ORIGINAL ARTICLE

Proposal of Color Index Ranges According to Ripeness Stages in Fruits

MSc. Minelkis Machado Molina^{I  *}  , Dr.C. Annia García Pereira^I  , Dr.C. Neili Machado García^I 

^IUniversidad Agraria de La Habana, Facultad de Ciencias Técnicas, San José de las Lajas, Mayabeque Cuba.

ABSTRACT

Maturity is the most important and complex stage in the development of the fruit, where they acquire sensory characteristics that define it as edible. The objective of this research is to determine the color indices associated with the different stages of maturation of the pumpkin fruit (Carica papaya L, variety MaradolRoja), guava (Psidiumguajava L, variety EnanaRoja EEA1-23) and tomato (Solanumlycopersicum, variety Vyta). To carry out the study, 32 pumpkins and 40 guava fruits were used. Fruits pumpkin and guava, were harvested in the Agricultural Company Ho Chi Minh, in the municipality of Jaruco, and tomato in the farm "The Papas" belonging to the National Institute of Agricultural Sciences (INCA), located in the municipality from San José de lasLajas, Mayabeque. The chemical, physical-mechanical and organoleptic properties were monitored in the different stages of maturation. In addition, the fruits were photographed and the images were analyzed to obtain the color indexes in the different stages of maturation. Finally, for the pump fruit and the tomato, four ripening stages and five for the guava are defined, in addition to the ranges of the color index associated with each one of them.

Key words: Color; pumpfruit; guava; tomato; ripening stage

INTRODUCTION

Over the last years, there has been a greater consensus on the need to determine the ripening states of agricultural products, based on the need for the consumer to have at their disposal, edible fruits with optimum quality parameters. This demand for fresh fruits and vegetables obliges industry professionals and researchers to carry out tests that provide an accurate assessment of these parameters.

The quality of agricultural products refers to a series of characteristics that determine their degree of acceptance by the consumer, fundamentally related to their general health and commercial life. Based on this, knowledge of the chemical, physico-mechanical and organoleptic properties of any agricultural product play an indispensable role in achieving a good presentation and conservation of it, allowing to define the most appropriate management during the periods of pre-harvest, harvest and postharvest (^{Thompson, 1998}).The maturation process involves physical-chemical and sensory changes that respond to those properties previously mentioned that define some essential aspects for consumption, focused on the deterioration of firmness, the variation in soluble solids content (SSC ), as well as enzymatic darkening (^{Aranceta y Pérez-Rodrigo, 2006}).

The organoleptic properties that are currently considered more important with regard to consumer appreciation are aroma, taste and color. Color is usually considered a psychological factor of appreciation and a valuable criterion for choosing a food product; even in products of plant origin, it is related to the possibility of choosing ripening and its suitability (^{Mathias-Rettig y Ah-Hen, 2014}).

Color is the most notorious feature in many fruits during ripening, and therefore it is used as a criterion to define the maturity of a fruit according ^{Reid (2002)},it is one of the main criteria of acceptance by consumers (^{Slaughter, 2009}; Padrón et al. 2012). The most important transformation is the degradation of the green color, which is associated with the synthesis or unmasking of pigments whose colors range between yellow (carotenoids) and red-purple (anthocyanins).The Color Index (CI *) is one of the most efficient ways to assess color in agricultural products according ^{Vignoni et al.(2006)}, which acquires significant importance for monitoring fruit quality through digital image processing; This constitutes an effective method to be able to describe the analysis points in the fruit where the eye is not able to distinguish hundreds of determining colors in the ripening scale. The use of IC * as a non-destructive tool allows through the use of techniques such as photography to establish the stages of maturation, it has been addressed by ^{Vignoni et al. (2006)}; ^{Yirat et al. (2009)}; Yam y Papadakis (2004); Vazquez (2015), among others based exclusively for this on the benefits of specialized software Adobe Photoshop v. 2007.

Some researchers have studied the correlation between the fruit skin color parameters according to different color models, acidity, SSC, citric acid and anthocyanin content (^{Dafny-Yalin et al., 2010}).^{Manera et al. (2011)} studied the correlation between the parameters of the pomegranate skincolor and the air temperature; also ^{Manera et al. (2012)} worked on the relationship between the air temperature and the color dusting in thelemon peel during ripening. Some authors studied the color changes that occur during the post-harvest storage of various fruits (^{Shwartz et al., 2009}).

The IC * is the quantitative property that represents the color, which is a dimensionless number that relates the different values ​​of the color components (Ex .: RGB - Components R, G and B; independently). L * a * b * is the international color measurement standard adapted by the International Lighting Commission (CIE) (^{León et al., 2006}). Although there are different color spaces, the most widely used in food color measurement is the color space L * a * b *, because a uniform distribution of colors is obtained and is very close to the characteristics of human perception of color (^{Velioglu et al., 2011}, Larraín et al, 2008; Wu y Sun, 2013).

The use of color models facilitates the specification of the evaluated points of the fruits in colors, being located in a three-dimensional coordinate system defined in a subspace; each color is defined by a single point, which is obtained from the average true color of the epidermis of the fruit. This property describes the coloring of the epidermis of the fruit, allowing to follow the evolution of the ripening and for this it returns three parameters L *, a *, b *, following the standard of illumination of the spectral scale, where L * describes the luminosity, a * and b * evaluate the saturation that gives the purity of the color and the color tone itself (^{Francis y Clydesdale, 1975}). Axis (a) that goes from green to red by measuring the purity of color, while axis (b) goes from blue to yellow by measuring the color tone itself. The Color Index according to ^{Thompson (1998)},is governed by the expression (1):

$I{C}^{*}=\frac{a\times 1000}{L\times b}$ (1)

where:

a	- zone of variation between green and red of the spectrum;
L	- color intensity;
b	- zone of variation between blue and yellow of the spectrum.

The equation that represents the IC *, then looks for the most relevant variation between the different values (L *, a *, b *), in order to find an optimal representation of the color in the fruit (^{Bonilla-González y Prieto-Ortíz, 2016}).The IC * of an entire fruit can vary along its entire surface due to the appearance of veins characteristic of the ripening process, hence it is convenient to establish ranges of IC * according to the EM given by the standard used as a reference .

The current investigation is aimed to determine the IC * associated with each ripeness stages in papay (Carica papaya L, Maradol Roja var.), guava (Psidiumguajava L, EnanaRoja EEA1-23 var.) and tomato (Solanumlycopersicum, Vyta var.)

METHODS

The papayfruit and guava, were harvested in the Ho Chi Minh Agricultural Company, in the municipality of Jaruco and the tomato on the farm "Las Papas" belonging to the National Institute of Agricultural Sciences (INCA), located in the municipality of San José de Las Lajas, Mayabeque. The fruits were randomly collected without presenting physical or mechanical damage and guaranteeing the representativeness of each EM. They were carefully placed vertically on the fruit-peduncle junction, in commercial cardboard boxes with breathing holes to ensure homogeneity between indoor and outdoor temperatures. The boxes are covered with a tarp to avoid the dust and foreign particles adhesion during transportation, and then they were transported to the Quality Laboratory at the Faculty of Technical Sciences of the Agrarian University of Havana, in the province of Mayabeque to develop the experiments and data processing.Upon arriving at the Quality laboratory, the total sample was washed and dried, latter anexpert panel proceeds to select and sort the final sample. The fruits were carefully placed on a plateau, always ensuring that they remained in an upright position (fruit-peduncle break down). The average temperature of the room was 25.4 ° C and the environmental relative humidity of 74%.

The total sample for the determination of the IC * values ​​and their ranges consisted of 32 papay and 32 tomatoes, randomly selected, separated into four groups of eight (8) and 40 guava fruits separated into five groups of eight fruits for each maturation state . The IC * is obtained by the method of image capture, according to ^{Vignoniet al.(2006)}, expression (1).

The images of each fruit were captured from the front using a Canon Rebel XS W18-55ls digital camera, Taiwan, located on a professional tripod raised 1.40 meters from the ground surface and connected to a computer (CPU: 2.5 GHz, 4 GB of RAM, 64-bit Windows operating system).The fruits were arranged in front of an achromatic background, resting on the fruit-peduncle junction. The distance fruits - camera was approximately 1.5 meters. The fruits were lightened by means of a 120W incandescent bulb located parallel to the camera, 0.50 m above it. The size of the images was 780 x 640 pixels. The resolution of levels of each color plane R, G and B was 256 levels.Four images of each fruit were taken by means ofrotating the fruit 90( around an imaginary vertical axis, seeking to have some redundancy in the colors that are captured from the fruit, which is always advantageous. The images are exported to Photoshop portable software (Adobe SysteEM, 2007), to get the numerical representation of the variables L *, a * and b * of 10 randomly selected points and finally obtain the average value for the whole fruit. The value of the IC * is determined according to the mathematical expression (1) and is located in a color plane defined by two axes that allows to visualize the correspondence of this property with the colors of the maturation scale (Figure 1).

FIGURE 1 Spectral color diagram (L * a * b *) for fruit analysis. 

The ranges are defined based on the Mexican standard ^{NMX-FF-041-SCFI (2007)}, for the papay that considers seven EM and the Mexican standard ^{NMX-FF-031 (1997)},for the tomato establishes six EM. In the case of Guava was considered the Ecuadorian Technical Standard ^{NTE-INEN-1911 (2009)} and the research carried out by ^{Yirat (2009)},that establishes five EM. It also takes into account the results of the properties, the values of L*, a* and b* obtained from the images and the assessment criteria of the experts.

RESULTS AND DISCUSSION

Tables 1, 2 and 3 show the average experimental values ​​of the physical-chemical, mechanical and organoleptic properties of the Papay (Maradol Roja var.), the guava (Enana Roja EEA1-23 var.) and the tomato(Vyta var.)such as: pH, soluble solids content (SCC), firmness and IC *, calculated for the different ripeness stages. The behavior of the properties coincides with that described by ^{Yirat et al. (2009)}, ^{Aranceta& Pérez-Rodrigo (2006)}; ^{Bonilla-González & Prieto-Ortíz, (2016)}, showing a markedtendencytodecrease in the case of firmnessas thefruitgetsripeand theincrease in pH, SSC and IC in thesameprocess. The firmness and IC * values ​​coincide with those obtained by ^{Yirat (2009)}, for the guava and ^{Muñiz et al. (2011)}for the papay, while a slight appreciable dispersion in the pH and SSC for both properties is observed being less noticeable in the papay. In all cases it coincides with the values ​​described in the literature for the fruits under study ^{Yirat et al. (2009)}, ^{Muñiz et al. (2011)}, Padrón et al. (2012), Vazquez (2015).The results shown corroborate the correspondence with the EM in which they were classified for the subsequent obtaining of the IC * ranges.

TABLE 1 Physico-chemical, mechanical and organoleptic properties of the papay Maradol Roja variety 

EM	pH	SSC ^oBrix	Firmness (kgf/cm^2)	IC*
I	4.60	8.36	31.02	-10.25
II	4.90	8.51	18.7	-4.89
III	5.25	8.8	6.1	1.81
IV	5.65	9.0	3.9	25.04

TABLE 2 Physico-chemical, mechanical and organoleptic properties of the guava Enana Roja variety 

EM	pH	SSC ^oBrix	Firmness (kgf/cm^2)	IC*
I	3.265	6.64	2.20	-9.13
II	4.27	6.94	1.97	-4.2
III	5.13	7.06	1.84	0.92
IV	6.24	7.94	1.69	3.31
V	6.96	8.89	1.50	5.39

TABLE 3 Physico-chemical, mechanical and organoleptic properties of the tomato Vyta variety 

EM	pH	SSC ^oBrix	Firmness (kgf/cm^2)	IC*
I	3.931	2.372	1.94	-3.21
II	4.313	3.73	1.80	-1.06
III	4.645	5.63	1.71	4.73
IV	5.198	7.77	1.39	20.98

According to the obtained results, it can be observed that as the maturation state increases, the IC * values also increase (see, figure 2), which demonstrates a high dependence between the maturation state and this organoleptic property

Figure 2 illustrates the variation of the average IC values according to the EM for fruits where a more marked color transformation in the papay was noticeable according to this fruit goes through the whole ripening process, followed by the tomato and then the guava.

FIGURE 2 Average IC * variation according to the EM of each studied fruit. 

The color index ranges proposed for each EM according to the modifications realized to the ^{NMX-FF-041-SCFI (2007)}, which describes seven EMfor the papayand the ^{NMX-FF-031 (1997)} with six EM for tomato remind as fallow. In the case of the Red Maradol variety, it is observed that the color depends on the percentages of presence of green, yellow or orange outer stripes. When establishing the correspondence between the IC * and the maturation status, the IC * values in states 1 and 2 and similarly in states 3, 4 and 5 did not vary significantly. Therefore, because it has a large burden of subjectivity, according to experts, it is proposed as a result of this investigation to consider only four ripeness stages for the papay (Table 4). In the case of tomato, the same conclusion was reached for similar small color variations, so that, from six maturation stages, four maturation states are defined (Table 6).

For the determination of the IC * ranges of guava, the ripening states are described in the Ecuadorian Technical Standard ^{NTE-INEN-1911 (2009)} with six EM. However, the results obtained correspond to those achieved by ^{Yirat (2009)}when studying the EnanaRojavariety EEA1-23, which establishes five EM, so that the same amount of EM is maintained, they are shown in the Table 5. The simplification or reduction of the quantity of EM facilitates the implementation of fruits classification tools in an automated way based on techniques such as those employed by ^{Vignoniet al.(2006)}; ^{Yirat et al.(2009)}; Yam & Papadakis (2004);^{León et al.(2006)};^{Hassanafshari-Jouybaria(2011)}; Padrón et al. (2012) y Vazquez (2015). Through the following IC* intervals the EM proposed are defined and shown below for papay(Table 1) four EM, for guava (Table 2) five EM and for tomato (Table 3) four EM.

TABLE 4 Ripeness stages for papayMaradolRoja variety 

IC*ranges	Fruitskin color description	RipenessStage (EM)
-13.28 ≤ IC* ≤ -7	Darkgreen	EM 1. Physiological maturity (all green)
-7 < IC* ≤-1	Clear green with incipient yellow vein	EM 2. Green
-1 < IC* ≤ 2.5	Slight green tonality, prevalence of yellow color	EM 3. Middleripe
2.5 < IC* ≤ 48.31	Orange color 90 to100%.	EM 4. Ripe

TABLE 5 Ripeness stage for Enana Roja EEA1-23 variety 

IC* ranges	Fruitskin color description	RipenessStage (EM)
-12.34 ≤ IC* ≤ -7	Green	EM 1. Green
-7 < IC* ≤-1	Green yellowish	EM 2. Middleripe
-1 < IC* ≤ 1.9	Yellow with incipient tonalities of green	EM 3. Ripe
1.9 < IC* ≤ 4	Yellow pale with tonalities orange	EM 4. Overripe
4 < IC* > 6.10	Yellow reddish with Carmelite tonalities	EM 5. Industrialripeness

TABLE 6 Ripeness stage for tomato Vyta variety 

IC* ranges	Fruitskin color description	RipenessStage (EM)
-5.21 ≤ IC* ≤ -2	Darkgreen	EM 1. Physiological maturity (all green)
-2 < IC* ≤ 0	Clear green with incipient yellow vein	EM 2. Green
0 < IC* ≤ 8.9	Slight green tonality, prevalence of red color.	EM 3. Middleripe
8.9 < IC* ≤ 32.51	Orange reddish 90 to 100%.	EM 4. Ripe

CONCLUSIONS

• Four EM were established for thepapayMaradolRoja : physiological maturity (all green), green, middle ripe and ripe; with IC* average for EM of -10.25; -4.89; 1.81 and 25.64, with IC* ranges from -13, 28 to -7; -7 to -1; from -1 to 2.5 and from 2.25 to 48, 31 for EM from I to IV respectively.

• Five EM were established for the Guava EnanaRoja EEA1-23: green, middle ripe, ripe, overripe and industrial ripeness, with an IC* average of EM of -9.13; -4.2; 0.92, 3.31 and 5.39, with IC* ranges from -12, 34 to -7; -7 to -1; from -1 to 1.9; 1.9 to 4 and from 4 to 6.10 for EM from I to V respectively.

• Four EM were established for the tomato Vyta variety:physiological maturity (all green), green, middle ripe and ripe; with anIC* average for EM of -3.21; -1.06; 4.73 and 20.98, with IC* ranges from -5.21 to-2; -2.0 to 0; from 0 to 8.9 and from 8.9 to 32.51 for EM from I to IV respectively.

• The decrease in firmness and pH is ratified, as well as an increase in the soluble solids content and IC* valuesas the fruits ripening process progresses, while the greatest variation in IC * was obtained in papay, followed by tomato and guava in that order.

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Received: January 29, 2019; Accepted: September 02, 2019

^*Author for correspondence: Minelkis Machado Molina, e-mail: [e-mail]

Minelkis Machado Molina,Profesora,Universidad Agraria de La Habana, Facultad de Ciencias Técnicas, Departamento de Ingeniería Informática,Carretera Tapaste, km 22 ½, San José de las Lajas, Mayabeque, Cuba, e-mail: [e-mail]

Annia García Pereira, ^, Profesora Titular, Universidad Agraria de La Habana, Facultad de Ciencias Técnicas, Departamento de Ingeniería,Carretera Tapaste, km 22 ½, San José de las Lajas, Mayabeque, Cuba, e-mail: [e-mail]

Neili Machado García, Profesora Titular, Universidad Agraria de La Habana, Facultad de Ciencias Técnicas, Departamento de Ingeniería Informática,Carretera Tapaste, km 22 ½, San José de las Lajas, Mayabeque, Cubae-mail: [e-mail]