The market for ready-to-eat salads has grown rapidly in recent decades as a result of changes in consumer attitudes towards convenience, health and choice. Minimally processed horticultural products, such as fresh-cut prepared salads, are generally subjected to simple operations - including cleaning, washing, drying, cutting and packaging - soon after harvest that make them ready to use.
But as a result of these operations, the harvest life of ready-to-eat salads can be limited compared with the whole product. It should be remembered that minimally processed products are still living tissues - even after preparation and treatment.
Cutting is the main factor responsible for deterioration of these products since damaged plant tissues usually respire (release carbon dioxide and water) more rapidly. Generally, products that have a higher respiration rate have shorter post-harvest lives. This is why the basic concept of product-specific respiration rates (eg respiratory quotients) stands true as a fundamental principle of fresh produce post-harvest technology.
Despite the growth in ready-to-eat salads, the industry still faces significant challenges. It must find ways of maintaining product quality and availability by keeping deterioration and associated physiological disorders to a minimum and reducing microbial load using alternative treatments.
Enzymatic browning is the most commonly observed disorder of fresh-cut salads. It is believed that the operation of cutting stimulates the activity of an enzyme called phenylalanine ammonia lyase (PAL).
PAL is the principal enzyme that governs downstream metabolism of a group of organic compounds named phenylpropanoids in plants. It is thought that the increased activity of PAL results in altered phenol metabolism. Phenols are organic compounds that can be oxidised by the enzyme polyphenoloxidase (PPO) to quinones, which form the brown to reddish pigments responsible for tissue browning. Increased PAL activity has been correlated with increased susceptibility to browning in fresh-cut lettuce. For this reason, PAL could be used as a marker for shelf-life of some fresh-cut salads since increased synthesis of brown pigment stems phenol oxidation.
The content of phenols (ie the substrate for browning disorders) in vegetables is associated with their browning sensitivity. Fortunately, it has been shown that ascorbic acid (AsA) is a highly effective inhibitor of enzymatic browning. This is thought to be because AsA can reduce quinones to phenolic compounds and thus suppress the production of endogenous browning pigments. AsA may also inhibit PPO by reducing the pH of plants' cells. High-constitutive AsA content in cut-salad leaves is associated with increased resistance to browning during storage1.
Various different treatments have been evaluated to reduce the incidence and severity of browning in salads. In addition to maintaining cold storage, proper modifications of oxygen, carbon dioxide, and other gases inside the packaging may extend post-harvest life and maintain the salad's nutritional value. The use of modified-atmosphere packaging (MAP) is used ubiquitously across the industry to extend the post-harvest life of ready-to-eat salads. These are usually sold in the "pillow bag" format. The modified atmosphere can be either achieved passively, or actively - by flushing with a gas mixture before the package is sealed2. Many products, once removed from the altered gaseous atmosphere, tend to degrade faster than if stored in air.
The concept of modified atmosphere is simple, however, optimum gaseous composition is difficult to achieve throughout storage as once the package is closed further control over the gaseous composition is reliant on the product respiration rate (which changes over time) and on the specific permeability and nature of the packaging material used - such as the type of plastic and its thickness.
Reduced oxygen concentrations not only reduce respiration rates but will tend to reduce browning caused by PPO activity. As responses to atmospheric modification vary among salad species and cultivars, it is vital to understand the conditions and limitations of these techniques on the supply chains of each salad type - especially where mixed-salad products are used. The applicability of MAP to mixed salads has been under-researched as most published research has focused on single varieties.
Proper low-temperature storage and management of relative humidity should be pre-requisites to MAP3. Inappropriate MAP (eg oxygen concentration is too low) can lead to the production of off-flavours caused by anaerobic respiration (when plants release energy without using oxygen). Fresh-cut products tend to be more resistant to high carbon dioxide levels compared with their intact counterparts.
A new book on controlled atmosphere and MAP is to be published this year by Taylor & Francis Group, entitled Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities. The book is edited by Elhadi M Yahia.
Preservation technologies are based on two key principles - reducing the respiration rate of product and potential microbes and decreasing the microbial load by washing and/or disinfection. A number of treatments and technologies (viz controlling temperature, pH, water activity, redox potential, preservatives and modified atmosphere) are available to extend post-harvest life and maintain quality of fresh-cut salads; but a combination of preservation techniques is the best way to achieve a successful preservation strategy. This practice is referred to as the "Hurdle Concept"4, whereby combining individual preservation techniques or "hurdles" at low intensity can result in an overall impact on microbial growth while minimising the loss in quality. The selection of hurdles needs to be product-specific and chosen according to consumer demand and supply chain management needs.
The growth in sales of salads has been dramatic. However, so that this growth can be maintained a greater appreciation of the fundamental principles behind post-harvest technology is required such that industry can make informed decisions based on empirical data and sound hypothesis.
NEW TECHNOLOGIES - elicitors of induced resistance in post-harvest horticultural produce
The increasing loss of conventional fungicides because of pathogen resistance and general unacceptability in terms of public and environmental risk has favoured the introduction of integrated pest management (IPM) programmes.
Inducing natural disease resistance (NDR) in harvested horticultural crops using physical, biological and/or chemical elicitors has received increasing attention - it being considered a preferred strategy for disease management.
However, the enhancement of constitutive and inducible anti-fungal compounds and suppression of post-harvest diseases through using elicitors is under-researched.
How the timing of pre- and/or post-harvest elicitor treatment - and environment - can affect the degree of elicitation and the potential for inducing resistance to reduce post-harvest disease is also poorly understood.
More applied and basic research is required to understand the role that induced NDR can play in achieving the practical suppression of post-harvest diseases as part of an IPM approach.
During the development of commercially valuable plant organs and after their harvest, NDR generally declines - leading to inevitable infection, disease and ultimately death. In horticultural produce, post-harvest diseases caused by fungi usually begin as either latent infections established in the field or wound infections during subsequent harvesting and handling. A decline in NDR can activate quiescent (dormant) infections and increase disease incidence/severity. Four sets of factors affect the decline of NDR in produce after harvest:
1. nutritional requirements for the pathogen;
2. pre-formed antifungal compounds (phytoanticipins);
3. the potential for inducible antifungal compounds (phytoalexins);
4. activation of fungal pathogenicity factors.
Factors 1 and 4 - which can help pathogens to thrive - tend to be enhanced as tissues ripen and/or senesce, while factors 2 and 3 - which can help disease resistance - tend to be suppressed. Many pre-formed and/or inducible defence mechanisms are involved in the NDR of harvested horticultural fruit, vegetable and ornamental crops5.
Elicitors of NDR may be biological, chemical or physical and may induce local acquired resistance (LAR), systemic acquired resistance (SAR) or induced systemic resistance (ISR). Luckey coined the concept of inducing NDR "plant hormesis"6. Hormesis involves the stimulation of a beneficial plant response by low or sub-lethal doses of an elicitor/agent, such as a chemical inducer or a physical stress. The importance of induced or acquired resistance in plants has been long documented5.
Non-ionising radiation, among the various physical treatments available, has potential for controlling postharvest diseases5,7. Low doses of short-wave ultraviolet light (UV-C, 190-280 nm wavelength) can control many storage rots of fruit and vegetables5. UV-C irradiation at low doses (0.25 - 8.0 kJ m-2) target the DNA of micro-organisms.
For this reason, UV-C treatment has been used as a germicidal or mutagenic agent. In addition to this direct germicidal activity, UV-C irradiation can modulate induced defence in plants. At appropriate wavelength and dose rates, UV-C irradiation can stimulate accumulation of stress-induced phenylpropanoids (eg phenolics). Many phenylpropanoids have been associated with induced disease resistance and/or health-promoting properties. The concept of reducing waste by using UV-C is nearly 30-years-old.
The first demonstration that the technology was capable of reducing post-harvest disease was published by Lu et al on onions8. It remains to be seen whether this technology will be taken up by the UK horticulture industry as it is not used widely overseas. This said, the concept has gained support from the Defra (Hortlink HL0188: UV-induced hormesis: a novel approach to cut waste and maintain quality in fresh produce) and so fortunes may change.
1. Degl'Innocenti, E, Pardossi, A, Tognoni, F and Guidi, L (2007). Physiological basis of sensitivity to enzymatic browning in 'lettuce', 'escarole' and 'rocket salad' when stored as fresh-cut products. Food Chem. 104, 209-215. 2. Rico, D, Martin-Diana, AB, Barat, JM and Barry-Ryan, C (2007). Extending and measuring the quality of fresh-cut fruit and vegetables: a review. Trends Food Sci. Technol. 18, 373-386. 3. Terry, LA, Cristoso, CH and Forney, CF (2008). In Elhadi M Yahia (Ed), Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities Taylor and Francis Group (in press) 4. Leistner, L (1999). Combined methods for food preservation. In M. Shafiur Rahman (Ed), Food preservation handbook (pp. 457-485). New York: Marcel Dekker. 5. Terry, LA and Joyce, DC (2004). Elicitors of induced resistance in post-harvest horticultural crops: a brief review. Postharvest Biol. Technol. 32, 1-13. 6. Luckey, TD (1980). Hormesis with ionizing radiation, CRC Press, Boca Raton, Florida. 7. Sharma, G (2007). Process challenges in applying low doses of ultraviolet light to fresh produce for eliciting beneficial hormetic responses. Post-harvest Biol. Technol. 44, 1-8. 8. Lu, JY, Stevens, C, Yakabu, P, Loretan, PA, Eakin, D (1987). Gamma, electron beam and ultraviolet radiation on control of storage rots and quality of Walla Walla onions. J Food Process. Preserv. 12, 53-62.
1. Degl'Innocenti, E, Pardossi, A, Tognoni, F and Guidi, L (2007). Physiological basis of sensitivity to enzymatic browning in 'lettuce', 'escarole' and 'rocket salad' when stored as fresh-cut products. Food Chem. 104, 209-215.
2. Rico, D, Martin-Diana, AB, Barat, JM and Barry-Ryan, C (2007). Extending and measuring the quality of fresh-cut fruit and vegetables: a review. Trends Food Sci. Technol. 18, 373-386.
3. Terry, LA, Cristoso, CH and Forney, CF (2008). In Elhadi M Yahia (Ed), Modified and Controlled Atmospheres for the Storage, Transportation, and Packaging of Horticultural Commodities Taylor and Francis Group (in press)
4. Leistner, L (1999). Combined methods for food preservation. In M. Shafiur Rahman (Ed), Food preservation handbook (pp. 457-485). New York: Marcel Dekker.
5. Terry, LA and Joyce, DC (2004). Elicitors of induced resistance in post-harvest horticultural crops: a brief review. Postharvest Biol. Technol. 32, 1-13.
6. Luckey, TD (1980). Hormesis with ionizing radiation, CRC Press, Boca Raton, Florida.
7. Sharma, G (2007). Process challenges in applying low doses of ultraviolet light to fresh produce for eliciting beneficial hormetic responses. Post-harvest Biol. Technol. 44, 1-8.
8. Lu, JY, Stevens, C, Yakabu, P, Loretan, PA, Eakin, D (1987). Gamma, electron beam and ultraviolet radiation on control of storage rots and quality of Walla Walla onions. J Food Process. Preserv. 12, 53-62.