Abstract

Partial least squares (PLS) regression models were constructed from near infrared spectra to predict weight loss in mushrooms stored at ambient conditions (20°C, 78% RH). Regression coefficients were examined with the aim of estimating the water absorbance pattern for a better understanding of the process of mechanical damage in mushrooms. Sequential spectra of 32 mushrooms, harvested at two different times, were obtained over the 400–2498 nm range; half of the samples were physically damaged by vibration. Data analysis focussed on the region of the combination of asymmetric and symmetric stretching modes of water molecules (i.e. 1300–1600 nm). Standard normal variate (SNV) transformation of second derivative spectra performed better than other spectral pre-treatments tested to correct the baseline shift and scatter present in the data; mean centring of both weight loss and pre-treated spectra was necessary to correct variability between mushroom samples (i.e. size, shape, initial moisture content, maturity). Main spectral variations were found at the following wavelengths: 1408–1410 nm, 1434 nm, 1462–1464 nm and 1520nm. Previous researchers have assigned these bands to free (1408–1410 nm), intermediate strongly-bound (1434nm and 1462–1464nm) and strongly-bound (1520 nm) water molecules. The regression coefficient vector of undamaged mushrooms presented relevant features at all four bands; that for damaged mushrooms lacked significant coefficients around 1520 nm. Additionally, the regression vector and weight loss of damaged mushrooms exhibited opposing trends around the 1460 nm band.

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