By: Hector Garcia O., Co-Founder and General Manager of Laboratorios Diagnofruit Ltda, SOCHIFIT and AMICH Member. [email protected]
The title of this article is the basis of a sometimes irreconcilable struggle between the technical teams of the pre-harvest fruit exporters versus the post-harvest team. For those of us who have worked in a post-harvest technical department, we constantly had to put on the table the idea that “post-harvest is not a hospital”, which implies that many pre-harvest factors are those involved in the performance of the fruit during storage. Thus, components such as pathogen load in the field, maturity problems, high nitrogen concentrations, low calcium levels, poor harvest (bruises, tears, etc.), are usually the most relevant characteristics, regardless of the type of fruit, that most frequently become the determinants of a negative production process.
Despite what has been described, all post-harvest handling is important and must be controlled to perfectly preserve fruit that has been optimally produced and harvested and at the same time contain or try to prolong the life of fruit that has deficiencies or imbalances.
How to identify imbalances, deficiencies or other negative aspects in cherries?
Prior to harvesting, the post-harvest technician must know the fruit of each productive unit that he will receive, so even if he does not go to the orchard, he must establish certain diagnostic patterns to evaluate all seasons, just as a doctor must monitor someone who is going to run a marathon for the first time. Under this scheme, there are some critical points for cherries:
- Nitrogen and eventually calcium content in fruits, which helps us to look at the balance of the plants. Without going into the field, high levels of nitrogen and low calcium can describe overloaded orchards, which are generally more susceptible to diseases, bruises and pitting; making the analogy with the doctor, it would be like discovering clogged arteries that would put at risk the patient who is preparing for a marathon.
- Pathogen load such as Botrytis, Alternaria and Geotrichum. Different techniques can be applied to determine this parameter, conventional ones such as humid chambers are relatively successful, but they are slow, or alternative molecular analysis based on qPCR that can give us results in a couple of days. The important thing is to choose a technique and always use it, standardize the area of the plot, the sampling date, all with the objective of being able to compare year after year. The recommendation is to sample at color break, in this way we have the opportunity to analyze the results before harvest and eventually generate changes in the phytosanitary program or destination of the fruit.
Then, to complete the diagnosis, we must observe and compare with historical results, and add, once the fruit is in the packing house, the values of reception of parameters of maturity and quality, such as soluble solids, hardness, size, bruises, etc., in this way we generate an entry grade for the fruit, with which we segregate. Once this process is finished, which is not without difficulties, especially due to the order of the information and capacity of analysis, we must worry about post-harvest management; today we will discuss the technical bases of two key stages: cooling and chlorination.
Temperature management
As is widely known, modifying the post-harvest environment is one of the key tools that allow us to have a longer life for the fruit. Due to the delicate nature of the cherry, this is probably the fruit species within Chilean fruit production that receives the most attention in terms of temperature and humidity control. In general terms, lowering the temperature involves a decrease in metabolic processes, and not only in the fruit, but also in phytopathogenic fungi (Figure 1), which transforms this technique into the first barrier to containing infection.
Mushrooms like Botrytis, Penicillium and Monilinia, are mesophilic, this means that their optimal development occurs between 15ºC and 30ºC, therefore they are affected by very low and also very high temperatures. So, as we can conclude, lowering the temperature of the cherries, a process that begins with hydrocooling, is crucial to keep the fungi at bay and must be done as soon as possible; delays in entering this process can have negative consequences on the final result (Figure 2). Many times, in the weeks peak In the case of harvesting, the waiting time between the arrival and processing of fruit can exceed several days. In this particular case, it is recommended to integrate fungicides into the hydrocooling that slow down infection processes triggered in the first stage of post-harvest and protect healthy fruit from contagion by contact or contamination.
Figure 1. Development of brown rot caused by Monilinia fructicola in peach fruits at constant temperatures. (Taken from Brooks and Cooley 1928 by Kader, A. 2002)
Figure 2. Delays in refrigeration and subsequent development of rot in peaches stored at 0ºC immediately or after 24 or 36 hours of storage at 20ºC. The data indicate the development of the disease in 3 days (A) and 6 days (B) after being removed from refrigeration (Kader, A. 2002)
Water disinfection and monitoring of active chlorine
The cherry packing process is one of those that has the most relation with water, from hydrocooling to calibration, this element is present in high volumes. All this water recirculates, so contamination with pathogens is very easy to occur; lots with a high inoculum load could end up infecting others with an optimal level at harvest. So, chlorination takes on absolute relevance as a water disinfection agent. The process is relatively simple, the aqueous solutions obtained from sodium hypochlorite (NaOCL) or calcium hypochlorite (Ca(OCl)2) produce the biocide hypochlorous acid, interacting with pathogens through rapid and non-specific oxidation. Its virtues include: high efficacy if used properly, low cost and reduced contamination.
However, something very important to consider is that, as it reacts without specificity, hypochlorous acid is very inefficient in stopping a fungal infection that has developed in the fruit. The same compounds or exposed fruit cells inhibit the disinfection process. Therefore, it should not be considered a curative element or continuous protection, as are certain fungicides that we also use in post-harvest and will characterize in another chapter.
As Professor Adel Kader explains in his book “Postharvest Technology for Horticultural Crops”, when calcium or sodium hypochlorite is added to water, the following are produced: hypochlorous acid and calcium or sodium hydroxide. The magic of chlorine disinfection lies in the balance of this reaction and it tells us that we must take care of the presence of hypochlorous acid in the solution.
NaOCl + H2O ↔ NaOH + HOCl
Sodium hydroxide dissociates and OH- is neutralized with HCO3-, which is found naturally in water, and in Chile it is more common than we would like. Depending on the pH, hypochlorous acid is in equilibrium with the ions, as shown in the following equation:
HOCl ↔ H+ + OCl–
So, pay close attention to this fact: The pH of the solution determines the proportion of “active” chlorine., as opposed to the “inactive” hypochlorite ions (OCl–). HOCl and OCl– Together they represent the amount of “free” chlorine, which is what we must routinely measure in packing. Hypochlorous acid or active chlorine is an active disinfectant, while OCl- is a non-active disinfectant.
In summary, the concentration of active chlorine determines the oxidation potential of the solution and its disinfecting power. At a higher pH, the amount of active chlorine is dramatically reduced (Table 1). However, a low pH (e.g. pH 3-6) will result in the volatilization of chlorine in the form of chloramines; therefore, monitoring Free Chlorine and the pH of the water is a critical point to review periodically, considering that it can also be affected by temperature and residues in the water (plant material, soil, etc.) (Table 1).
Table 1. Effect of pH and temperature on the concentration of active chlorine (HOCl) (modified from White, 2009)
Now that we know the physical-chemical bases of chlorine disinfection, the importance of monitoring is revealed to us more easily. To achieve this objective, the first thing is to establish an optimal threshold of ppm of free chlorine in water, which should be in a range close to 100 ppm; this value depends on the work area, due to the possibility of volatilization and problems for operators since it can be irritating. Once the packing has entered the fruit process, active chlorine and pH curves must be made based on kilos of processed fruit, with this we will establish the moments of hypochlorite replacement based on kilos of processed fruit. Today there are chlorine injection equipment, which must also be monitored to know its limitations.
A variant of the physical-chemical analysis is the biological monitoring of waters, which means evaluating the load of phytopathogens in solution. Both monitoring methods must be combined to generate absolute control of what happens to our waters. We can use two techniques for biological monitoring, both based on water sampling from the different process wells or even hydrocooling (Figure 4). The first technique is conventional, based on plate sowing and visual quantification of colonies (Figure 5); the second involves the qPCR tool, which through specific primers allows us to quantify, for example, conidia of Botrytis in a volume of water and from this create contamination or risk thresholds. The conventional technique has some disadvantages over the molecular one, 7 and 2 days of response respectively, transforming qPCR into the most appropriate for the fast-paced cherry process.
In conclusion, although post-harvest does not improve the condition of the fruit, it is the culminating process of our production chain and must be carried out with the maximum possible control, with the purpose of extending the life of our product, in ideal conditions for consumption. The decrease in temperature of the cherries is the beginning of a series of post-harvest management procedures that we carry out that aim at conservation. The initial cooling and all the movement within the calibration machine is carried out with water, which must be constantly monitored for free chlorine levels, pH and amount of phytopathogen inoculum. To do this, we must establish a system of periodic sampling that allows us to analyze the risks and reduce them based on historical and local information; each machine and water is different depending on the area in which it is installed, therefore the analysis of own data is crucial to successfully overcome the season that is already beginning.
