USING COLORIMETRY AS ANALYTICAL TECHNIQUE FOR

QUALITY ASSESSMENT OF GREEN COFFEE BEANS

Gail Nelson, Research Department Coffee Industry Board

Kingston, Jamaica

July 14-15, 2005

XXI LATIN AMERICAN COFFEE INDUSTRY SYMPOSIUM

San Salvador, El Salvador 

Email: gnelson@ciboj.org

INTRODUCTION• Coffee is the second most traded

commodity after oil.

• Coffee is sold primarily in the green bean form.

• Assessment of quality involves organoleptic appraisal as well as physical examination of the beans.

• Colour is one important quality index for green coffee beans.

INTRODUCTION

Premium grades of Jamaican coffee must exhibit

structurally sound beans of a blue-green colour.

Traditional Approach to Colour Assessment

• Involves the use of the human eye to view samples and categorize beans based on:

perception colour;

reference samples;

and group consensus.

Categories of Coffee Derived from Traditional Approach

- Blue green - Yellowish

- Green - Yellow

- Greenish - Whitish

- Pale green - Brownish

- Yellow-green - Mottled

Premium Grade Jamaican Coffee Beans

Greenish Coloured Beans

Pale/ Whitish/Mottled Beans

Yellow/ Brownish Beans

Categories… cont’d

• Yellowness and whiteness are undesirable characteristics of green coffee beans. They occur naturally over time and are usually indicative of diminishing quality in coffee beans.

• Usually associated with diminished flavour/aroma and development of age-related taints in the cup.

Categories… cont’d

• Brownish discolourations are usually associated with over- fermentation or improperly handled coffee.

• Mottling is usually associated with improper drying or poor storage conditions leading to moisture loss/gain in the beans.

Limitations of Traditional Approach

1. It is difficult to quantify and report colour in verbal descriptive terms.

2. Word definitions may be ambiguous. Different words have different meanings to particular individuals.

3. Requires skilled inspectors and may take months of training.

Limitations of Traditional Approach

4. Perception of colour is a highly personalized experience being influenced by factors such as:

– Light source– Sensitivity of the analyst’s eye– Biases towards certain colours– Sample size– Viewing angle– background

A New Approach

Colorimetry is one technology being used by Coffee Industry Board of Jamaica (CIB) to standardize color measurement and colour reporting.

Colorimetry

• Makes use of a colour-meter (colorimeter), an electronic instrument, to measure colour.

• Colour of samples are measured in a standard manner and the reported colour is given in “colour units” in a standard numerical format.

‘Minolta CR-310 Chroma Meter’

Advantages of Colorimetry

• Samples are measured using the same light source, illumination method and instrument conditions for every measurement;

• Making it possible to obtain accurate and consistent measurements regardless of the analyst;

• Thereby eliminating human biases.

Advantages …2

• Colour meters are designed to have sensitivity corresponding to the human eye.

• Measurements are reported in a standard numerical format which enables precise communication and interpretation of results.

Colour Reporting…1

• The CIB use the ‘L a b’ reporting format for colour reporting in accordance with standards set by the “Commission International de l’Eclaire” (CIE).

• CIE is an international organization concerned with light and colour.

Colour Reporting…2

- ‘L’ measures the lightness/brightness of the colour (e.g. the yellow of a lemon is brighter than the yellow of a grapefruit).

-‘a’ measures hue- - - (+a) is the degree of redness and (-a) is the degree of greenness.

- ‘b’ measures hue- - - (+b) is the degree of yellowness and (-b) is the degree of blueness.

Use of Colour meter for Quality Assessment of Coffee

• Research was conducted by CIB to determine the usefulness of colour meters for colour assessment of green beans.

• This was done by investigating whether colour meters could distinguish and report differences in visually distinct categories of green beans.

Reference Samples• Three categories of beans were

analyzed:

I. Premium grade . This was a new crop/current crop sample which was was close to the ideal vivid ‘blue-green’ colour.

II. Prime grade. This was also a new crop sample, but had begun to lose its vivid green colour. Described as greenish.

III. Pale/White. This was a sample of old beans which had lost all green colour and had a chalky appearance.

Visual Images of Reference Samples

Premium-Green Coloured Prime-Greenish Coloured Old- Whitish Coloured

Experimental…1

1. The instrument was calibrated as per the manufacturer’s specification.

2. Coffee beans were mixed together in a large container and a 100 gram portion was removed and placed in a covered Petri dish (transparent).

3. The ‘viewing eye’ of the colorimeter was placed on top of the Petri dish cover.

Experimental…2

4. Six readings were taken in different locations for each sample. The mean of these measurements was then used as the colour value for the sample.

5. The three reference samples were measured individually and combined in the ratios shown in Table 1.

6. The results were then analyzed and are discussed further.

Table 1. Colour Assessment of Different Ratios of Reference Sample Coffee Beans

 

COLOUR 

METER 

MEASUREMNTS

Ratios of reference samples

L a b

Green beans (100%) 51.29 +1.30 +5.15

Greenish beans (100%)

52.14 +1.33 +6.43

Green beans (75%)White beans (25%)

53.14 +1.4 +7.2

Greenish beans (75%)White beans (25%)

55.43 +1.39 +8.28

Green beans (50%)White beans (50%)

55.91 +1.37 +8.16

White beans (75%)Green beans (25%)

58.82 +1.52 +10.40

White beans (100%) 60.47 +1.45 +11.06

CATEGORY OF BEANS

Observations

1. The colour of green coffee beans is not a pure green colour. It is a combination of hues. The optimum colour values must be established by the use of reference samples.

2. From data in Table 1. As the ratio of white beans to green beans increases, there is a corresponding increase in ‘L’ value.

3. In terms of ‘L a b’ values the trend is

Green < Greenish < Whitish

Conclusions and Recommendations

• Based on Observations (2) and (3), it is apparent that the colour meter is able to:– ‘recognize’ changes in the quantity

of pale/white beans in a coffee sample.

– differentiate between green beans, greenish beans and pale/white beans.

• This technology can therefore be used to describe and categorize green coffee beans based on colour measurements.

• Once maximum/minimum allowable ‘L a b’ values have been set for particular grades of coffee (using reference samples), these values can be used as an acceptance/rejection/reclassification criterion.

Conclusions and Recommendations

• Samples which are non- homogenous (e.g. a mixture of very green and very white beans) may receive an overall passing grade, although unappealing to the eye;

• It is recommended therefore that this technique be used in conjunction with viewing the sample to ensure that the final product not only has desirable colour but is homogenous as well.

Conclusions and Recommendations

Conclusions and Recommendations

Homogenous colour Non-Homogenous colour

The Colour- meter can therefore find use as a quality assessment tool in industries where colour is a selection criterion/quality index for coffee beans.

Conclusions and Recommendations

References1. A Comparison of the Traditional and

Colorimetric Methods of Quantifying Pale Beans in Green Coffee Samples. Gail Nelson. June 2001.

2. Precise Color Communication. Colour Control from Perception to Instrumentation. Minolta Co., Ltd. 1998.

3. Coffee Industry Board “Quality Assessment Sheet for Green Coffee Beans”

‘Muchas Gracias’

Gail Nelson, Research Officer, Coffee Industry Board of Jamaica