Keeping Quality of Green Soybeans

I. The Factors Concerning Postharvest Quality Deterioration and the Effects of Atmosphere and Whole Plant Packaging

Abstract

Green soybeans, a common crop in Japan, is harvested at the green-pod stage and its somewhat immature seeds are eaten as a snack etc. The taste of the seeds, however, deteriorates much more rapidly than the visual quality. This paper reports the investigation on the factors concerning the post-harvest quality deterioration and the means of keeping quality, especially applicable under non-refrigerated conditions for practical use. Shirayama-and Komagi-dadachamame cultivars were used and their pods were packaged in perforated polyethylene (PE) bags and held at 0°, 5° and 20°C, respectively.
Organoleptic test showed that the taste of soybean became flat in 2 days at 20°C. Yellowing or browning of pods began after several days at 20°C, and 2 weeks at 5 °C. At 0°C, the loss of freshness of pods was noticed after about a month, while green color remained.
Sugar content, index of free amino acid content (ninhydrin-positive substances), and L-ascorbic acid content in seeds drastically decreased in 2 days at 20°C. At 5 °C, marked decrease in sugar and amino acid was found after a week. At 0°C, amino acid considerably decreased after 2 weeks of storage while sugar content showed little change. Post-harvest deterioration in the taste of the green soybeans is supposed to be principally attributable to the decrease of sugar and amino acid.
When metabolic inhibitors were applied to the seeds by vacuum infiltration, change of the sugar content was retarded with NaF and 2, 4-dinitrophenol (DNP), and that of amino acid with cycloheximide and DNP.
Some other pods were packed in PE bags of 0.07mm film-thickness, and the atmosphere in bags was replaced by N₂or CO₂, or withdrawn under vacuum. These treatments brought about a retardation of sugar loss, and an increase of amino acid in contrast with a sharp decrease in non-treated pods. Simple sealing also had a comparable effect to the a bove-mentioned treatments, showing high CO₂ and low O₂ concentrations in the bag during the days succeeding the sealing. At 20°C, however, shelf life reduced to 3-4 days since a symptom of gas injury developed on the surface of pod and seed. At 5°C, no injury was found during 2 weeks of storage and loss of sugar and amino acid was considerably retarded.
Packaging of a whole plant with leaves and pods attached in a PE bag of 0.03mm-thickness exhibited a marked effect on keeping quality, and preferable taste was kept for several days even at 20°C. External appearance was also maintained by this means, and pod color remained green without any gas injury for more than 10 days at 20°C.