Food Technology Information Center


Submitted to the Office of the Science Advisor U.S. Agency for International development

Self Regulated Atmospheres to Prevent Fungal Damage in Moist Paddy

Principal Investigator: Ezra (Jonathan) Donahaye. Grantee Institution: Agricultural Research Organization (Volcani Center) Israel
Collaborator: Silvestre C. Andales. Bureau of Postharvest Research and Extension (formerly NAPHIRE) Philippines
Project Number: C12-057
Grant Number: TA-MOU-94-C12-057
USAID Program Coordinator: Dr. Judith Chambers
Project Duration: 27th January 1995 - 30th November 1999

PROJECT STAFF: Israel: Jonathan Donahaye - Principal Investigator. Shlomo Navarro - Co Investigator. Nachman Paster - Co investigator. Miriam Rindner - Technical Staff. Avi Azrieli - Technical Staff. Mazal Menasherov - Technical Staff. David Chemoguz - Technical Staff. Rafael Dias - Technical Staff. Philippines: Sivestre C. Andales Principal Co-Investigator. Filipinas M. Caliboso- Co-Investigator. Glory C. Sabio - Research Officer. Gemma Mallo - Technical Staff. Don David Julian - Technical Staff. Emelie C. Ablaza - Technical Staff. Joel V. Dator - Technical Staff.

Executive Summary: The objective of this project was to provide a solution to the acute problem in far-eastem Asia where paddyrice is harvested at high moisture contents (MCs) during the monsoon season. This paddy must then be dried rapidly to a safe MC in order to pievent it from molding and rotting. However, if the paddy is dried rapidly from about 30% to the required "safe" MC, the grains suffer stress resulting in cracking and breakage. To overcome this problem a two-stage drying procedure has been advocated where the paddy is initially dried to 18% (intermediate MC), at which stage yeast and bacterial activity are suppressed, followed by a second stage drying from 18 to 14% MC to prevent the development of storage molds. However, the drying problem is compounded by the fact that most farmers do not have flash dryers and are obliged to sell their grain directly to the traders. Even if flash dryers are available, insufficient capacity of second stage dryers creates a bottleneck at harvest time.

Our project was designed to develop a technology that would enable farmers to overcome this bottleneck at the second drying stage by providing them with a means of storing the intermediate MC paddy under tightly sealed conditions and thereby prevent spoilage for prolonged periods until drying by sun or machine is again an available option.

The present policy of the Filipino govemment is directed at providing small scale farmer cooperatives with onsite storage units so as to decentralize storage of the national grain reserve as well as provide rural communities with a higher level of food security. Implementation of this policy is under way, and already the concept of sealed storage to protect dry grain from insect infestation has been widely promoted together with the distribution of flexible plastic outdoor storage cubes that were developed by ARO and BPRE as the outcome of a previous CDR project (C7-053). In 1998, about 200 units of these storage structures were purchased and distributed to farmers' cooperative recipients nationwide. Recently, the govemment has purchased an additional 300 units for distribution among farmer cooperatives through soft-loans. This is being done as a mitigating measure in anticipation of "La Niiia".

However, the problem of harvesting moist paddy in the rainy season still remains. Although both the previously developed storage technology, and the present one are based on the same principle of hermetic storage, the objective of the present project was to employ the principle of self-regulated atmospheres caused by aerobic metabolism in order to arrest fungal development and preserve grain quality in paddy of intermediate MC. Here, to prevent spoilage, oxygen depletion must be much greater and more rapid than that required to control insects, and it was anticipated that this would require a higher level of hermetic seal than that required for insect disinfestation.

The first two questions to be answered, before field trials could be initiated, were - do the rates of oxygen depletion obtainable by sealed storage of moist paddy prevent mold proliferation sufficiently in the damp grain; and, - can hermetic storage of intermediate MC grain, be carried out without having a deleterious influence on the aroma, taste and cooking qualities of the rice?

Both these aspects were studied during the first and second years with laboratory studies in the Philippines being undertaken on the effect of hermetically sealed "moist" paddy stored for different time periods on different quality parameters including milling and organoleptic characteristics; while in Israel, studies with the same paddy and also wheat were directed at evaluating rates of aerobic metabolism at different moisture contents and temperatures as a basis for determining rates of oxygen depletion within the storage structures. In the first year a flexible storage structure of 10 tons capacity was manufactured from a plastic laminate chosen from a series of materials that were screened to test their permeability to oxygen and carbon dioxide. This structure was field-tested prior to shipping for paddy storage trials to be undertaken in the Philippines. Although calculations indicated that the low permeability of the liner material would give a sufficient seal to reproduce laboratory conditions, an additional factor was anticipated to have an influence on the storage environment under field conditions. This was the development of air convection currents within the stack that carry moisture and deposit it at the top layer. These currents develop when temperature gradients are formed as a result of diurnal temperature fluctuations. This phenomenon was noted in the previous study when storage cubes were set-up in un-shaded sites. As a counter measure, an insulating layer of rice hulls was placed over the top layer of bags and this solution was adopted as standard procedure. However, for the storage of intermediate MC grain the situation is much more critical since any rise in MC above 18% is liable to enable the anaerobic metabolism of bacteria and yeasts that have a strong influence on grain quality, particularly taste and aroma.

An improved solution developed during this project was the use of an external reflective shade cover placed over the storage cube in order to reduce temperature gradients within the grain and thereby minimize the moisture migration phenomenon. Initial trials that were carried out in both countries in the second year, were inconclusive. However, after modifications, further trials carried out during the third year in both countries gave positive results that have led to adoption of this concept for all outdoor storage in plastic liners, and inclusion of reflective covers in the standard commercial kits.

It was clearly demonstrated in the first year that the rates of oxygen depletion in hermetically sealed moist paddy could prevent mold proliferation, but the effects of hermetic storage upon paddy quality took longer to evaluate than planned, as this required repeat experiments to enable in-depth evaluations of cooking and acceptability parameters that were carried out by Prof. Del Mundo at the University of the Philippines, Los Bafios after 1, 3 and 6 months of storage. The project findings indicated that after 1 month, quality of sealed paddy stored at up to 18%MC had not deteriorated. However, further evaluations made on paddy stored hermetically for 1, 3 and 6 months under both laboratory and field conditions confirmed that after the first month of storage the quality of moist paddy (16-18% MC) deteriorated progressively and the grain was no longer acceptable by the taste panels. These findings enable the following tentative recommendations to be made for paddy storage duration:

(Not confirmed by field trials) For 18% MCone (1) month
For 17% MCone (1) month
For 16% MCcan be extended to two (2) months
For 15% MCcan be extended to three (3) months
For 14% MCstill the recommended level for long term paddy storage

In conclusion, the present widespread implementation of the hermetic storage technology at the cooperative and village level throughout the Philippines has been backed up with BPRE initiated "on-the-spot" extension courses. Many aspects of this technology have not yet been explored especially field validation of the laboratory findings at 15 - 17% MC. However will we anticipate that this enterprise will serve as a starting point for the adoption of hermetic storage to protect paddy of intermediate MC until it can be dried.

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