Quality improvement through postharvest management

 

 

Carlos Sanz

Instituto de la Grasa (CSIC), Department of Physiology and Technology of Plant Products, Padre GarcíaTejero, 4, 41012-Seville, Spain.

 

 

Abstract

Purpose of review: Postharvest operations are currently aimed at maintaining harvest quality as well as possible for as long as practical. This review shows a different perspective, considering postharvest management as a way to improve quality, especially organoleptic quality, thus making the product more attractive to the consumer.

Findings: A large number of papers have been published during the past few years describing the best postharvest conditions that may be used to maintain the quality of fruits and vegetables. In some cases an improvement in quality to meet consumer demands is met by applying specific postharvest conditions.

Directions for future research: Specific conditions for postharvest treatment and storage of horticultural commodities should be explored in order to enhance the organoleptic attributes of products. This might reduce the pressure that currently exists to find new cultivars with an improved flavour.

 

Keywords: fruits; vegetables; quality improvement; postharvest management

 

Stewart Postharvest Review 2005, 3: 8

Published online 01 October 2005

DOI: 10.2212/spr.2005.3.8

 

 

 

Introduction

The aim of the production, handling and distribution of fruit and vegetable products is to satisfy consumers. Based on an improved understanding of postharvest biology, significant advances in postharvest technology that help to maintain quality and safety of fresh fruits and their products have been made. Traditionally, different tools have been used in postharvest management to prolong product shelf-life attending almost exclusively to the visual appeal of fruits and vegetables (colour, defects, and decay). As a result, many desirable attributes, such as aroma and taste that impart the fresh-like characteristics to fruits and vegetables are lost in the process. Washington State `Delicious' apple sales and market shares have experienced significant losses in recent years, which has been partially attributed to lack of post-storage quality and its influence on consumer acceptance. Controlled atmosphere (CA) storage conditions for apples have reduced post-storage volatile production when compared with those stored in refrigerated air [1–3].

 

Historically, most scientific efforts were aimed at limiting undesirable storage disorders throughout extended storage periods and retaining the more intuitive aspects of apple quality such as colour, texture, sweetness, and acidity rather than general palatability for highest-grade fruit. There are other examples of adverse effects of CA storage on quality attributes though extending shelf-life. Pomegranates stored in CO₂-enriched air showed a lighter colour associated with a lower anthocyanin concentration probably due to suppression of the biosynthesis of these pigments [4]. Similarly, limitations in marketability were found in red raspberries after eight days storage in 20% CO₂; in this case losses were due to discoloration [5]. Gil et al. [6] also reported a reduction in anthocyanin content in the internal tissues of strawberries as a consequence of storage at 5ºC in CO₂-enriched air. Ahumada et al. [7] have also reported that treatment of grapes with insecticidal CAs (0–0.5% O₂, 35–100% CO₂) provided significant insect control although negatively affecting consumer preference; this was mainly due to the decrease in soluble solids content and a high production of fermentative products.

 

 

Postharvest quality improvement

One of the challenges for postharvest science in the future is not only to add the fresh-like characteristics back to horticultural crops but also to enhance these qualities. Introduction of desirable traits into horticultural crops by genetic engineering technology makes commercialisation of this type of product quite unlikely because of the current public debate over genetically modified horticultural commodities. An alternative is to look for specific postharvest treatments that will improve the organoleptic attributes of the product to meet consumer demands.

 

Modified atmosphere

There are several examples in literature where an improvement in quality is obtained through the use of different techniques. Among treatments with plant regulators or hormones, the classical and very well-known example is the use of ethylene for the degreening of citrus fruits. There is an improvement in appearance since the consumer demands oranges with an orange colour. Besides ethylene, exposure to methyl jasmonate (MJ) vapours seems to be very promising for altering pigment changes in fruit peel. Our group has observed that MJ produced a degreening effect on apples [8], and Fan et al. [9] proposed a combination of MJ and ethephon treatments to promote commercially desirable colour changes in apples while minimizing adverse effects on other fruit quality attributes. On the other hand, González-Aguilar et al. [10] found that treatments with MJ, alone or in combination with modified atmosphere (MA), inhibits fungal decay and prevents chilling injury development of papaya fruit during storage at 10ºC, prolonging shelf-life and increasing visual appearance. The authors found similar results when MJ was applied vapours to mango fruits [11]. MJ induced resistance to chilling injury and fungal development in mangoes during storage at 5ºC. Fruits attained a better overall quality mainly due to improved colour development.

 

The use of non-conventional MA could provide an enhancement of fruit quality. Rocculi et al. [12] demonstrated that the use of nitrous oxide (65–90%) gave rise to an increase in firmness, total soluble content, and whiteness index in apple slices and a decrease in browning. On the other hand, conventional MA storage also promoted changes in the content of fruit pigments. Recommended MA conditions for strawberry (5% O₂ and 20% CO₂) causes a decrease in pigment contents according to Gil et al. [6], but this effect can be overcome when using MA storage with high levels of O₂ and CO₂ that produce higher levels of anthocyanins [13]. This phenomenon may be related to the higher antioxidant capacity of fruits in these conditions found by Stewart et al. [14]. However, the benefits of high O₂ in enhancing anthocyanin content and controlling Botrytis cinerea might be outweighed by detrimental effects on flavor quality due to the accumulation of fermentative volatiles [13, 15]. It has been observed that MA with high O₂, alone or in combination with high CO₂, produced an increase in the contents of acetaldehyde, ethanol and ethyl acetate as it occurred in anoxia conditions. Further biochemical and molecular studies in strawberry showed that high CO₂ conditions induced the expression of a pyruvate decarboxylase gene while alcohol dehydrogenase activity remained almost unaltered [16]. Besides, either high or low O₂ conditions gave rise to an increase of strawberry alcohol dehydrogenase activity, while alcohol the ester-forming activity, acyltranferase, suffered just a slight increase. These data explained the high levels of fermentative products in MA either with low or high O₂ and the different aroma volatile patterns found among esters [17].

 

The beneficial effect of CA on resveratrol content in grapes was recently reported by Artés-Hernández et al. [18]. This effect is higher when using O₃ shocks, providing an increase in total anthocyanins. Resveratrol is a stilbene compound that showed antimelanoma properties [19] and is inducible in response to stress conditions, so that it is a potential target for increasing health-promoting properties of fruits or fruit products through postharvest treatments.

 

Ultraviolet radiation

A patent has been developed for the postharvest treatment of fruits and vegetables using pulses of ultraviolet (UV) radiation [20] and has been applied for the stilbene-enrichment of red wines [21]. This novel technique has already been proposed for improvement of strawberry appearance [22]. Treatment of strawberries with UV light reduced rots and alleviated colour problems in two strawberry cultivars characterised for a low level of anthocyanins, both internal and external, without impairing other fruit quality attributes. It has been suggested that there is a link between anthocyanin synthesis and antifungal resistance, probably related to the level of proanthocyanidins [23]. UV-C irradiation has also been proposed as a new tool for controlling the decay of grapefruit without affecting soluble solids concentration and titratable acidity [24]. This effect may be related to the observed accumulation of phytoalexins. Its use is recommended for controlling decay in citrus fruits since the current use of fungicides is continuously challenged by health authorities. Similarly, González-Aguilar et al. [25] proposed exposure of ‘Tommy Atkins’ mangoes to UV-C irradiation before cold storage as an effective and rapid method to reduce decay without affecting quality attributes.

Temperature control

Another way in which produce quality can be improved is by controlling the temperature of the product, either using cold storage or by means of heat treatments. Strawberry colour increased around 10% after harvest due to the accumulation of anthocyanins during the first two days and then declined slowly coinciding with the senescence of the fruit. However, lowering the temperature of the fruit to 2ºC gives rise to an increase in the content of these pigments close to 30% during the first three days, keeping a high level of anthocyanins up to 10 days of cold storage when the fruit appears senescent [26].

 

Postharvest rot is a major factor limiting the extension of storage life of many freshly harvested fruits and vegetables and heat treatment appears to be one of the most promising means for its control [27–29]. However, there are some examples where heat treatments are useful not only for decay control, but also for improving other fruit quality parameters. Physical treatments applied to the fruits might alter the enzymatic systems by partial or complete inhibition of their activities causing changes in the aroma or pigment profiles [30–33]. Heat treatment of ‘Hass’ avocado fruit during commercial disinfestation for fruit fly improved both internal and external quality while reducing rots in ripe fruits [34]. Cold disinfestation causes severe skin damage in this fruit, thus impairing avocado marketing. A patent was developed for simultaneously rinsing and disinfecting bell pepper using hot water [35]. This method significantly reduced incidence of decay and improved the general appearance and firmness of different cultivars of these fruits [36]. On the other hand, Fernández-Trujillo and Artés [37] obtained a one-week prolongation of the shelf-life of ‘Paraguayo’ peaches over a storage period of three weeks’ by applying intermittent warming during cold storage. Fruits attained better visual quality and flavour than fruits at harvest. Intermittent warming induced a controlled reduction in flesh firmness which resembled that obtained in normal ripening, and which could enhance customer satisfaction. Similarly, intermittent warming was observed to strongly reduce chilling injury in sweet pomegranates, allowing maintenance of the visual appearance and flavour of fruit after 90 days of storage, significantly increasing anthocyanin concentrations in the juice [38]. The quality of apple fruits, traditionally well-stored at low temperatures, could also benefit from heat treatments. Klein and Lurie [39] found that ‘Anna’ and ‘Granny Smith’ apples stored at 46ºC for 12 hours or at 46ºC for 24 hours, before storage at 0ºC, were firmer at the end of storage and had a higher soluble solids-acid ratio and a lower incidence of superficial scald than unheated fruit.

 

Heat treatments ­can also be applied to fresh fruits for improving the quality of fruit-derived products. In the scope of current search being carried out by our group for treatments on olive fruits to modulate bitterness intensity in virgin olive oils, it was found that water-heat treatments prior to processing considerably reduced bitterness in virgin olive oil. Bitterness is a common and desirable attribute in these oils when present at low to moderate levels, but it is rejected by consumers when present at high intensity. Besides, water-heat treatments also affect other quality traits. These treatments provide an increase in both of chlorophyllic compounds and carotenoids in virgin olive oil [33], so that the final product has an enhanced fruit-juice-colour appeal. Aroma volatile composition is also modified by water-heat treatments, characterised by a strong decrease of five- and six-carbon aldehydes and alcohols [30]. This fact and the unaltered levels of esters help the oils to attain a more fruity aroma due to a decrease of the green-odor notes. Results obtained suggest that the temperature of the olive fruit when entering the crusher, the first step for virgin olive oil production, is the only factor responsible for these effects, and that increasing fruit temperature might reduce the activity level of the lipoxygenase-hydroperoxide lyase system during crushing. Research is now aimed at achieving a better control of olive fruit temperature at crushing in order to modulate taste, aroma, and colour of virgin olive oil to satisfy consumer demands.

 

Conclusions

Postharvest operations are currently designed to maintain visual appeal of fruits and vegetables, so that many desirable attributes, such aroma and taste, that impart the fresh-like characteristics, are lost in the process. One of the challenges for postharvest science in the future is to add those fresh-like characteristics back to horticultural crops. In addition, there are many examples in the literature that point out to the possibility of an improvement of product quality, especially organoleptic quality, through postharvest management by means of using specific postharvest treatments in order to enhance the sensory attributes of the product according to the consumer demands.

 

Acknowledgements

Part of the data presented belongs to a work supported by Research Project AGL2002-02307 from Programa Nacional de Recursos y Tecnologías Alimentarias funded by the Spanish Government, and Research Project CAO01-004 from Consejería de Agricultura y Pesca from the Andalusian Government.

 

 

 

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