Near infrared technology predicts browning in white seedless table grapes

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DUARTION: Two years
LEAD INSTITUTION: Stellenbosch University (Department of Viticulture and Oenology)
BENEFICIARY: The table grape industry

The browning of white table grapes has been a main reason for rejections in overseas markets and subsequent losses to the South African table grape industry (SATI). It is regarded as the most critical quality issue to be resolved for the industry to remain competitive and prosper financially. Browning typically develops during the later stages of cold storage and is generally not noticed at harvest or during packing. It affects specific berries in a bunch but not the bunch as a whole and it can therefore be assumed that browning potential is related to the particular characteristics located in individual berries.

Investigating the potential of near infrared technology

The Post-Harvest Innovation Programme and SATI financed a project, conducted by Pieter Raath, Dr Hélène Nieuwoudt and Andries Daniels (MSc student and ARC employee) at the Department of Viticulture and Oenology of Stellenbosch University. The main objective of the project was to investigate the potential of near infrared (NIR) technology to predict the browning potential of packed white, seedless table grape varieties.

  • ‘Regal’ and ‘Thompson Seedless’ grapes of different maturity levels were used for testing.
  • A representative sample of individual berries of both table grape varieties was scanned at harvest, after four weeks of cold storage at -0.5°C, followed by one week at 7.5°C.
  • Spectra were compared with chemometric methods to discriminately assess the potential of NIR spectroscopy for detecting browning.
  • Chemical parameters like glucose, fructose, pH, Brix and total titratable acids were quantified in the grapes to test fruit quality.
  • Mid infrared (MIR) spectroscopy was used in transmission and attenuated reflection mode (FTMIR-ATR) to establish calibration models that can be used to predict the concentration of chemical compounds in table grapes.
  • NIR spectra were generated from opposite sides of 72 intact ‘Regal’ table grape berries, from bunches (soluble solids content of 16% to 18%) harvested in the Hex River Valley.
  • NIR measurements were taken in the wave number range, 12493 – 4000cm¹ and full spectra (2209 data points) were generated by using a NIR optic probe with a 10mm diameter tip.
  • Browning phenotypes were determined by visual inspection and classified according to the SATI pamphlet.

Benefits to industry

  • Research results prove that NIR can be used to predict the browning potential of white, seedless table grapes at harvest.
  • This enables producers to make informed decisions. By not exporting fruit with browning potential, they save on shipping costs and potential overseas claims.
  • NIR also provides a rapid, non-destructive method for measuring sugar and acid in table grapes.
  • The project provides calibration models (algorithms for individual chemical compounds) that can be utilised to develop a commercial method of testing. In other words, producers should soon be able to measure the sugar and acid dynamics non-destructively in the vineyard via handheld scanning devices.
  • Further study is planned to verify the current calibration models for commercial implementation and to generate algorithms for additional chemical compounds.