The Importance of Leaf Maturity to Shelf-life and Quality of Fresh-cut Kale Stored at Two Temperatures

Kale (Brassica oleracea var. acephala) prepared as fresh cut salads has increased in recent years due to its high nutritional content. An attribute that concerns the industry and consumers is the shelf life of the product, which directly affects the nutritional content and visual quality. Currently kale leaves at different maturity stages may be harvested, resulting in a heterogeneous product in terms of composition and shelf life. In the case of the fresh-cut salads this may be more critical due to enhanced deterioration caused by mechanical damage during processing. Changes in composition and visual parameters of fresh-cut kale leaves (var. Lacinato) were investigated for three maturity stages at harvest (immature, mature, overmature), two temperatures (0 and 5 °C) and five periods of storage (0, 14, 21, and 28 d; up to 42 d for 0 °C product).  Product was cut manually into 2 cm strips, washed in chlorinated water, manually centrifuged, and packaged in unsealed LDPE bags. Total chlorophyll content (mg/g FW) decreased during storage, with the lowest concentration found in pieces from overmature leaves at 5 °C after 28 days, while the total carotenoid content did not vary among the different conditions studied. Ammonia content (mg/g FW), an indicator of membrane degradation caused by abiotic stress, remained low for pieces from all maturity stages stored at 0 °C during 42 days, but increased dramatically in pieces from mature and overmature leaves at 5 °C between 21 and 28 days of storage.  Color (L*, a* and b* values determined with a reflectance colorimeter) as well as marketability indicators (off-odors, overall visual quality, yellowing, decay, and browning on cut ends based on rating scales) presented significant differences in response to the postharvest conditions studied.  In general, the loss of composition and visual quality of fresh-cut kale leaves increased with temperature, leaf maturity stage at harvest and days of storage.