Ann Peirs

Light penetration properties of NIR radiation in fruit with respect to non-destructive quality assessment

Abstract Some issues related to the non-destructive measurement of apple quality attributes by means of NIR reflectance spectroscopy are addressed. A comparison was made between two optical configurations, which can be used to perform NIR-spectroscopic measurements: the bifurcated and the 0°/45° optical configuration. A relationship was established between the reflectance spectra (880–1650 nm) and the soluble solids content by means of the partial least squares technique. Depending on the data pre-processing method, correlation coefficients between 79 and 91% were obtained. The results obtained with the bifurcated fibre were only marginally better than those obtained with the 0°/45° configuration. The apple skin reflectance and skin transmission properties with regard to NIR radiation were also investigated. The intensity of the light source was high enough to penetrate through the apple skin and gather information about the apple parenchyma tissue. A method was developed to calculate the light penetration depth for each wavelength in the range from 500 to 1900 nm. This method was applied to measure the light penetration depth in ‘Jonagold’ apple fruit tissue. The penetration depth is wavelength dependent: up to 4 mm in the 700–900 nm range and between 2 and 3 mm in the 900–1900 nm range.

Prediction of the optimal picking date of different apple cultivars by means of VIS/NIR-spectroscopy

Abstract The use of visible/near infrared (VIS/NIR) spectroscopy was evaluated to determine the internal quality and the optimal harvest dates of apples non-destructively. Calibration models were constructed with data from eight cultivars, three orchards and 2 years, in order to make the models as robust as possible for future use. The prediction of the maturity, defined as the number of days before commercial harvest, was reasonably accurate. The most robust model predicted the maturity with a validation correlation of 0.90 (SEP=7.4 days). The prediction of maturity, according to the Streif index, showed a validation correlation of 0.84 (SEP=0.18 kg/% brix×starch index) for one orchard. Maturity was orchard-dependent, however, and as a consequence, a combined prediction equation was not accurate. Individual quality characteristics (soluble solids, Streif index, acidity and firmness) were well predicted. The calibration model for soluble solids content resulted in a validation correlation of 0.84 (SEP=0.73% brix) for the results over 2 years from one orchard, but like the Streif index, was orchard-dependent and appeared to account largely for the orchard dependence of the latter. Acidity and firmness were predicted with a validation correlation of 0.80 and 0.78 and SEPs of 2.07 ml NaOH and 1.13 kg for, respectively, two and three orchards over the 2 years.

Temperature compensation for near infrared reflectance measurement of apple fruit soluble solids contents

Near infrared reflectance (NIR) spectroscopy has been used successfully to measure soluble solids in apple fruit. However, for practical implementation, the technique needs to be able to compensate for fruit temperature fluctuations, as it was observed that the sample temperature affects the near infrared reflectance spectrum in a non-linear way. Temperature fluctuations may occur in practice because of varying weather conditions or improper conditioning of the fruit immediately after harvest. Two techniques were found well suited to control the accuracy of the calibration models for soluble solids with respect to temperature fluctuations. The first, and most practical one, consisted of developing a global robust calibration model to cover the temperature range expected in the future. The second method involved the development of a range of temperature dedicated calibration models. The drawback of the latter approach is that the required data collection is very large. When no precautions are taken, the error on the soluble solids content reading may be as large as 4%brix.

Modelling fruit characteristics during apple maturation: A stochastic approach

At present, mathematical models to predict the change of fruit quality attributes during apple maturation are deterministic and do not take into account the large natural variability of fruit quality attributes during the growing season. In this work a stochastic system approach was developed to describe the quality evolution of fruit. The basic dynamics of fruit quality evolution was represented by means of a stochastic system, in which the initial conditions and the model parameters were specified as random variables together with their probability density functions. A fundamental approach from stochastic systems theory was used to compute the propagation of the probability density functions of fruit quality attributes, which requires the numerical solution of the Fokker–Planck equation.

Non-destructive and destructive firmness measurements on apples and peaches.

Abstract A non-destructive acoustic impulse response test was performed on different apple varieties at different maturity stages and on recently picked peaches. High natural frequencies were measured solely on fruits with low mass. The firmness was determined destructively on the fruits with the hand-operated penetrometer and a universal testing machine. A weak correlation between the results of the non-destructive and the destructive method could be derived. The stiffness measured with the acoustic impulse response technique tended to change with maturation of the apples. A drop in stiffness in a short time period was ascribed to the weather.

Numerical solution of the Fokker-Planck equation for postharvest applications

In recent years many mathematical models have been developed to predict the change of fruit quality attributes such as firmness during maturation and cold storage. Instead of modelling the full chain of underlying biochemical processes, typically models of minimum complexity are developed to describe the basic dynamics of fruit quality attributes by means of a limited number of state variables. However, these models are deterministic and do not take into account the large natural variability of fruit quality attributes. In this research the evolution of fruit quality attributes was represented by means of a system of differential equations in which the initial conditions and the model parameters were specified as random variables together with their probability density functions. A fundamental approach from stochastic systems theory is used to compute the propagation of these probability density functions through a dynamic model for apple firmness during maturation. This approach considers the solution of the Fokker-Planck or forward diffusion equation. Because the Fokker-Planck equation is a first order partial differential equation of hyperbolic type, usually the solution can not be derived in closed form. Numerical tools are developed to discretise the computational domain and are based on special finite difference schemes. The main features of this stochastic approach were demonstrated for the time evolution of soluble solids content during apple maturation.