Non-Invasive Monitoring of Berry Ripening Using On-the-Go Hyperspectral Imaging in the Vineyard

María Paz Diago,Juan Fernández-Novales,Ignacio Barrio

doi:10.3390/agronomy11122534

Abstract

Hyperspectral imaging offers enormous potential for measuring grape composition with a high degree of representativity, allowing all exposed grapes from the cluster to be examined non-destructively. On-the-go hyperspectral images were acquired using a push broom hyperspectral camera (400–100 nm) that was mounted in the front part of a motorized platform moving at 5 km/h in a commercial Tempranillo vineyard in La Rioja, Spain. Measurements were collected on three dates during grape ripening in 2018 on the east side of the canopy, which was defoliated in the basal fruiting zone. A total of 144 grape clusters were measured for Total soluble solids (TSS), Titratable acidity (TA), pH, Tartaric and Malic acid, Anthocyanins and Total polyphenols, using standard wet chemistry reference methods, throughout the entire experiment. Partial Least Squares (PLS) regression was used to build calibration, cross validation and prediction models for the grape composition parameters. The best performances returned determination coefficients values of external validation (R2p) of 0.82 for TSS, 0.81 for Titratable acidity, 0.61 for pH, 0.62 for Tartaric acid, 0.84 for Malic acid, 0.88 for Anthocyanins and 0.55 for Total polyphenols. The promising results exposed in this work disclosed a notable methodology on-the-go for the non-destructive, in-field assessment of grape quality composition parameters along the ripening period.

Highlights

Grape ripening is usually monitored to track the accumulation and/or catabolism of primary and secondary metabolites in the pulp, seeds and skins of the berries, in order to identify maturity advancements or delays, and to determine the optimal composition of the fruit towards the designation of harvest time
total soluble solids (TSS) ranged between 11.10 ◦Brix to 24.5 ◦Brix, pH varied between 2.69 and 4.50, Titratable acidity (TA) ranged from 2.20 to 13.40 g/L tartaric acid, malic acid varied from 0.84 to 11.1 g/L, tartaric acid concentration ranged from 4.55 to 12.47 g/L, while anthocyanins and total polyphenols ranged from 0.10 mg/g berry and 0.35 absorbance units (AU)/g berry to 2.28 mg/g berry and 2.24 AU/g berry, respectively
These boxplots represent the variability of data along the ripening period, with the following statistical metrics: minimum, maximum, median, first and third quartile, and the mean value of each grape composition parameter for each sampling date

Summary

Introduction

Grape ripening is usually monitored to track the accumulation and/or catabolism of primary and secondary metabolites in the pulp, seeds and skins of the berries, in order to identify maturity advancements or delays, and to determine the optimal composition of the fruit towards the designation of harvest time. Among the main berry compositional parameters, total soluble solids (TSS), berry acidity, often expressed as pH and Titratable acidity (TA), and concentrations of the main organic acids in the berry, such as tartaric and malic acid, as well as the anthocyanin and total phenol concentrations (these in red varieties only) are usually analysed using wet chemistry procedures on periodically sampled fruit during five to six weeks before harvest [1]. These analytical methods are destructive, require time-consuming berry sampling, as well as sample preparation in most instances. These facts highlight the relevance of the design, definition and representativeness of fruit monitoring if the aim is an accurate and robust estimation of its compositional evolution along ripening

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Conclusion