VG03092 Lettuce – Shelf Life

Fresh cut lettuce is an expanding category for supermarkets in Australia.

In an effort to meet this market demand, processors need a year round supply of good quality product. OneHarvest P/L create a year round supply by sourcing Australian Iceberg and Cos lettuce from different regions throughout the year.

There is a summer and winter location in both Queensland and Victoria to ensure a year round supply of fresh product. The lettuce is processed in the winter in Brisbane (Harvest Fresh Cuts) and in Bairnsdale (VEGCO) in the summer.

The processed products are then sold Australia-wide, mainly through the supermarket chain stores. The key to growing this category is to supply customers with a consistent quality product. The difficulty comes in achieving this when environmental conditions change between districts and seasons.

This project aims to identify management strategies that can help growers produce a quality product during sub-optimal seasonal periods.

Key Results :

1. Identification of the most productive times and shelf life for each of the main lettuce production regions in Qld and Vic.
2. New Varieties – The identification and agronomic work on important new lettuce varieties including Cyclone, Challenger and Nr varieties.
3. Effect that trimming and time held before processing has on shelf life of Cos lettuce.
4. Later harvesting – improvements in yield without adverse effect on shelf life or core length by later harvesting (main production season).
5. Development of crop scheduling for predicting days to harvest and yield.
6. Trickle irrigation can be used to produce high yielding and quality lettuce with significant water savings compared to overhead irrigation.
7. Rapidly cooling lettuce as soon as possible after harvest significantly improves shelf life.

Project Findings :

Crop Scheduling

• Preliminary Scheduling models have been prepared for Cos and Iceberg lettuce grown in several districts in Qld and Victoria.
• For Iceberg lettuce planted in Gatton, Qld the time from transplanting to harvest varies from as little as 49 days for the May harvest extending out to 70 – 77 days for the midwinter harvest and then decreasing back to 63 and 56 days for September/October harvesting.
• During this time yields can vary from 30t/ha (350g/head) in the May and October period, up to 45 – 50t/ha (1.5 kg) in the optimum harvest period in July through to August.
• For Cos lettuce grown in East Gippsland, Vic the time from transplanting to harvest varies from 95 –100 days for the early part of the season decreasing to 42 – 49 days in mid-summer and then extending out to 70 – 75 days for the late autumn harvest period.
• The yield per hectare during this time varies from a lower yield in mid-summer of 500 – 550g/head compared to 750 – 800g/head produced in the autumn and spring period.
• Bolting is a major consideration for crop scheduling. The potential causes for bolting could include:

• Day length (especially long days)
• High diurnal temperature range e.g. maximum daily temperatures equal to or greater than 27 degrees C and minimum temperatures dropping to 5 degrees C.
• A combination of long days, high temperatures and transplanting into dry soil.
• Soil temperatures, both waterlogged and drought conditions.
• Number of leaves when transplanted (especially if the leaf number is greater than 7).
• Length of time Cos seedlings held in soil-less media once the optimum root ball has been developed.
• Stress, especially moisture, soil compaction and salt.
• Genetics, with some varieties exhibiting slow bolting tendencies

Harvest maturity

• The trials in Gatton in 2005 demonstrated how yields can be increased in this region and time of year without significant detriment to lettuce processing quality.
• This change in production practice is best suited to cooler periods in the season when growing conditions are optimal to avoid the issue of bolting.
• The results showed that yield increases as high as 60% could be obtained for Iceberg lettuce (cv. Titanic transplanted 26-04-05) and a 35% increase in Cos lettuce.
• There was no impact on shelf-life when iceberg or Cos lettuce was grown for an extra seven days and the average core length in the plants from the later harvest, was well within the specification for lettuce processed as fresh-cut salads.
• These findings are specific to the conditions experienced during these trials and research encompassing a range of regions, seasonal timing and varieties is required to verify a broader application.

Postharvest handling Storing and Trimming

• The results showed that storing the unprocessed heads for 5 days prior to processing resulted in a reduction in the quality by 22-31% .
• The harder trimmed lettuce (55%) was able to maintain close to initial quality until day 8 whereas the normal trimmed lettuce (70%) was declining in quality from day 4.
• Once the lettuce quality started to decline, it seemed to be programmed on that track, and this may mean that shelf life assessment protocols could be developed with less assessment points, but which were aimed to identify the beginning of the decline in quality and the rate of change.
• The effect of storing lettuce for 5 days prior to processing results in a reduction in overall score of about 28%.
• Storing heads reduces the starting quality by about 28% compared to using fresh heads for processing.

Reducing moisture in the bags

• The results show that having an absorbent sachet in the bag increased the shelf life of the product by about 3 day.
• This is an important result as it demonstrates the fact that the current processing can be improved.

Postharvest temperature Management

• Lettuce must be vacuum cooled with in half an hour of harvest to ensure maximum shelf life.
• Vacuum cooling 6hours after harvest did not improve shelf life compared to the forced air cooled samples.

Vacuum cooling improved shelf life compared to forced air cooling

• A break in the cool chain (2 hrs at 15 degrees C) negated the benefits of on farm cooling and reduced the overall shelf life of the product

The physical impact of processing lettuce

• Other research shows that more cuts and blunter equipment increase the response and increase the rate of deterioration.
• The translicer used in this processing line was not having a significant impact on the shelf life of the product compared to manual cutting with a sharp knife.
• The results in relation to the degree of trimming support those from the previous section.
• Trimming lettuce to 55% prior to processing significantly improves the shelf life of the processed product.

Variety Selection

• Trials with lettuce varieties are relatively easy to design, establish and grow to harvest. They are very time consuming making observations, collecting data, analysing and interpreting before making a decision on adopting a new variety as shown over the three years of the project.
• Attributes other than marketable yield (Nasonovia resistance for iceberg & slow bolting for Cos) must be taken into account when adopting new varieties.

Crop Nutrition

• It is essential that nutrients are applied at the appropriate ratio with other nutrients rather than simply applying elements independently if optimal yield and quality is to be achieved.
• Excessive nitrogen application can reduce both yield and quality (shelf-life)
• Phosphorous management is important for sustaining the maximum, long-term performance of the crop
• Phosphorous supply can be an issue on soils new to horticultural production, where relatively high rates may need to be applied
• Calcium foliar sprays are ineffectual in tip burn alleviation and cultural practices that aim to prevent excessive growth rates are a better strategy for reducing the incidence of tip burn.

Crop Establishment

• If the potential value of Cyclone, or similar varieties, is to be fully explored for the freshcut salad industry, specific cultural practices which optimise yield must be considered.
• cv. Cyclone yield has been shown to increase significantly when planting density is raised to 80,000 – 100,000 plants per hectare and the appropriate, supporting nutrition is applied.
• The ability of direct-seeded lettuce crops to produce yields comparable to that of transplanted crops has been demonstrated, presenting a means to reduce planting costs.

Soil Moisture Management

• Trickle irrigation can be used successfully to grow lettuce with higher water use efficiency than using sprinkler irrigation.
• Lettuce plants should be maintained free of water stress right up to harvest for maximum yields.

      See Also :   “Influences on shelf life in Cos
and Iceberg”
                              AUSVEG Conference – May 2006   

Acknowledgements :

The Authors wish to thank :

• Co-operating lettuce suppliers – Nelson Cox, Bill Taylor, Andrew Young, Max Durham, Jay Parchert, Ian Thorne, Lester Hamblin.
• Withcott Seedlings and Boomaroo Seedlings.
• Numerous Vegetable Seed companies.
• Kim Martin (ex. Vegco P/L) for the initial project concept with Vegco & HFC staff

This project was jointly funded by OneHarvest and Horticulture Australia Limited.and Horticulture Australia through the National Vegetable R&D Levy.

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