Effect of storage conditions on physical and biochemical properties of distinct kale (Brassica oleracea var. acephala) cultivars: A comparative study on Lacinato and KPTL-19

Introduction

Globalization and shifts in economic standards across developing countries, particularly India, have significantly restructured consumer food perceptions. Modern generations increasingly prioritize nutrient-dense functional foods that contribute directly to physiological health, metabolic regulation, and the prevention of chronic nutritional diseases. Among the cruciferous vegetables (genus Brassica), kale (Brassica oleracea var. acephala) has garnered profound scientific and commercial attention. Characterized by non-heading leafy rosettes, kale is recognized as a premier superfood owing to its high concentration of vitamins (A, C, B6, E, and K), essential dietary minerals (calcium, iron, potassium, magnesium, and phosphorus), and diverse bioactive phytochemicals, including glucosinolates, polyphenols, carotenoids (lutein, zeaxanthin), and anthocyanins. In India, kale is cultivated as an exotic winter crop under cool, dry climates, primarily across Jammu and Kashmir, Himachal Pradesh, Punjab, Uttar Pradesh, and Haryana. Despite its high agricultural potential and ease of cultivation, kale remains an underexploited commodity within the domestic food processing industry due to its high postharvest vulnerability. At harvest, the fresh foliage exhibits an elevated moisture content (approximately 84.5% to 86.0% wet basis) and large surface-to-volume ratios, driving intense physiological respiration and transpiration.

Consequently, under ambient storage parameters (20-25 °C), kale suffers rapid water depletion, wilting, enzyme-mediated yellowing, and degradation of labile bioactive components within a few days, destroying its marketability. To optimize domestic commercial logistics and reduce high supply-chain losses, selecting stable cultivars and instituting scientifically validated postharvest handling infrastructure are paramount. Polymeric modified atmosphere packaging, such as low-density polyethylene (LDPE), acts as a partial moisture and gas barrier, establishing an equilibrium gas composition that reduces transpiration and respiration. However, the precise interaction between modern exotic cultivars and temperature treatments remains inadequately characterized in Indian processing research. The present investigation was therefore executed to evaluate the physiological and biochemical properties of two prominent kale cultivars: 'Lacinato' (Tuscan/Dinosaur kale), celebrated for its narrow, elongated, intensely dark bluegreen blistered leaves; and 'KPTL-19', an heirloom variety developed by the ICAR-Indian Agricultural Research Institute (Himachal Pradesh, Kullu Valley) characterized by flat, frilly 'oak leaf' shapes with distinctive purple stems and veins rich in anthocyanins. This research directly establishes the physical and biochemical kinetics of both cultivars under ambient atmospheric storage versus refrigeration (5 ± 1 °C) to provide definitive processing frameworks for shelf-life extension.

Materials and Methods

Plant Material Procurement and Sampling

Freshly harvested, uniform, and disease-free leaves of Lacinato kale (Brassica oleracea L. var. acephala) were obtained directly from the experimental fields of the Department of Vegetable Science, Punjab Agricultural University (PAU), Ludhiana. Concurrently, leaves of the heirloom kale cultivar KPTL-19 were sourced from the ICAR-Indian Agricultural Research Institute regional center, Kullu Valley, Himachal Pradesh, and transferred under controlled temperature protocols to the laboratory. All primary experimental testing and quantitative analyses were executed at the Department of Processing and Food Engineering (College of Agricultural Engineering and Technology) and the Punjab Horticultural Post-Harvest Technology Centre (PHPTC), PAU, Ludhiana.

Preliminary Physical parameters Analysis

Prior to storage allotment, initial structural attributes including leaf length, width, and surface area were systematically quantified from randomly selected fresh replicates. Linear measurements (length and width) were obtained using a calibrated metric scale. For the irregularly surfaced leaves, leaf surface area was determined via the standard grid tracing method. Outlines of individual leaves were precisely mapped on 1 mm resolution graph grids, and total leaf area was calculated by quantifying the sum of bounded squares (assuming a base grid resolution of 1x1 cm). Partial boundary squares covering more than 50% of the grid area were registered as integer square units, while stem/petiole surface areas were excluded from the final computational summation.

Experimental Design and Storage Treatment Packages

Freshly harvested leaves were subjected to a standardized preparatory washing process utilizing tap water to eliminate field debris, followed by ambient air-drying to completely remove surface moisture. Sample units weighing precisely 50 grams were grouped and enclosed within moistureretentive Low-Density Polyethylene (LDPE) packaging film pouches. The sample packages were securely enclosed using a manual impulse heat sealing packaging machine. The experimental matrices were distributed into two independent storage environments for a 15-days trial: 1. Ambient Storage: The kale leaves were stored in the month of February and March 2020 at an average ambient temperature of 20 ± 2 °C. 2. Refrigerated Storage: Maintained inside a cold room chamber at temperature of 5 ± 1 °C.

Destructive and non-destructive sample evaluations were conducted at four synchronized intervals: Day 0 (Fresh baseline), Day 4, Day 8, and Day 12.

Analytical Procedures for Quality and Biochemical Parameters 2.4.1. Headspace Gas Composition Analysis

To track internal packaging atmosphere dynamics driven by respiration, the volumetric concentration (%) of gaseous Oxygen (O₂) and Carbon Dioxide (CO₂) inside the sealed pouches was measured non-destructively using a GS6600 O₂ & CO₂ Headspace Analyzer (Systech Illinois/Gaspace 6000). The dual detector infrared and zirconia sensor unit was calibrated using ambient atmospheric baseline standards. An airtight hypodermic sampling needle probe was driven through an adhesive rubber septum fixed to the external pouch wall, automatically aspirating fixed gas volumes to display stabilized concentration data on screen.

Physiological Weight Loss (PWL)

Gravimetric moisture loss kinetics were tracked using a precise digital weighing balance. The cumulative physiological weight loss percentage (PWL) relative to the fundamental baseline mass was calculated for individual sampling dates via the following standard equation:

where W0 represents the baseline fresh packaging weight (50 g) at Day 0, and Wt denotes the absolute package weight recorded at specific testing intervals (t = 4, 8, 12 days).

Surface Color Evaluation (CIELAB)

Surface color changes were tracked utilizing a Konica Minolta CR-400 Handheld Portable Chroma-Meter, calibrated against a standard white tile configuration. The CIELAB tristimulus color space values were captured across three primary coordinates: L* (lightness ranging from 0 = black to 100 = white), a* (chromaticity defining red when positive and green when negative), and b* (chromaticity defining yellow when positive and blue when negative). Multiple surface point readings were executed at designated leaf blade and midrib coordinates to construct accurate averages. The Total Color Difference (ΔE) representing cumulative deviation relative to fresh baseline tissue was determined mathematically via standard Euclidean distance algorithm tracking over time. titration method using a 0.4% oxalic acid stabilizing extract phase. Total chlorophyll content was determined spectrophotometrically at absorbance wavelengths of 660 nm and 642.5 nm following exhaustive tissue tissue extraction inside an 85% acetone extracting medium matrix.

Cutting Strength

Results and Discussion

Morphological and Baseline Characteristics of Fresh Cultivars

The two cultivars exhibited striking morphological distinctions at harvest. Fresh 'Lacinato' leaves exhibited an elongated, narrow, blade-like architecture with an average baseline length of 34.41 cm and width of 9.06 cm, resulting in a mean individual leaf surface area of 220.71 cm² (Table 1). Visually, Lacinato displayed an intensely dark, bluegreen color with a profoundly crinkled, embossed surface texture. In contrast, 'KPTL-19' leaves presented an 'oak leaf' morphology with deeply frilly, toothed margins and distinctive purple coloration along the stem and primary veins, indicating a high concentration of vacuolar anthocyanin pigments. KPTL-19 leaves possessed a baseline length of 22.5 cm and width of 14.0 cm, providing a mean surface area of 235.0 cm² per leaf.

Physiological Weight Loss (PWL)

Weight loss in harvested leafy vegetables is driven primarily by transpirational water loss, which directly degrades visual quality, mass, and freshness. Under ambient storage conditions, both cultivars suffered rapid and severe moisture loss. By Day 4, Lacinato leaves lost 11.2% of their initial weight, while KPTL-19 lost 16.9% (Table 3). Since weight loss exceeding 5% to 10% causes visible wilting and shriveling, both cultivars became entirely unmarketable and were discarded after Day 4 under ambient conditions.

Cutting Strength

The peak mechanical cutting force required to shear kale leaves increased across all experimental groups, driven by water loss and cell wall lignification. Under ambient storage, Lacinato's cutting strength increased from 2.70 N to 3.39 N by Day 4 (Table 5). This change was even more pronounced in KPTL-19, which surged from an initial 4.26 N to 7.74 N by Day 4. This rapid increase in cutting force under ambient conditions stems from severe moisture depletion, which leaves the vascular ribs tough, fibrous, and leathery, thereby reducing consumer acceptability. This research confirms that combining polymeric packaging with mechanical refrigeration is essential to maintain the quality, maximize shelf life, and reduce postharvest losses of kale within the commercial supply chain.

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