Knowledge for a sustainable world

BSc, PhD

Dr Debbie Rees has been working for the Natural Resources Institute since 1994. For the first few years most of her work was focused on improving postharvest handling of sweet potato in East Africa. However, she has now broadened her research area to cover postharvest technology for a wide range of perishable crops both in the UK and overseas. She currently has projects working on apple, pear, broccoli, swede and cabbage in the UK, on yams in West Africa, sweet potato in East Africa as well as high value commodities imported into the UK such as avocado, grape. Debbie was appointed as a Reader in Plant Physiology in 2003.

Together with her colleague Dr Richard Colgan, Debbie has been instrumental in setting up the Produce Quality Centre (PQC), a collaborative initiative with East Malling Research. By bringing together the expertise and facilities of two institutes, the PQC provides the widest range of expertise and the best facilities for postharvest research on perishable commodities in the UK.

Debbie was lead editor for the book; “Crop Post-Harvest: Science and Technology Volume 3: Perishables” which was published by Wiley Blackwell in 2012

Debbie started her scientific career studying for her PhD at Oxford University investigating the use of phosphorus Nuclear Magnetic Resonance to study energy metabolism in both human muscle and bacteria. Following a one year fellowship at the Baylor College of Medicine in Houston, Texas, she then spent five years as a post-doctoral researcher studying photosynthetic mechanisms at Sheffield University. During her work at Sheffield she gained expertise on the analysis of chlorophyll fluorescence signals, which has been very useful in her later career. She spent two years working in the Wheat programme at CIMMYT (International Centre for Wheat and Maize) in Mexico immediately before joining NRI.

Debbie Rees has a range of research interests all relating to finding ways to retain the quality of fresh produce and reduce postharvest losses.

She has a particular interest in understanding the cultivar differences in postharvest behaviour of root crops and the potential for breeding for improved postharvest characteristics. During the 1990s Debbie managed a range of projects, which looked at the postharvest behaviour of sweet potatoes; a crop which is genetically very diverse. These projects demonstrated that the wide variability in keeping qualities during marketing of sweet potato roots in East Africa related to efficiency of wound-healing; a characteristic that can be relatively easily selected for within breeding programmes. Lessons learnt from this work are now being applied to yams (Dioscorea spp) in West Africa.

Sprout control is very important for root and tuber crops. Debbie has a particular interest in understanding biochemical mechanisms for the control of dormancy break and sprout growth. She is involved in projects on sprout control in potato tubers, in collaboration with Sutton Bridge Crop Storage Research and the James Hutton Institute. Information obtained from these projects can be extremely valuable in improving our understanding of sprout control in crops that are less extensively researched such as sweet potato and yams.

The potential for using chlorophyll fluorescence as a practical tool during handling of fruit and vegetables is a key interest. Debbie worked on chlorophyll fluorescence as a tool to investigate photosynthetic mechanisms in the 1980s at Sheffield University. She also used the technique to study photosynthetic efficiency of wheat in CIMMYT. She is currently involved in a range of projects to test the potential of the technology to assess the maturity of vegetables and fruit (particularly important for optimising harvest time) as well as detection of storage stresses.

Ethylene is a natural plant hormone that controls a wide range of physiological processes. With the recognition that many vegetables are sensitive to concentrations of ethylene well below 100 parts per billion, there is growing interest in the management of ethylene throughout the fresh produce handling chain. Debbie is involved in a range of projects to investigate ethylene management for vegetables such as broccoli, cabbage, swede as well as fruit such as apples and pears.

There is growing interest within the fresh produce industry in the use of gaseous ozone as a disinfesting agent against both fungal and bacterial pathogens. Debbie is managing a project to test the efficacy of gaseous ozone for extending storage life for a range of produce, with a particular focus on the effects and the mechanisms of the effects of ozone on the produce quality.

For produce such as apples, pears and potatoes which can be stored for many months, it is important to be understand and manage stresses imposed by the storage environment. Debbie is part of a team within the PQC that is investigating biochemical markers and other indicators of tissue stress.

Yam Improvement for Income and Food Security (2012-16): Role - Objective leader.

This project, funded by the Bill and Melinda Gates Foundation, is located in Ghana and Nigeria, and aims over the next 5 years to (a) increase yam productivity (yield and net output) by 40% for 200,000 small-holder yam farmers (90% with less than 2 acres) in Ghana and Nigeria; and (b) deliver key global good research products that will contribute to the longer term vision of improving yam productivity and livelihoods of yam dependent farmers.

Debbie Rees leads one of the project objectives which aims “To reduce post-harvest losses of yam tubers on farm and during marketing, and improve produce quality”.
This includes four specific activities:

Activity 4-1: Develop, validate, and promote technologies to reduce on-farm tuber storage losses of existing varieties for tubers used for seed yam, home consumption, and marketing.
Activity 4-2: Develop and promote technologies to reduce tuber losses during marketing.
Activity 4-3: Improve post-harvest characteristics through germplasm assessment and selection.
Activity 4-4: Improve farm/small-scale processing.

More information about this project can be found at

Ethylene and fungal control strategies to reduce waste of fresh produce (2009-2010): Role– Technical input

WRAP is working with industry to identify ways of reducing wastage of fresh fruit and vegetables through the whole supply chain. From the packhouse, through distribution depots and retail outlets, to domestic households, every step of the chain offers opportunities to realise the benefits of avoiding waste.

Both ethylene and microbial contamination, in the form of fungal spores, can increase the rate of product deterioration. This WRAP funded project focused on the impact of the levels of ethylene gas and fungal spores on fresh fruit and vegetables, as they make their way through the supply chain. It measured these levels, assessed their impact on produce deterioration, and signalled where the potential for improvement may lie through the introduction of technology, or where further investigation may be beneficial.
The main recommendations arising from this project were:

  • The fresh produce industry should consider installing ethylene removal/scrubbing equipment within storage facilities, particularly for produce most susceptible to ethylene;
  • Ethylene concentrations within packaging, and the effects of ethylene controls in packaging, should be measured, with an initial focus on broccoli and melon;
  • Studies should be conducted on the most susceptible produce to define the effect of continuous exposure to low concentrations of ethylene; and
  • The fresh produce industry should consider introducing fungal spore control technologies within packhouses, potentially including the use of covered bins to reduce atmospheric spore counts

A full report on this project can be found at

Storage trials for UK fruit and vegetables – Technical input

The Produce Quality Centre ( is undertaking a range of projects to optimise storage of UK fruit and vegetables.  UK apples and pears can be stored for several months, sometimes up to a year using controlled atmospheres.  The Jim Mount facility of the Produce Quality Centre has a research scale controlled atmosphere system capable of testing fifty atmospheric compositions simultaneously.  Through funding from the Horticulture Development Company, Sainsbury’s, Agrofresh, this is currently being used to optimise storage protocols for key UK varieties including Bramley, Gala and Braeburn apples and Conference pears.  The objectives are to provide fruit of optimum eating quality with as low energy input as possible.

Trials are also underway to improve storage of vegetables such as broccoli, cabbage and swede.  In this case, as for fruit, ethylene management is a key factor. Technologies, including chlorophyll fluorescence to predict storage life/detect stress and thereby improve storage management are being tested.

Potato sprout control

Debbie Rees is part of a team working to improve our understanding of the control of dormancy break and sprout growth in potato tubers.  She coordinated a project funded through the DEFRA Link programme “Reducing energy usage and wastage by improving ethylene control of potato sprouting.” 2009-2012.  This project involved a collaborative team including the James Hutton Institute, Sutton Bridge Crop Storage Research and commercial partners; Pepsico Ltd, Greenvale AP Ltd, Cygnet Potato Breeders Ltd and Landseer Ltd.

The project objective was to develop strategies for improved sprout control that reduce quality losses while opening the way for storage at higher temperatures for both processing and fresh marketed potatoes.  In particular the project sought  alternative strategies to the widely used sprout suppressant, CIPC.

Continuous application of ethylene (4-10 ppm) to potatoes during storage has been developed as an alternative to CIPC prior to the initiation of this project, as it inhibits the growth of sprouts once they have initiated.  However, an important constraint to ethylene use for sprout suppression is that varietal variability in ethylene sensitivity means that most varieties need to be held at low temperature for effective sprout control. In addition to this, ethylene can induce increased respiration and sugar accumulation in some varieties. This is not acceptable for processing varieties as sugar accumulation causes fried products to become unacceptably dark.  Sugar accumulation also impacts on the increased potential to produce acrylamide, identified as a potential carcinogen, during potato processing.

The project followed two approaches:

Approach 1: using a range of advanced molecular biology techniques, comparison of ethylene sensitive cultivars with non-responders was used to enhance our understanding of the molecular mechanism by which ethylene exerts its control on sprout growth with the aim of identifying the key potato genes involved and developing markers for these genes.

Approach 2: Developing storage strategies by combining ethylene treatment with other chemicals

The results from this project are being further developed within a subsequent BBSRC funded project “Controlling dormancy and sprouting in potato and onion” (2013 – 2016) funded through the BBSRC Horticulture and Potato Initiative and involving a consortium that includes the James Hutton Institute, Cranfield University and Imperial College.

Sweet potato cultivars with improved keeping qualities for East Africa FCDO Crop Postharvest Programme (1999 – 2002)

The objective of this project was to facilitate development of sweetpotato cultivars with improved post-harvest qualities, especially improved shelf-life and storability, thus improving food security and income generation through better marketing and storage.  Breeding initiatives for sweetpotato are at an early stage compared to other staples, so there is particular potential for crop improvement, and a need to understand quality characteristics and how to select them.

During an earlier project a wide range in perishability was found among cultivars (R6507), underlining the potential for improvement by breeding. A major factor was the ability of a cultivar to heal wounds at moderate humidities, which was found to correlate with low dry matter (DM) content. Given that the preferred sensory characteristic of mealiness relates to high DM content, identification of cultivars with high DM content and efficient wound-healing would be beneficial.

We screened sets of germplasm covering 47 sweetpotato cultivars from many origins.  It was shown that cultivars fall into groups depending on origin; East African cultivars have higher DM content and lower wound healing efficiency than cultivars from other locations.  Wound healing efficiency is linked to DM content, but exceptional cultivars with high DM content and high wound healing efficiency were identified.

Research to determine the physiological basis for differences in wound healing efficiency, and the link with DM found that when roots are wounded at moderate/low humidity there is an initial accumulation of sugars close to the wound.  The level of accumulated sugars is cultivar dependent and a major factor in controlling efficiency of healing.  We hypothesize that the accumulation of sugars protects tissues from desiccation by increasing the osmotic potential. Although we cannot demonstrate a difference in initial weight loss, it is possible that specific cellular compartments are protected against water loss.  A relationship between DM content and wound healing efficiency is a consequence of the fact that cultivars with rapid starch mobilisation tend to have lower DM content. We do not know if the good healing/high DM cultivars have some additional characteristic to improve wound healing.

A book entitled “Sweetpotato post-harvest assessment:
Experiences from East Africa” was produced as a collaboration between the Natural Resources Institute, the Tanzanian Ministry of Agriculture and the International Potato Center  with input from the National Agricultural Research Laboratories of Kenya and the National Agricultural Research Organisation of Uganda.  This book consolidates information and expertise gained in this and earlier projects.

Leader of the Postharvest Science and Technology Research Group

  • Member of the Board of the European Fruit Research Institute Network since 2012
  • Member of the Association of Applied Biologists
  • Member of the International Horticultural Society
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