Apples still represent one of the most important fresh produce types within the fresh fruit and vegetable category. Standard quality control for apples is usually based on setting a numerical lower limit for firmness, soluble solids content, starch pattern index, colour and mass. All these factors affect quality but are commonly measured using rather rudimentary equipment which is known to have some flaws.
This said, recent published research has shown that apple firmness is the most important factor in determining consumer acceptance of apples (Harker et al, 20081). Soluble solids content and acidity also play a role in defining consumer preference within specific cultivars. Mealiness is a negative attribute. There has been a positive trend towards consumers choosing to purchase firm/crisp apples, while a small proportion still value softer and more aromatic cultivars, which are inherently more challenging to store.
However, is the consumer driving the selection of firmer varieties or is it availability (ie what they are presented with) and/or standardisation/consistency? Product quality is only one aspect that governs consumer choice - price, promotion, place and packaging are also important.
In addition, availability is also key. This has been increased through cold storage, ethylene control/suppression and controlled atmosphere (CA) storage such that consumers are often faced with fruit from both hemispheres.
Consumers thus have to choose - consciously or not - between short- and long-term stored fruit (Varela et al, 20082). However, the notion that longer-stored produce is less fresh and thus inferior does not always stand true.
Time is only one factor that governs post-harvest quality. The storage conditions, post-harvest treatments and handling are the defining criteria which ultimately affect quality.
Significant advances in post-harvest technology have reduced wastage and better maintained quality, yet extension of storage life and thus availability of a consistent product are still primary concerns. Since the first work by Kidd and West after World War One on developing CA storage to the recent advances in usage of the ethylene binding inhibitor 1-methylcyclopropene (1-MCP) it could be argued that many of the fundamental challenges faced today by the apple industry have changed little in the past 90 years.
A 1930 report by the now-defunct Department of Scientific & Industrial Research (Food Investigation Report No. 38, J Baker) stated that for apples: "The most important losses are caused by over-ripeness, fungal rotting, bitter pit and internal breakdown, freezing and development of scald; Jonathan spot and brown heart are also found. Bitter pit and internal breakdown are most common in certain varieties, but all varieties are liable to suffer from fungal rotting. A survey of the present knowledge with regard to each of these abnormalities is available, with suggested remedies, but it is emphasised that in many cases more work is required before marked improvement may be expected."
Superficial scald is a physiological storage disorder which still presents a serious problem to the apple industry, especially for certain susceptible cultivars that are valued by consumers (eg Bramley and Granny Smith).
Scald was one of the major causes of wastage until the discovery of diphenylamine (DPA).
This antioxidant was routinely used by apple growers as a post-harvest dip to control scald. DPA is no longer used ubiquitously in the EU. Early work by US researchers (cf reviews by Blankenship and Dole, 20033; Watkins, 20064) and at Cranfield University (Dauny and Joyce, 20025) showed that 1-MCP could not only inhibit scald development but also increase storability (maintain firmness).
The commercial introduction of 1-MCP (SmartFresh) has revolutionised some sectors of the apple industry; however the strengths and weaknesses of the technology for other climacteric fruit need also to be considered.
In the second part of this feature, we look at the part ethylene can play to preserve the storage life of onions and potatoes.
NEW TECHNOLOGIES - IMPORTANCE OF ETHYLENE TO NON-CLIMACTERIC PRODUCE
Ethylene is a plant-growth regulator and is clearly fundamental to the post-harvest physiology of many fresh produce types; however the literature on the role of ethylene in non-climacteric produce is far from comprehensive.
Conventional wisdom stated that non-climacteric fruits and vegetables such as strawberry, onion, pineapple and potato were not responsive to ethylene. This is not the case, yet the magnitude of the ethylene-induced response as compared to climacteric produce (ie avocado, apple and banana) is less pronounced. More fundamental research is required.
It is likely that dormancy and sprout suppression in onions are under the control of a combination of factors, and the role of ethylene in these processes is as yet unknown.
The interaction between abscisic acid (ABA; Chope et al, 20066) and ethylene depends on tissue type and developmental stage. A single dose of the ethylene perception inhibitor 1-MCP was shown to reduce sprout growth in short-storing onion cv SS1 bulbs (Chope et al, 20077).
It was observed that higher concentrations of glucose, fructose and sucrose were maintained in 1-MCP-treated bulbs stored at 12 degsC (see graphs, p28).
Further investigation of the mechanism by which this occurred would be valuable, particularly since systems to maintain continuous low levels of ethylene (ca. 10 (mu)l l-1; equivalent to parts per million) in onion stores have recently been marketed as a revolutionary new method to suppress sprout growth in stored onions.
The contrast between the use of ethylene to extend storage commercially, and the use of an inhibitor of ethylene perception to extend storage demonstrates that there may be a dichotomy in the role that ethylene has to play in onion dormancy.
Potatoes are also low producers of ethylene, and have low sensitivity to it, but continuous exposure to ethylene has been shown (like with onions) to be an effective method of sprout control in potatoes (Chope and Terry, 20088).
Exposure of some varieties of stored potatoes to ethylene has the undesired side-effect of darkening the fry colour upon processing (such as Maillard reaction).
This is caused by an accumulation of fructose and glucose in tubers stored in the presence of ethylene.
Fry colour darkening can be prevented in potatoes stored at 9 degsC by application of 1-MCP prior to ethylene exposure and at subsequent monthly or bimonthly intervals. Thus, 1-MCP has been shown to reduce the rate of ethylene-induced sugar accumulation in potato tubers.
This contrasts with results reported by Chope et al (20077) where 1-MCP treated onions stored at 12 degsC maintained a higher concentration of carbohydrates, probably due to reduced carbohydrate catabolism compared with untreated controls.
It would be useful to study the carbohydrate concentration (including fructans) in onion bulbs exposed to continuous ethylene in store.
If, as in potatoes, the simple sugar concentration rises due to hydrolysis of storage carbohydrates then this could have a positive impact on taste perception and could be used as a marketing tool.
Although ethylene can be used as a method of sprout control for potatoes, it reduces the true dormant period (defined by the number of days from planting to shoot emergence) in comparison with control tubers stored in air.
Taken together, this suggests that ethylene breaks dormancy, but suppresses sprout elongation, and that 1-MCP may act differentially on different tissues, or impact on metabolic pathways other than those concerned with ethylene.
Research within the Plant Science Laboratory, Cranfield University, is ongoing to further elucidate the role ethylene plays in both onion and potato storage. Current projects are as follows:
- Defra HortLink HL0182 - Sustaining UK Fresh Onion Supply By Improving Consumer Acceptability, Quality and Availability. 2007-2010. Sponsors of this project are Defra and the Horticultural Development Company, along with UK industry partners.
- R412 - Understanding the Fundamental Role of Ethylene in Potato Storage 2008-2011. This project is sponsored by the Potato Council.
L Both of these projects will be discussed in more detail in forthcoming articles in Grower.
1. Harker, FR et al, 2008. Eating Quality Standards for Apples Based on Consumer Preferences. Postharvest Biology and Technology 50, 70-78.
2. Varela, P, Salvador, A and Fiszman, S, 2008. Shelf-life Estimation of 'Fuji' Apples. Postharvest Biology and Technology 50, 64-69.
3. Blankenship, SM and Dole, JM, 2003. 1-Methylcyclopropene: A Review. Postharvest Biology and Technology 28, 1-25.
4. Watkins, CB, 2006. The Use of 1-Methylcylcopropene (1-MCP) on Fruit and Vegetables. Biotechnology Advances 24, 389-409.
5. Dauny, PT and Joyce, DC, 2002. 1-MCP Improves Storability of 'Queen Cox' and 'Bramley' Apple Fruit. HortScience 37, 1082-1085.*
6. Chope, GA, Terry, LA and White, PJ, 2006. Onion Bulb Storage Is Related to a Temporal Decline In Abscisic Acid Concentration. Postharvest Biology and Technology 39, 233-242.*
7. Chope, GA, Terry, LA and White, PJ, 2007. The Effect Of 1-Methylcyclopropene (1-MCP) On The Physical and Biochemical Characteristics of Onion cv SS1 Bulbs During Storage. Postharvest Biology and Technology 44, 131-140.*
8. Chope, GA and Terry, LA, 2008. The Role of Abscisic Acid and Ethylene in Onion Bulb Dormancy and Sprout Suppression. Stewart Postharvest Review 2: 5.*
* Articles available on request from Dr Leon Terry at Cranfield University.