Baby leaf salads are an excellent convenience food. In Australia these products are marketed as either packaged fresh cut, minimally processed or fresh product and the baby leaf category is growing rapidly.
Baby leaf salad crops such as lettuce and spinach are harvested and marketed in an immature stage of development compared to other vegetable crops.
This means growing and handling practices which were developed for the mature selections of these crops are no longer valid.
The term baby leaf describes a wide range of leafy vegetables harvested when only a few weeks old. However, this project focussed on the main components commercially blended, packaged and sold in major supermarkets and included lettuce (Lactuca spp.), spinach (Spinacia oleracea), wild rocket (Diplotaxis tenuifolia) and cultivated rocket (Eruca sativa),
The baby leaf category also includes some new salad lines such as rocket, tatsoi and mizuna.
This project was a collaboration between OneHarvest, a commercial baby leaf manufacturer and Applied Horticultural Research.
This report describes the optimum pre and post harvest management of baby leaf spinach, lettuce and rocket to ensure maximum shelf life and quality for the consumer.
James Le Budd
Key Outcomes :
A crop life of at least 30 days, from seeding to harvest produced higher quality. This was achieved by using slower growing varieties under warmer conditions or by only growing other varieties when the season was cool enough for varieties to attain the 30 day minimum growth period.
The growth rate of spinach was temperature dependant. The best quality was achieved when the average daily maximum temperature did not exceed 23° C and the average night temperature did not exceed 8-10°C.
A growing degree day (GDD) model was developed with a base temperature of 0°C and high temperature cut off of 27°C. The requirement from seeding to harvest for slow, medium and fast growth rate spinach varieties was 443, 420 and 404 GDD respectively.
In lettuce, nitrogen had a significant effect on shelf life. Concentrations of greater that 4% in the leaves at harvest resulted in a shorter shelf life.
Rapid, vacuum cooling of the product after harvest maximised shelf life. Cooling within half an hour of harvest was best, any delay shortened the shelf life.
The stability of Vitamin C in the harvested product was dependent on storage temperature. When held at 0°C, spinach retained 90% Vitamin C content for 7 days whereas at 4°–7°C, about 50% of the Vitamin C was lost.
For European wild rocket, alternating higher day (24oC) and cooler night (<10oC) temperatures partially overcame low temperature inhibition of germination. Cultivated rocket germination was not sensitive to low soil temperature and germinated down to 5°C.
The highest yields of spinach were obtained at a density of 1000 plants/m2 but this reduced average leaf weight to 1g and reduced shelf life (stored at 2°C) from 23 days at 300 plants/m2 to 16 days at 900 plants/m2.
Transplanted multi-leaf type lettuce were compared with more traditional; single leaf direct seeded lettuce types and found to yield better and displayed good quality attributes.
Finally the agronomic modelling data was used to select a suitable new growing area. This resulted in a commercial pilot-to-prove farm being set up at Mt Gambier in South Australia and was shown to successfully produce market quality baby leaf products including spinach.
Project Findings :
The most significant finding of the project was the effect of growth rate on quality. The results showed that for spinach to attain maximum quality, it needs to be grown for at least 30 days from seeding to harvest.
This 30-32 day minimum growth can be achieved by either selecting slow-growing varieties during warmer periods, or by growing the crop in a climate cool enough to give at least 30 days from seeding to harvest.
The effect of temperature on spinach growth rate was found to be a combination of maximum day temperature and minimum night temperature.
High day temperatures can be offset to some extent by cooler nights, resulting in slower overall growth.
The most rapid growth occurs when there is a combination of high day and high night temperatures.
The crop growth rate of spinach for any given area and time of year can be predicted by using the crop growth model developed as part of the project utilising historical average temperature data.
This model is based on accumulated growing degree days or heat units over the growing season with a base temperature of 0oC.
The data shows that good quality spinach is grown in areas and seasons where the accumulated heat units are between 400 and 500 from emergence to harvest.
Other agronomic factors such as variety evaluation, germination temperature tolerance, plant nutrition in particular nitrogen application, spacing and weed control were all investigated.
The overall results from these studies were that for maximum quality it is important to chose the right variety for the right time, don’t over apply nitrogen or plant at a too high a density and be mindful of the herbicide you use as some are phytotoxic.
Any agronomic factor that promoted rapid growth such as high density or high nitrogen resulted in “weaker” plants that did not have maximum shelf life.
The overall message is that slow growth ensures maximum shelf life until more robust baby leaf cultivars become available.
The postharvest research carried out in this project validated the principles of best practice for leafy vegetables.
The overall postharvest principle is that pre-harvest factors determine potential shelf life, while post-harvest handling determines to what extent this potential shelf life is realised.
For maintaining postharvest shelf life of baby leaf products harvested product must be vacuum must to 0oC within half an hour of harvest to achieve the longest shelf life and the longer vacuum cooling is delayed, the shorter the shelf life of the product.
Storage temperature is also critical with storage at 0oC maintaining both visual and nutritional quality (Vitamin C) for a longer period than 4oC and 7oC.
Unfortunately this is not commercial practice with products often stored between 4 and 7oC.
Another principle that was validated was that it is important to pack dry product as free moisture encourages bacterial breakdown in the package.
Finally the agronomic and postharvest bench marks developed in the project were used to search for, select and establish a successful commercial pilot-to-prove farm at Mt Gambier in South Australia.
This project was conducted over three years and so further work is required to validate some of the results reported.
This is particularly true for the data that is based on temperature and seasonal climate.
Many of the results reported are novel and have not been previously reported in the scientific literature.
See Also : "Agronomic ... improvement in ... lettuce ... for fresh cuts salads"
R&D Project VG03092 - Dec 2006
The authors wish to thank the following growers who provided land, farming inputs and
time to host the trial work for the project :
We thank Tom McAuliffe, OneHarvest and both management and processing staff at OneHarvest factories at Wacol, Qld and Bairnsdale, Vic for their assistance in
- Ryan McLeod, Australian Fresh Salads, Gympie, Stanthorpe and Warwick,
- Colin Britton, Britton Produce, Stanthorpe, SE Qld
- Edgar Grech, Grech Farms, Ellis Lane, Camden, NSW
- Bill Taylor Jnr, Taylor Farms P/L, Boisdale, Vic
- Nelson Cox, Riviera Salads, Lindenow, Vic
We thank seed companies Rijk Zwaan, South Pacific Seeds, TerraNova, Fairbanks,
Lefroy Valley and Seminis for the provision of seed and baby leaf varietal
We also acknowledge the research contribution of the following 4th Year
undergraduate students from Sydney University, NSW.
- Scott Graham - Salanova nutrition
- Ben Gudex - Growing temperature and the shelf life of baby leaf spinach
- Matt Hall - Spinach scheduling and European Wild Rocket – enrolled PhD
student on European Wild Rocket
This project was jointly funded by OneHarvest and Horticulture Australia Limited through the National Vegetable R&D Levy.
The Australian Government provides matched funding for all HAL’s R&D activities.