A group of Spanish researchers is conducting studies into sustainable approaches for using chlorine dioxide to disinfect surface irrigation water for produce fields. But they say their results should be useful to growers worldwide, regardless of where they farm.
"We want this to be applicable for all growing conditions, all climatic conditions and all water sources for any growing area," said Ana Allende, Ph.D. and a senior researcher with the Department of Food Science and Technology of the CEBAS-CSIC in Murcia, Spain, and principal investigator. CSIC - or Consejo Superior de Investigaciones Científicas - is the Spanish National Research Council.
The study, titled "Demonstration of practical, effective and environmentally sustainable agricultural water treatments to achieve compliance with microbiological criteria," seeks to characterize the efficacy of chlorine dioxide as an agricultural water disinfectant among E. coli, water and the disinfectant, chlorine dioxide.
To that end, Allende and co-investigator, Mabel Gil, Ph.D. also a senior researcher with CEBAS-CSIC, are working with one of Spain's largest vegetable growers to field-test their mathematical models for validation of minimal doses under commercial conditions.
During the first year of the two-year project, the researchers characterized E. coli strains from three water sources - streams, reservoirs and surface water. When they weren't able to culture E. coli on petri plates, they used DNA to determine E. coli levels. They found the pathogen to be more abundant in smaller streams.
At the same time, the two characterized the physical and chemical parameters of the three water sources, including pH, organic matter, alkalinity and ultraviolet absorbance. In the end, they found no significant differences among the sources as far as the physical or chemical characteristics except for UV absorbance.
The researchers compared their results to those reported in the literature to determine similarities or differences with irrigation water sources in other parts of the world.
"We were comparing them to the worst-case scenarios in the irrigation water, so we were selecting for three types of water to really cover a very wide range of microbial qualities," Allende said. "We had data from very clean water and very dirty water. By doing these three, I think we will be able to cover all of the situations that growers will find in different parts of the world."
They used the new-found data to develop a mathematical model that includes dosing rates of chlorine dioxide to meet E. coli log reduction levels contained in most food safety guidances. The model also will factor in chlorine dioxide disinfection byproducts to ensure they don't exceed established regulatory threshold levels and won't affect the quality of the produce.
In addition, Gil said they plan to use those mathematical equations to develop a user-friendly online tool for growers to determine chlorine dioxide injection rates. Users will input a few parameters from their water quality tests, such as E. coli counts and the UV absorbance, and the online tool will provide a chlorine dioxide dose range required to reduce microbial loads to the levels recommended in most food safety guidances, Gil said.
Putting model to a real-world test
In October 2015, the researchers started the experimental studies with one of Spain's largest produce growers who had cooperated with them on trials in the past. Allende said working with grower-cooperators has been mutually beneficial because they can test these treatments to disinfect irrigation water under real-world conditions.
The grower benefits, too. "It's not just that we're looking at the efficacy of the disinfectant, but we also will give them science-based results about whether this is useful under real conditions and what the environmental impacts are," she said.
The grower volunteered about 4 hectares of baby spinach that were sprinkler irrigated. The field was divided into four replicated plots for comparison of treated to untreated checks.
Guided by their mathematical equation, the researchers applied chlorine dioxide throughout the crop's growing season.
The grower-cooperator met with the researchers regularly to exchange information about their progress as well as the field cultural activities.
Throughout the season, the researchers pulled water, soil and crop samples every week from at least five sampling points to analyze for microbial diversity as well as the physiological status of the plants. Analyses also included quantification of the chlorine disinfection byproducts in irrigation water, plant tissue and soil. In order to determine changes in the natural microbiota of plant and soil, the researchers will use next generation sequencing techniques (NGS) such as Illumina (Solexa) sequencing.
The data, which is still being analyzed, also will be used to fine-tune their mathematical equation before the researchers repeat the field trial this spring to see what, if any, effect warmer growing conditions have on chlorine dioxide activity.
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