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The influence of endogenous enzymes on the flavour quality and shelf life ofvegetables and herbs

Objective

1.To identify the major enzyme systems in vegetables and herbs which effect flavour quality;

2.To understand the effect of processing, packaging and storage stress on these enzyme systems;

3.To utilize this understanding to tailor processing, packaging and storage regimes to improve flavour quality.
Research has been carried out in order to identify themajor enzyme systems in vegetables and herbs which effect flavour quality and to understand the effect of processing, packaging and storage stress on these enzyme systems.
Parsley, green beans and potato were selected as models to study peroxidase, lipoxygenase and polyphenol oxidase respectivly. Tomato is the composite system in which 3 enzyme systems are operative.

With respect to parsley, 2 analytical procedures have been designed to permit the quantification of aroma and pigments in tissue. These methods involved solubilization of the analyses using typically chloroform and methanol under an environment designed to stabilize interfering processes.

The primary quality issue for potato is the darkening of tissue during processing and storage due to the formation of melanin from the action of polyphenol oxidase (PPO) on tyrosine coupled to a series of nonenzymic reactions.
Partially purified PPO was produced and analyzed with subsequent characterization in terms of:
influence of dissolved oxygen;
substrate specificity and enzyme kinetics;
effect of monophenol and diphenol molar ratio on activity;
pH optimum;
temperature optimum;
influence of ionic strength using magnesium chloride and potassium chloride;
chemical inhibitors;
heat inactivation at a range of temperatures.

Quality issues for frozen green beans are based on aroma and texture, and involve the lipoxygenase and pectinase enzyme systems.
The work on aroma involved the production of crude peroxidase and lipoxygenase extracts from green beans and their subsequent experimental characterization in terms of:
optimum temperature and pH range;
enzyme kinetics;
thermal stability;
activity regernation following thermal treatment.

Quality issues related to tomato are based on aroma and colour. The chemical species associated to these quality components are the volatile C6 aldehydes and alcohols and the carotenoid pigments and are associ ated to 3 operative enzyme systems, nanely lipoxygenase (LPO), polyphenol oxidase (PPO), and peroxidase (POD).
Partially purified enzymes have been produced and characterized in terms of:
optimum temperature;
optimum pH;
inhibition temperature;
distribution within the tomato fruit;
characterization of endogenous substrates in the presence and absence of Iycopene as an oxidative couple.
The objective of the project is to determine the biological/biochemical basis for flavour quality change in plant tissues by developing an understanding of the enzymes, substrates and product interactions responsible for such changes in herbs and vegetables, particularly parsley, horseradish, green beans and tomatoes. On the basis of the above findings a product specific treatments for these tissues to control the balance of flavour components contributing to their overall flavour quality will be developed.
Identify tissues
The tissue selection is based on a combination of group specific interest, commercial importance of particular crops and on a biochemical, specific or general interest basis.
The vegetable and herbs selected were:
- tomato fruit,
- horseradish root crop,
- leguminous green or french beans,
- parsley herb.
These represent a spectrum of vegetables, fruits and herb types with widely differing flavour character.
Relevant flavour components
The key flavour components of the above tissues will be described by reference to the literature and from the knowledge and experience of the four partners. A similar approach will be taken for identification of the enzymes potentially involved in the generation or degradation of the key tissue flavour components.
Model systems
- Enzyme systems
The identification of the quality influencing enzyme systems will focus these studies towards the isolation of the appropriate enzymes. The degree of purification will be dependent on the known stability of the system examined. - Model studies
The substrate, cofactor, cosubstrate, inhibitor and synergist for the established enzymes will be determined. Using model systems of crude, enriched or purified enzymes the substrate/product relationships will be established in model systems.
Tissue systems
- Tissue relevant enzyme systems
The four tissues described will be subjected to accelerated deteriorative changes using tissue damage methods including freeze/thaw and mechanical communication and bruising. Changes will be assessed in detail by the above described physical and sensory flavour methodologies.
- Tissue treatments
Using this knowledge of flavour deterioration treatment regimes tailored to generate specific flavour components and/or to inhibit or restrict degradation of the same can be devised and evaluated. These include: - thermal regimes for enzyme inactivation,
- pH control,
- inert gas atmospheres (N2, CO2),
- oxygen scavenging systems (enzymic and chemical),
- barrier packaging,
- chemical inhibitor systems.
Implicit in these studies are the effect of these treatments on the overall tissue quality (texture and colour) in addition to flavour.
Tissue strategies
Strategies will be evolved for best optimisation of the flavour quality of the described tissues and will involve specification of:
- the harvesting method,
- the treatment or processing routine,
- the packaging type,
- storage conditions.
The procedures employed above should utilise methods of treatment and packaging that are preferably physical and/or biological (consumer and environmentally friendly) rather than chemical.

Coordinator

UNILEVER UK CENTRAL RESOURCES LTD
Address
Unilever House, Blackfriars
EC4P 4BQ London
United Kingdom

Participants (3)

Fraunhofer-Gesellschaft zur Förderungder Angewandten Forschung e.V.
Germany
Address
Steinerstraße 15
81369 München
NATIONAL LABORATORY FOR ENGINEERING AND INDUSTRIAL TECHNOLOGY
Portugal
Address
Estrada Do Paco Do Lumiar 22
1649-038 Lisboa
Università degli Studi di Perugia
Italy
Address
San Costanzo
06121 Perugia