WO2006135267A2

WO2006135267A2 - A new method and apparatus for monitoring fruit quality and ripeness using light-induced luminescence

Info

Publication number: WO2006135267A2
Application number: PCT/PT2006/000014
Authority: WO
Inventors: Hugh Douglas Burrows; Alexander Kharlamov
Original assignee: Universidade De Coimbra
Priority date: 2005-06-14
Filing date: 2006-06-14
Publication date: 2006-12-21
Also published as: WO2006135267B1; WO2006135267A3; PT103290B; PT103290A

Abstract

The present application relates to a method and apparatus for in-vivo characterisation of the quality and ripeness of fruits and vegetables in which the aroma at the surface of the fruit or vegetable is excited by light of a specific wavelength so as to induce luminescence of said aroma wherein the spectral distribution of the luminescence is detected. The values of parameters based on the intensity of the distribution are determined and changes are compared with predetermined deviation values to draw conclusions on the state of the tested quality of the fruit or vegetable.

Description

DESCRIPTION

"A NEW METHOD AND APPARATUS FOR MONITORING FRUIT QUALITY AND RIPENESS USING LIGHT-INDUCED LUMINESCENCE"

Scope of the invention

The invention concerns a new method and apparatus for the real time in vivo characterization of the aroma of fruits and vegetable species using light- induced luminescence.

An apparatus is developed which uses the said method and makes it possible to monitor the ripeness and quality of a variety of fruits, as well as the quality and state of certain vegetable species. The description concentrates on the case of apples, but the method and apparatus are applicable to the characterization of other fruits and vegetable species (plants).

Prior art

The monitoring and improvement of the quality of fruit and vegetables is a matter of recognized importance. Although volatile compounds usually constitute a small fraction in food, the analysis of these compounds is becoming increasingly necessary since it has been recognized that their odorant properties can be used in the quality control of food products and in the identification of their geographical origin. Since aromas influence consumer acceptance, they are important quality attributes and knowledge of their properties is of considerable value.

Aromas are constituted by a large variety of compounds with diverse chemical structures and they are present in a wide range of concentrations. The compounds responsible for a particular aroma are normally accompanied by many other volatile components. The relative abundance of identified emitted volatiles is a fingerprint of a particular species. The real time monitoring of the volatiles emitted by fruit and vegetables can provide key information to the agricultural and food industries for determining optimal harvesting dates and storage conditions to maintain and improve quality.

The most important methods currently used can be separated into three groups.

1. Isolation methods. Analysis of aromas based on methods including extraction, separation and identification of volatile organic compounds by high resolution gas chromatography (GC) or high performance liquid chromatography (HPLC) hyphenated with mass spectrometry and/or infrared spectroscopy. However, in any isolation method, the final extract is never completely faithful to the true aroma and since it is often necessary to prepare chemical derivatives of such extract, these methods are very time consuming. It must also be mentioned that analysis is further complicated since it has been reported, based on isolation methods, that fruits can produce complex mixtures of volatiles with more than two hundred identified compounds.

2. An electronic aroma sensing system (electronic nose or electronic sniffer) based on aroma sensor array has already been developed. The sensor array responds to the presence of specific aromas by resistance changes and it can be used for real time monitoring of the total concentration of authentic volatiles.

3. A method based on proton- transfer-reaction-mass spectrometry has been introduced for real time trace gas detection. This method has been successfully applied to real time monitoring of organic volatiles emitted by fruits during their ageing process. It has been demonstrated that the concentrations of volatiles emitted by fruits can be related to a sequence of compounds: methanol, ethanol, acetaldehyde, methyl acetate, acetone, acetic acid, ethyl acetate, mixture of ketones and aldehydes, and esters.

These methods are expensive and time consuming.

As references, we can cite examples of patents describing non-destructive optical methods of testing (monitoring) of fruits, vegetables and seeds for post- harvest quality, using absorption (EP97402493 and US5708271), transmission (EP99110130 and DK8000028), reflection (EP87302018, Cl, US6020587) and fluorescence of the skin and leaves (US5822068, WO99/56127, US6512577, US5822068) and seeds (EP97919750) or direct visualization of scattered light from seeds (Cl and EP86112627), all these methods having been developed for operating in ultraviolet (UV), visible (VIS) and near infrared (NIR) spectral regions. These methods are based on the interaction of light with a surface and/or volume of the samples (fruits, vegetables or seeds).

In patent US5708271, the absorption of light in NIR optical regions is used for the non-destructive determination of the sugar content of a fruit or vegetable. The application of light transmission has been described for testing the internal quality of a fruit or vegetable by visible light (EP99110130) and for meat (in individual carcasses) in the infrared region (DK8000028).

The detection of light reflected by a sample in the VIS-NIR region has been used for the quality control of seeds such as grains and rice (Cl, EP87302018) and in a plant chlorophyll content meter (US6020587).

Any scattered or fluorescent light detected from the sample (under optical irradiation) can be analysed for identifying characteristic light from biochemical components in a sample, on the surface or in the volume, while the presence of certain components determines the quality of the sample. Due to its high sensitivity, fluorimetry has been used in the quality control of products such as fish and meat (EP84850177) and for the detection of algae in water by analysing chlorophyll fluorescence (US4500641). For example, the presence of chlorophyll- like compounds in the skin of fruit, vegetables and seeds has been used for indirectly estimating the quality of samples by chlorophyll fluorescence (US5822068, WO99/56127, US6512577). The quality of the sample can be estimated as a function of the intensity of the chlorophyll fluorescence. Since the amount of chlorophyll is directly related to the quality of the sample, it is possible to select samples according to their quality.

Summary of the invention

The present invention claims [1-3] that luminescence of the organic volatiles emitted from the surface of various species of fruits and vegetables can be used for the in vivo monitoring of chemical evolution in such plants. This new approach uses the ability of volatile compounds produced by fruit or vegetables to luminesce in the vicinity of their surface when irradiated by a light of the appropriate wavelength. While the origin of this emission has not yet been confirmed, its existence allows minute amounts of a substance to be successfully detected and characterized. Furthermore, the spectroscopic capacity of such method allows for a reliable discrimination between different physiological states of living species, while the optical focusing and scanning of excitation light allows the mapping of emitting regions as small as a few microns of the plant surface. As shown in [1], most organic volatiles can luminesce effectively during their release through the skin of the fruit, which makes this remote, optically based "artificial nose" a very generic approach and an extremely powerful diagnostic tool to be used in the control of plants.

Description of the figures

The description that follows is based on the drawings attached hereto, which represent without any restrictive character:

In figure 1, a diagram of an apparatus for monitoring the quality and state of ripeness of fruit and vegetables by the light-induced luminescence of their aroma.

In figure 2, the emission spectrum of an apple from unripe to overripe.

In figure 3, the emission spectrum of an apple after one month of ageing.

Detailed description

According to the present invention, a new method and apparatus are claimed for monitoring the quality and state of ripeness of fruit and vegetables by the light-induced luminescence of their aroma, comprising: detecting the luminescence spectrum excited by light of a wavelength able to induce luminescence, comparing the spectrum with predetermined threshold curves for ripe fruit (or vegetables) and good quality samples, and drawing conclusions on the state of the tested fruit (or vegetables).

The method involves: excitation of the surface of the fruit or vegetable by the light of a specific wavelength in the range of 500-650 nm to induce luminescence; detection of the spectral distribution (i.e. intensities) of the light emitted in the range of 650-800 nm from an irradiated interfacial region; measuring the values of parameters based on the intensity of the spectral distribution of the emission at a discrete wavelength in steps of 10-20 nm; monitoring the values of the said parameters; detecting changes in the values of the said parameters; comparing changes in the values of the parameters with determined deviation values; interpretation of changes in the values of the parameters above the said deviations as variables dependent on both the quality of the fruit or vegetable and its degree of ripeness.

The apparatus as shown in figure 1 for monitoring the quality and state of ripeness of fruit and vegetables by the light-induced luminescence of their aroma comprises: a monochromatic light emitter 1; a first optical fibre 2 connected to the said emitter 1, used for excitation of luminescence; a second optical fibre 3 connected to a spectrometer 4, used to detect the luminescence spectrum; certain means are used for measuring the values of parameters based on the intensity, at discrete wavelengths, of the spectral distribution of the emission; and a personal computer 5, spectrometer 4 and computer program 6 are used to compare the said measured spectral intensities with predetermined threshold curves in steps of 10- 20 nm. The apparatus has a monochromatic light emitter 1 connected to an excitation optical fibre 2, which is used for irradiation of the region near to the surface of the sample 7. The sample 7 is placed on an appropriate holder 8. Luminescence from the interfacial region of the sample 7 is detected by the use of the second optical fibre 3 connected to the spectrometer 4 controlled by a PC 5. The light exciting and detecting ends of the optical fibres, 2 and 3 respectively, are located at a distance of 2-5 mm from the surface of the sample, as shown by dashed lines I and II.

The apparatus can include the following experimental and illustrative components: photoexcitation wavelengths (±2 nm) in the range of 500-650 nm from a suitable source, such as a 150 W Xenon arc lamp, which can be isolated by an appropriate monochromator with filter combinations or using excitation by a laser or laser diode with wavelengths of λ_exc-514.5, 632.8, 650 nm (in order to avoid any light-induced damage to the fruit, the output can be around 1 mW); optical fibres are used for excitation and detection of luminescence; measurements can be performed in an unpolarized configuration using angles from 90° up to back- scattering geometry, from an irradiated interfacial region of the fruit or vegetable, with the orientation of the excitation light varying from tangential angles to right angles in relation to the surface of the fruit or vegetable; the luminescence spectrum is measured at room temperature with an appropriate monochromator at an approximately 2 nm resolution, equipped with a CCD or photomultiplier detector, sensitive in the region of 600-800 nm for detection of the molecular luminescence flux of 10^"9 photons per second; detection is carried out in a photon counting mode with a count rate in the detector of about 10³ counts per second.

Control of the ripeness and quality of fruits The effect of the evolution of the luminescence spectrum monitored during the ripening and ageing process of fruits was characterized, as described in [1,3].

The following are two illustrative but not restrictive examples of the method according to the present invention.

1. A number of apples at different stages of ripeness were harvested on the days of measurement and these results are presented in figure 2. The spectra were taken in the same experimental conditions and under the same excitation levels of the laser diode (excitation wavelength of λ_exc=650 nm). The ripening of apples from unripe to ripe and then to overripe was accompanied by modification of the spectrum and a significant increase in luminescence in high energy regions (blue shift).

2. The ageing of apples is also accompanied by changes in the luminescence spectrum excited at wavelengths of λ_exc=632.8 nm using a He-Ne laser. Apples of various origins were purchased locally and were kept at room temperature in a non-controlled atmosphere for several weeks (no information is available on where these fruits were picked). The spectra were also taken in the same experimental conditions. The effect of one month of ageing of an apple is shown in figure 3. The ageing of apples is accompanied by changes in the bandshape in high energy regions and an increase (up to four times) of the luminescence intensity. References

1. A. A. Kharlamov, H.D. Burrows, "Visualisation of fruit odor by photoluminescence", J. Appl. Biochem. Microbiol., Vol. 37, No. 2 (2001) 206.

2. A.A. Kharlamov, H.D. Burrows, "About visualization of the aroma of fruits", Sensors and Actuators B, Vol. 77 (2001) 593.

3. A.A. Kharlamov, H.D. Burrows, "Monitoring of the aroma of fruits at their surface by luminescence", Progress in Colloid and Polymer Science, Springer- Verlag (2002), accepted for publication.

4. EP97402493

5. EP87302018

6. EP99110130

7. US5708271

8. EP86112267

9. US4713781 10. US5822068 11. EP84850177 12. WO99/56127 13. US6512577 14. US6020587 15. US4500641 16. DK8000028 17. EP97919750

Claims

1. A method for the in vivo characterization of the quality and state of ripeness of fruits and vegetables characterized in that it uses light-induced luminescence of their aroma as a characteristic feature and comprising the following stages:

- Excitation of the surface of fruit or vegetable by light of a specific wavelength in order to induce luminescence;

- Detection of the spectral distribution of the luminescence;

- Measuring of values of parameters based on the intensity of the spectral distribution of the emission,

- Monitoring the values of the said parameters,

- Detecting changes in the values of the said parameters,

- Comparing changes in the values of the said parameters with predetermined deviation values;

- Interpretation of changes in the values of the said parameters above the said deviations as dependent on both the quality and the state of ripeness of the fruit and vegetables.

2. A method according to claim 1, characterized in that the said parameters are related to the luminescence spectrum of the aroma of the fruit or vegetable which affects its quality or state of ripeness.

3. A method according to claim 1, characterized in that the measurement of the values of the said parameters based on the intensity of the spectral distribution of the emission is made in discrete wavelengths.

4. A method according to the previous claims, characterized in that the said comparison of the changes in the values of the said parameters is carried out by means of a comparison with threshold curves in steps of 10-20 nm.

5. A method according to claim 1, characterized in that the light emitted for luminescence excitation is monochromatic.

6. A method according to the previous claims, characterized in that real time measurements of the said parameters are made.

7. A method according to the previous claims, characterized in that the excitation time is reduced by the inducing light between each measurement.

8. An apparatus for the in vivo monitoring of the quality and state of ripeness of fruits and vegetables characterized in that it uses light-induced luminescence of their aroma as a characteristic feature and comprising:

- A holder 8 for supporting to the sample;

- A monochromatic light emitter 1 connected to a first optical fibre, used for luminescence excitation in an irradiated interfacial region of a vegetable or fruit 7;

- A second optical fibre 3 connected to a spectrometer 4, used to detect the luminescence spectrum;

- Measuring of values of parameters based on the intensity of the spectral distribution of the emission in discrete wavelengths;

- Computer 5 and spectrometer 4 and computer program 6 to compare the said spectral intensity measurements with threshold curves in steps of 10-20 nm.