. Further work is needed in this area.
The media used to heat the commodity can have an impact on both product tolerance and insect mortality. The heat load required with a hot water treatment is less than that required with a hot air treatment; however, the faster rate of heating in water does not completely explain the difference. Modification of the internal atmo-sphere of the commodity during hot water treat-ments enhances the effect of the heat treatment (Shellie, US). Identical heat doses from vapour-saturated forced air or from hot water can result in significantly different internal O2and CO2
lev-els in the fruit (citrus) and different mortalities of fruitfly larvae (greater with hot water); heat trans-fer is another factor which must be considered with regards to heating technology (Shellie (US); Shellie and Mangan, 2000 in this issue). High temperature CA treatments also impose a double stress on the insect. The modified atmosphere may effect the insect’s ability to adapt to the heat exposure (Neven, 2000 in this issue).
longer with a larger heat load, as slower heating rates are less effective. It is important not to assume that the response of a laboratory colony is indicative of wild populations that may be more or less susceptible. The method of insect infesta-tion can also affect the results.
Considerably more information is needed on the response of insects to heat and to multiple stresses such as combined heat and controlled atmospheres, or heat and cold. The response of insects to conditioning temperatures and lethal temperatures should be determined to allow mod-elling of mortality for the important quarantine insect pests. This will allow effective treatments to be predicted depending on the heating rate, but regardless of the commodity or method of apply-ing heat. However, the impact of modified atmo-spheres in heat treatments must also be considered. Additional work on insect response to heat plus controlled atmospheres is needed, in-cluding the effect of controlled atmospheres on the insect’s ability to adapt to heat stress. The rate of decay of conditioning effects at various temper-atures must be determined for quarantine pests and commodities to allow enhancement of product heat tolerance over insect heat tolerance.
5. Heat technology and economics
Discussion and interest in the research aspects of heat treatments can obscure the practicalities of establishing and running such a system. There is widespread use of hot air and hot water systems in several countries. The major requirements for heating technology are for systems which are ef-fective in terms of pathogen control or insect mortality whilst minimising thermal impact on product quality. At the same time, they must be economically viable.
The following information on a typical treat-ment system for mangoes provides some insight into the technicalities of a heating operation. Whitworth (US) described a typical plant with two tanks with four positions for a total of eight treatment positions. Treatment time ranges from 70 to 90 min, depending on fruit size. After treatment, the product will be :46°C. Cooling
may be required, taking between 35 and 45 min, with the aim of returning the product to the original ambient temperature (in this example, 26 – 27°C) as soon as possible. On a 90 min treat-ment cycle, using a two-tank, eight-position sys-tem, production will be :3200 – 3600 kg/h, based on a treatment batch of about 900 kg. In an 8 h day, production would be about 25 600 kg, and in a 12 h day, 45 500 kg.
An alternative controlled atmosphere tempera-ture treatment system (CATT) was described which utilized forced hot air (Black, US; Nevens, US). Parameters monitored by the system include temperature, dew point, air velocity, oxygen, and carbon dioxide. Thermister probes are used to monitor fruit centre temperatures, and tempera-ture can be controlled90.2°C. As an example of treatment requirements, a commodity at 20°C heated to 47°C would take 28 kilowatt hours of heat per tonne, which would not be prohibitive, and the same amount to cool it down. For 10 and 20 ton chambers, treatment time for apples would be 3 h at a heating rate of 12°C per hour up to a final temperature of 46°C. This would amount to a total of 4 h when loading and unloading time and purge time to set atmosphere, were added in. This would allow three loads in a 12 h or six loads in a 24 h period. CA is established and controlled by vacuum-based purging, conventional nitrogen generation, and liquid CO2.
6. Summary
There continues to be considerable interest in heat treatments for disease and insect control in the research community, but adoption by industry has been slow. The continuing loss of postharvest chemicals for decay and insect control will in-crease commercial interest in these treatments. The potential for product damage from effective heat treatments, particularly on a commercial scale, can however be great. Increased knowledge of the development and decay rate of thermotoler-ance in plants, pathogens and insects may allow the development of more commercially feasible treatments.
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