Bornimer Agrartechnische Berichte
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Abstract
Broccoli (Brassica oleracea var. italica) is a highly perishable crop. The aim of the work presented here is to find ways to predict storability of broccoli shortly after harvest. Several methods are examined regarding their suitability for predicting broccoli storability.
The non-destructive, optical measuring devices used in this work represent some of the common methods used for scientific quality assessment (color determination, chlorophyll fluorescence analysis). In addition, spectral data from diffuse reflection measurements, from both spot measurements and hyperspectral images, was evaluated. The generative development of broccoli curds was studied using diameters both of single buds and of whole curds, and was related to subsequent storability.
The concentrations of the chlorophylls a and b, green chlorophyll catabolites chlorophyllide a (Chlide), pheophytin a (Pheoin), pheophorbide a (Pheoide) and carotenoids ß-carotene (ß-Car), lutein (Lut), neoxanthin (Nx) and violaxanthin (Vx) were determined in floral buds of broccoli through the use of high pressure liquid chromatography (HPLC). The goal was to determine if chlorophyll catabolites may function as indicators of broccoli storability and, furthermore, to relate the concentrations of photosynthetic pigments to the color change of broccoli during storage.
In order to investigate effects from the field on the subsequent storability of broccoli, cultivation methods were altered. Plants were partly covered with covers of different materials, which affected the quantity and quality of incident global radiation. The application of fertilizers was specifically varied in one broccoli batch. Additionally, the influence of climatic events on the shelf life of broccoli was analyzed by querying an established database.
The non-destructive indexes that proved to be most sensitive to changes during storage were the levels of photosynthetic efficiency (Fv/Fm, ΦPSII), the CIELAB color space coordinate on a blue to yellow scale (b*) and the inflection point of reflection spectra in the transition region from dark red to infrared electromagnetic wavelength range (REIP). However, based on parameter sensitivity, the prediction of value levels after four days of storage at 16 °C and high relative humidity (RH) (~ 95 %) was unfeasible. Quality indexes that changed on the same storage day did not necessarily end up at the same value level. Additionally, values of both Fv/Fm and ΦPSII did not adequately reflect the quality status of broccoli when different harvest batches were compared.
In hyperspectral images, early quality changes were effectively rated based on Euclidean distances within broccoli curds and were displayed using reflection intensities at three wavelengths within the visible electromagnetic spectrum as RGB channels. The observed variation within broccoli curds was related to differences in the degree of senescence of single floral buds. However, the diameters of floral buds did not correspond to a rating of storability. In contrast, curd diameters within individual harvest batches were related to subsequent yellowing.
The yellowing of broccoli was based on the decomposition of chlorophyll, which unmasked the carotenoids present. No accumulation of chlorophyll catabolites was found during storage. A trend towards rapidly increasing chlorophyll a/b ratios was observed in a broccoli batch with particularly high storability.
Different N-fertilization of broccoli curds closely corresponded both with values of initial lightness L* of the CIELAB color space (R² = 0.70) and normalized difference vegetation index NDVI (R² = 0.73). Yet, N-fertilization had no impact on further storability. A negative impact on storability was observed when the spectrum of incident radiation on the field was temporarily reduced by using plastic covers with limited transmittance (λ = 400-500 nm). In a similar way, an increased number of hours with high solar radiation intensity (> 600 W m-2) during cultivation had a negative impact on broccoli storability (R² = 0.47). The impact of high irradiation partly corresponded with temperatures higher than 25 °C (R² = 0.29). A strong predetermination of curd storability was observed based on the water availability for broccoli plants in the last two weeks before harvest (R² = 0.71).
A successful prediction of broccoli storability is therefore most likely to be conducted by looking at effects during cultivation. For early non-destructive evaluation of the current quality status of broccoli, both the parameter b* and the spectral index REIP were highly suitable. Therefore, through the help of monitoring systems throughout cultivation (database of climate and cultivation incidents) in combination with reflection-based, hand-held devices during storage, the prediction and rating of broccoli storability can certainly be improved.