Postharvest sanitizers and fungicides

Postharvest sanitizers and fungicides

By: Francisca Barros, Militza Ivelic and Jessica Rodríguez – Kimün Spa Trio.

The cherry is a fruit sensitive to mechanical damage, so the movement in the line is carried out by water. The water, in turn, is used in recirculation ponds where we must ensure its safety in terms of controlling microorganisms that are dangerous to human health, as well as avoiding the presence of phytopathogenic fungi that could cause postharvest rot.

To sanitize the process water, the most used product in the industry has been chlorine, with sodium hypochlorite being the main source and currently calcium hypochlorite.

What are the advantages and disadvantages of this sanitizer?:

Hypochlorites are cheap products, highly reactive with organic matter, this being the main problem. Due to the way the cherry is harvested, the dirt present in these is washed away, dirtying the water, therefore having to constantly add more product to maintain an adequate concentration that ensures microbiological control.

 The technical information collected indicates that the control dose using hypochlorites for microorganisms (such as Eschericha coli, Listeria and Salmonella) that cause damage to humans is 1 to 2 ppm. The situation is different when we want to control phytopathogenic microorganisms where a minimum dose of 20ppm is required.

 A problem also mentioned in the technical literature refers to the decrease in the effectiveness of hypochlorites depending on the pH of the water. At acidic pH (<5) there is a risk of volatility, which could affect people’s work, while with very basic pH (>7.5) its effectiveness is lost. As a result of the above, the industry worked with these sanitizers at high concentrations, by not controlling the pH (100-120ppm). At present, the use of these sources is questioned due to the residues left by perchlorate and chlorate in the fruit. In Europe, for example, Regulation (EU) 2020/685 establishes a Maximum Residue Limit (MRL) for chlorate in fruits and vegetables of 0.05ppm.

Considering the indicated restrictions on residues in the fruit and because we must work in a more sustainable manner, hypochlorite applications have been changing towards automation, where the adjustment of pH and dosage have allowed concentrations to be lowered to levels of 20 to 40 ppm.

Most of the systems that are being used in cherry plants are ORP equipment (oxidation and reduction potential), but other chlorine dosing systems are also emerging (Photograph 1).

Photograph 1. Automatic equipment to verify sanitizer concentrations, ORP system, and direct ppm measurements.

To monitor the concentration of sanitizer in process water, the industry initially used the titration method with chemical reagents; currently, adapted photometers are used to measure high concentrations and to quickly verify the concentration, strips are also used. reactive (photo 2).

Photograph 2. Photometer and chlorine indicator strips to verify free chlorine concentrations in process line water.

Another way of providing a chlorinated source are halogenated compounds (sodium hypochlorite and chlorite), which have the advantage of being less reactive to organic matter and do not give off aromas, in addition to not requiring pH adjustment. The application of these products can also be done automatically and the concentrations used vary between 50-80 ppm.

  There are other sanitizers in the industry that are less questionable due to residues, such as peracetic acid. The formulations are varied and we must be clear that when using them we must consider the one that allows their contact with the fruit. Product of the strong odor in distribution containers, manual application is impractical and requires the use of automatic dosing equipment. The recommended working doses vary between 40 to 80 ppm.

Among the sanitizers offered in the cherry industry is ozone, but it is not widely used and requires equipment that generates the product “in situ” and injects it into the water.

Regarding the fungicides applied in the process lines, we must go by part and the first thing is to understand that said application is carried out with the aim of protecting the healthy fruit against contamination, by contact, of some fruit with latent infection and that we pack without we figure out.

In cherries, the way these products are applied has always been by immersion and has also varied over time, from a small drench well at the end of the line, to today when it is used in distribution ducts after calibration. (Photo 3). In said application, the exposure time is closely related to the residual left on the fruit, which we hope will remain around 1 ppm.

Photograph 3. Application of fungicide in post calibrator distribution gutters.

Regarding which fungicide to use in postharvest, we must ensure that it is allowed in the destination markets and that it has the authorizations for its use in postharvest and that it has the registration label in Chile for cherries.

As the fruit goes to different markets, we must try to use the one that is allowed in the largest number of countries and make it compatible with its effectiveness in controlling the phytopathogenic fungi of interest.

Currently we have five molecules to apply in postharvest (Table 1), being the most used “Fludioxonil”. This molecule has also been used in hydrocoolers or in showers immediately after the hydrocooler. These applications are made when the waits for processing are greater than two days of storage or in the event that during the harvest, a rain event occurs. The choice of this product has been for its efficiency in the control of phytopathogenic fungi but also because it does not present incompatibility with the chlorine concentrations in the water.

Table 1. Maximum residue limits by market for postharvest cherry. Extract Agenda Pesticides Asoex (August 2021).

Of the five alternatives mentioned above, two other molecules used in the industry are: “tebuconazole”, generally as a complement to another fungicide for the control of Geotrichum, which although not so common, appears in some cases in cherries. Unfortunately for Europe, this product only has an MRL of 1 ppm, which makes it restrictive and many times for safety reasons, in order not to have detections above said threshold, it is removed from the line programming.

The third molecule used is “pyrimethanil” which, when applied, cannot be used with chlorine since there is a reaction and in these cases the sanitizer used has been peracetic acid.

Regardless of the fungicide used, we must be clear that the application operation in large ponds brings with it some complications that we must be aware of. One of these complications is the generation of foam as a result of high turbulence, it can also happen that due to the unevenness of the water that the machines present, air enters the system and foam is generated. On the one hand, this foam carries away the active ingredient, and on the other hand, it runs the risk of staining the fruit.

Another problem is that the suspension due to low movement or agitation causes the precipitation of the ingredient and therefore we will have less concentration in suspension and therefore less residuals.

The fungicide applications in the lines are carried out manually, but progress is currently being made in the automation of the process. In this way, a more even residual has been achieved in the fruit and the risks of handling the product by the staff are minimized, since the machinery is in charge of dosing and products are only added to the extent that the reserves are consumed in the ponds.

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