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Agronomy: less water, better fruit

16/03/2018 - François-Xavier Branthôme - Lire en français
Reducing irrigation by 40% without losing yield

A 40% reduction in the irrigation water for processing tomato crops has led to an improvement in the technological value of the fruit without any loss of yield. This is the main outcome of a study combining pre-harvest and post-harvest analyses, with quality controls all the way from the cultivation field to the consumer's plate.

Around the world, the processing tomato industry must face a number of challenges, at varying levels: 
- An increasing demand for quality products on the part of consumers and processors.
- Dwindling water resources for irrigation. 
- Variations in the quality of the tomatoes harvested and in the criteria that actually do not provide much information about the technological value of fruit (yield and level of soluble solids (which mostly depend on soluble sugars and are determined by an index of light refraction expressed in °Brix)).

Thanks to a partnership between two INRA units at the Avignon center, specialized in the ecophysiology and the quality of fruit (the UR PSH for plants and horticultural cultivation systems) and in processing methods (the UMR SQPOV for safety and quality of plant-based products) and to other actors in the industry (the SONITO national interbranch organization for the tomato industry and the CTCPA food technology center), the way that quality is built into the manufacturing of tomato-based products has been analyzed throughout the production chain. 2 levels of irrigation, 4 commercial varieties, 3 stages of fruit maturity and 2 processing methods were studied for their combined effects on the quality of the tomatoes produced and on their suitability for the processing industry, with tests carried out over two production seasons near Avignon in 2016 and 2017. The effects of the different field factors were validated by setting up a similar experimental plot in semi-controlled conditions in a greenhouse.
Illustration of the installation and setup to study how quality is integrated into tomato crops, from cultivation to the processed product and at different industrial scales. A) Tomato plots made available for the project by the SONITO, B) greenhouse compartments of the UR PSH, C) industrial level processing units of the CTCPA and D) analysis of the processed pulps in the UMR SQPOV labs.

A moderate water deficit (-40% irrigation from flowering to harvest, compared to a control plot irrigated with sufficient water to completely replace all loss by evapotranspiration) only very slightly decreased the yield of raw fruit, and increased the yield of soluble solids by up to 27%. In this way, the efficiency of irrigation water usage was increased by an average of 20% throughout the growth cycle. The composition of soluble solids in the fruit at harvest time (levels of soluble sugars, organic acids and carotenoids) was not significantly affected by the water deficit.

Improving the quality of fruit
However, when the same fruit is processed according to industrial methods, the water deficit improves the viscosity of puréed tomato without affecting the color. This increase in the initial natural threshold of viscosity is an advantage in the technical process of manufacturing hot-break pastes, as it should allow for an adjustment of the parameters of thermal treatments and thereby limit the risk of browning the product.
The water deficit also limited the loss of viscosity observed when processing procedures used a maceration phase at low temperatures, generally lower than 70°C, the purpose of which is to prepare products of lower viscosity levels, referred to as "cold break".
This result shows that even if the fruit composition is not particularly affected by the water deficit, the enzymatic reactivity of the fruit tissue is reduced. By searching for indicators of purée quality at the fresh fruit stage, we were able to determine that the soluble solids content of the fruit is not correlated with viscosity, contrary to what is generally accepted in the profession. Furthermore, the lycopene levels in the fruit (red pigment) seem to have a positive impact on the viscosity of products. Studies should be carried out regarding the rheology of purées in order to better understand this observation.

This research opens the way for a control of irrigation methods that depends on a target level of soluble solids in the fruit. Acquired data will also lead to more accurate decisions in terms of cultivar choices and to a better timing of the start of harvesting operations depending on variety choice and processing method. Finally, studies are currently ongoing in order to better understand the mechanisms that determine the viscosity of the end products. Drawing up an overall pattern for viscosity and carrying out trials at different stages (laboratory, factory) will allow operators to adapt methods and processes according to the quality of the fruit arriving at the factory, in order to control quality from the field to the consumer's plate.

The results of this 2016 study were accredited by a publication in the periodical publication Frontiers in Plant Science. This research was also presented on two occasions, in 2016 and 2017, and was detailed in a report supplied to the interbranch organization.

(See also our article
"Tomato content positively influenced by deficit irrigation")

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