Abstract

Authors: M.I. Cantwell, K. Albornoz, G. Hong Keywords: damage, leaf maturity, temperature, visual quality, decay, off-odors DOI: 10.17660/ActaHortic.2019.1256.42 Abstract: Freshness is a very desirable attribute for all fresh produce, but especially leafy greens. Loss of freshness may occur due to postharvest factors such as time, temperature abuse, and mechanical injury. Ammonia, a product of protein catabolism associated with senescence of leafy greens, is toxic to plant cells and accumulates during postharvest handling. The time-course of ammonia accumulation (spectrophotometric determination) and marketable quality attributes (overall visual quality, decay, yellowing) was studied in relation to storage temperature and leaf maturity in fresh-cut kale, and in relation to storage temperature and mechanical damage in baby leaf spinach. Ammonia increased more rapidly in mature than immature kale leaves at 0, 5 or 7.5°C. Kale storage at 0°C clearly minimized quality loss as well as ammonia accumulation. At 7.5°C, differences in ammonia occurred before there were visible differences in kale quality attributes. At 0 and 5°C, measurable changes in ammonia coincided with visible quality changes. Young spinach leaves with greater mechanical damage had earlier increases in decay, off-odors and ammonia concentrations when stored at 2.5 or 7.5°C compared to leaves with minimal mechanical damage. Baby leaf spinach stored in perforated bags at 0, 5 or 10°C had ammonia increases coincident with or before changes in visual quality, decay and off-odors. Accumulation of ammonia in leafy green tissues should be associated with an increase in volatile ammonia when the product is in a package. This was effectively demonstrated in one spinach storage experiment using sensitive experimental volatile ammonia sensors. Spinach held for 6 days at 0°C had very little tissue or volatile ammonia accumulation compared to spinach samples held at 10°C for 6 days or 20°C for 2 days. Although there is probably no single measurement of freshness, there is evidence that for leafy vegetables, ammonia accumulation could be a useful freshness indicator. Ammonia can be determined in the leafy tissues or as a volatile in the packaged product.