VG98083 Lettuce – rots & browning

Rots and browning reduce the visual quality of the produce especially in fresh cut products.

Minor discolouration leads to loss of consumer appeal whilst majorr problems lead to rejection of the product

An investigation into the role of bacteria on rots and browning of cut lettuce is reported here.

Lettuce (Lactuca sativa L.) is an econorrically important food crop grown worldwide.

Demand far quality lettuce in the Australian domestic and export markets is high.

Visual quality of lettuce can be threatened by rots and browning, often leading to reduced shelf life and product quality issues with consumers.

We believe that the presence of plaM pathogenic bacteria may play a role in the elicitanon of the plant’s defence mechanisms and be a major cause of browning at the cut surface.


Sue Pascoe

Robert Premier

Postharvest bacterial rots and browning in lettuce - 2002
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This project has shown that fluorescent pseudomonads are capable of either causing a rapid browning reaction on the cut surface of lettuce or exacerbating the problem over a 48-hour period on bought lettuce or lettuce grown under sterile conditions.

The natural levels of fluorescent pseudomonads in soils of lettuce growing regions have been shown to vary over the seasons and also vary between crop types.

The finding that the after-harvest lettuce waste contained high populations of fluorescent pseudomonads resulted in a recommendation for a time lapse between harvest and replanting of seedlings.

Experiments conducted at Knoxfield have shown that different Pseudomonas species can induce browning of cut lettuce within 48 hours.

Browning is quite severe and rapid compared with other bacteria such as Erwima and Xanthomonas species.

The project has also shown that fluorescent pseudomonad levels on cut lettuce can be reduced, whilst maintaining good quality produce.

Also bacterial levels in the field were found to increase with the use of mulches, but the overall yield and quality of the lettuce heads were increased.

The results indicate that bacteria have an important role in not only rotting the cut surface of lettuce leaves but also in eliciting a browning response.

The cut surface of all leaves inoculated with bacteria turned brown whereas the cut surface of leaves that were not inoculated remained free of browning.

Recommendations :

  • Testing for fluorescent pseudomonads on the lettuce surface before processing to use as a warning system for lettuce that is likely to go brown.
  • Testing for fluorescent pseudomonads on lettuce as an indicator of the effectiveness of browning inhibition treatments, such as heat treatments, antimicrobial washes and chlorination.
  • Persistence of fluorescent pseudomonads on lettuce waste indicates that replanting should be conducted 10-14 days after the last harvest to reduce the chances of bacterial re-infection.
  • The use of plant based essential oils pre or post-harvest may be beneficial for the reduction of bacteria on the surface of fresh-cut lettuce (needs further research and development).
  • The use of mulches (of any type) are beneficial to lettuce production and increasing yields due to the soil water retention and the improved water infiltration in the field. They could provide large savings on water usage.
  • Maintenance of good postharvest conditions such as proper temperature management (fluorescent pseudomonads.

Acknowledgments :

The completion of this project would have not have been possible without the assistance of a large number of people :

Kon Koroneos – Werribee South, Vic. Paul Gazzola – Somerville, Vic.
Nelson Cox – Lindenow, Vic. Bill Taylor Jnr – Boisdale, Vic.
John Faragher, Bruce Tomkins, James Hutchinson, Julia Behrsing and the other members of the NRE-IHD Food safety, quality and nutrition team

This project was commissioned by Horticulture Australia Limited�with funds frrom the Vegetable R&D levy and the Victorian State Government..

The Australian Government provides matched funding for all HAL’s R&D activities.

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