Food Measure DOI 10.1007/s11694-017-9603-5
ORIGINAL PAPER
Simple on package indicator label for monitoring of grape ripening process using colorimetric pH sensor Bambang Kuswandi1 · Eka Ayu Murdyaningsih1
Received: 26 April 2017 / Accepted: 17 July 2017 © Springer Science+Business Media, LLC 2017
Abstract A simple on package indicator label for grapes (Vitis vinifera L.) ripeness has been developed based on chlorophenol red (CPR). The CPR was immobilized onto the filter paper via absorption method to make the CPR membrane. The membrane as a colorimetric label for grape ripeness works based on pH increase as the volatile organic acids decreased gradually in the package headspace due to the formation of sugars during berry ripening process. Subsequently, the color of the indicator label will change from white to beige then finally to yellow for over-ripe indication, which is easily detected by naked eye. The results show that the indicator label could be used to monitor the ripeness of grapes since the color changes of indicator label toward the ripeness of the grapes are in similar tendency and have a good linear correlation. Thus, the indicator label can be used as an effective tool for on-line ripeness monitoring of grapes packaging. Finally, the indicator label was successfully used as on-package indicator label for online ripeness monitoring of grapes in ambient and chilled conditions. Keywords Ripeness indicator · Chlorophenol red · Grapes · Fruit packaging · pH
* Bambang Kuswandi
[email protected] 1
Chemo and Biosensors Group, Faculty of Pharmacy, University of Jember, Jl. Kalimantan 37, Jember 68121, Indonesia
Introduction In order to avoid quality losses of fresh fruit products during the distribution postharvest chain from the garden to the table due to damage or unfavorable handling condition, quality monitoring of fruits products is usually used. The classical method to maintain the quality of fruits products is by continuous monitoring of ambient conditions, such as temperature and humidity. However, it only gives indirect information regarding the quality of fruits. Other information, such as ripening state of fruit products, is not covered. Ripening of fruit is an important phase, where the green fruit is converted into a highly palatable, rich in nutrition, and colored fruit [1]. The fruit ripening after harvest of non-climacteric fruit, such as grape, produce various volatile compounds. There are more than several hundred compounds producing in grape taste and flavor [2]. For instance, the ratio of acid to sugar at harvest is important for the taste of table grapes and for the sensory characteristics derived from wine grapes [3]. The well-known flavor compounds of Vitis vinifera are monoterpenes, which are responsible for the floral characteristics normally associated with grape varieties [4]. These compounds accumulate during the latter stages of ripening and are present in trace amounts at harvest. The classical analytical methods for the grape flavor profile mostly required an isolation step before identifying the compounds. Headspace solid-phase micro-extraction–gas chromatography–mass spectrometry [5, 6] for profiling free volatile compounds in grapes were usually employed. However, these analytical methods require sample preparation, expensive and need a large experimental set-up; therefore they are not suitable for monitoring in the real time changes of volatiles for grapes ripeness detection. Thus a novel analytical method is required for this purpose that is low-cost,
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simple, sensitive, and reliable for field and real-time applications. One alternative that can be used for this monitoring purpose is by employing new packaging technology such as smart packaging [7, 8], where the sensor is integrated into the packaging. By integrating the sensor/indicator into packaging system, therefore it can give information on the integrity of the food package that useful in assessing the food quality and safety inside the package [9, 10]. Mostly the proposed on package indicators for ripeness of fruits is based on ethylene as a marker for ripeness of climacteric fruit that emitted during ripening stage. The number of colorimetric sensors for ethylene detection has been applied for apple [11] and kiwi ripeness [12]. However, this method has limited application for climacteric fruit ripeness sensors, because of its cost and low stability against humidity and UV light. In the case of non-climacteric fruit, such as grapes (V. vinifera L.), during the ripening process, the increase in ethylene production is slight and the typical respiration peak does not occur [2]. In addition, in non-climacteric fruits where no burst in ethylene production during ripening is observed, organic acid (maleic and tartaric acids) play an important role during grape ripening [13–15]. Therefore, it is not suitable if ethylene is used as ripeness indication in this case. One promising simple method in detection of grapes ripeness is based on the volatile organic acid during ripening stages through the pH change as also proposed for guava [16] and strawberries packaging [17]. This is due to the fact that the volatile organic acids, such as maleic acid, play a prominent role during grape ripening process [14, 18]. The purpose of this study is to use the CPR membrane to construct simple and low-cost on-package indicator label for monitoring of grapes ripeness. The CPR membrane is a highly sensitive material toward pH change from white to yellow when interacts with higher pH due to a decrease in the volatile organic acids inside the atmospheric package, where monitored directly with indicator label. In addition, the indicator label shows a similar tendency and a linear correlation to the changes in pH, titratable acidity (TA), soluble solid content (SSC), hardness and weight loss as well as a similar trend towards the sensory evaluation of grapes. The performance of this indicator label was successfully tested directly for on-package ripeness monitoring of grapes during their shelf life in ambient and chilled conditions.
B. Kuswandi, E. A. Murdyaningsih
of ethanol (70%) to give the concentration of 0.5 mg/ ml. Glycerin (40 v/v%) and filter paper (Whatman, No.2) were obtained from a domestic market. All chemical used were of reagent grade (supplied by Merck, Sigma or Fluka) and used as supplied. Preparation of the CPR membrane The CPR membrane was prepared by immobilizing CPR on the filter paper (Whatman, No. 2) using absorption method. This procedure was carried out, simply by immersing the membrane sheet into 10 ml of a stock solution of CPR (0.5 mg/ml) addition 1 ml of glycerol (10%). The solution mixture was stirred for approximately 2 h. Then, the CPR membrane was washed with deionized water to remove unbound indicator with the membrane. Afterward, the membrane was conditioned with buffer at pH 4.0 to make the white membrane as original color of filter paper. The buffer at pH 4 was used phosphate the buffer in order to avoid the volatile or oxidative chemicals used in a buffer that may equilibrate with atmospheric in the head space inside the package that will interfere the color change of the membrane. The wet filter paper was dried at room temperature (25 °C) in the dark overnight. Afterward, the membrane was cut at desired shape according to the design as an on-package color indicator (Fig. 1). Preparation of the grape samples Fresh ripe grapes of normal pH (~4.31) purchased at a local fruit garden in Jember, were used in this study. Total weight around 200 g of grapes was used for the analysis, then it placed on styrofoam trays, and
Materials and methods Chemicals A stock solution of chlorophenol red (CPR) (Sigma, UK) was prepared by dissolving 5 mg of CPR in 10 ml
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Fig. 1 The design of color ripeness indicator label based on CPR membrane for grapes with color indication for just ripe (crunchy), still ok (firm) and over-ripe (juicy)
Simple on package indicator label for monitoring of grape ripening process using colorimetric…
low-permeability polyethylene (PE) plastic film was used to enclosed the grape packaging. The dimensions of the tray were 15 × 15 × 5 cm, so the head space volume would be around 15 × 15 × 1 cm. Since, the label work based on pH value; as long as certain pH value is reached the label will change the color, not only dictated by the volume of head space. Here, the package used was similar to the US or EU markets, where the grapes are mostly packaged using styrofoam tray or plate and covered by PE film, and for a bigger quantity of grape, the PE plastic bags are used. However, the styrofoam plate covered by PE film is most commonly compared to the PE bags. Therefore, we used the styrofoam plate covered by PE film in this work. Regarding the PE bags, the label could also be used in this type of package as long as the label could be placed and held in the same place. Since using the PE bags, it may easy for the label to displace due to the bag movement. For this purpose, the used of a thin film, such as acetyl cellulose film, could be the best choice, so that it cannot be easy to displace or remove during the bags distribution. The samples were stored at chiller conditions (4 ± 0.2 °C) in a low-temperature incubator (model MIR 153; Sanyo Electric Co., Japan) and at room temperature (28 ± 2 °C). The temperature of samples was maintained throughout the entire storage period in the freight and monitored using electronic temperature recording devices (Cox Tracer; Belmont, NC). Triplicate packages of the grapes product, from each storage temperature, were sampled at appropriate time intervals to allow for efficient kinetic analysis of pH measurements, SSC, TA, hardeners, and weight loss as well as sensory evaluation for the study of grape ripeness stored under chilled or room storage conditions. All experiments were conducted three times. Measurement of SSC, TA, pH, hardness, weight loss and color The determination of SSC was performed by homogenized the berries and the homogenate filtered through several layers of cheesecloth to obtain a clear juice. The SSC (%) was recorded with the refractometer (Fisher, Japan). The acidity of grapes is most often expressed in TA as an important parameter vintners use to evaluate the quality of juice and wine. For the TA analysis, 1 ml of juice was diluted into 50 ml of distilled water and titrated to the pH of 8.0 using a 0.05 M sodium hydroxide solution. The pH values were recorded by a pH meter (Russel, Moder RL150), with the glass electrode being immersed in the homogenate of berry meat after the end of the analysis. The hardness of the berry was measured using texture meter (Rheotex,UK). The weight loss (%) was measured using analytical balance (Sartorius, ED224S). Each analysis was repeated
three times. The berry color in term of green change to yellow was measured using a hand-held colorimeter (chroma meter CR-10, Minolta Inc., Japan) to determine the CIE color space coordinates, i.e. the visible colors to the human eye, as specified by the International Commission on Illumination (Commission Internationale d’Eclairage, CIE), L*, a*, b*, and c*. CIE L* (lightness), a* (redness), and b* (yellowness) values, and c*(color intensity), where in this case, yellowness (b*) was used for the measurements. Measurement of the indicator label The indicator label was placed inside the plastic covered package of the grapes samples for direct contact with the atmosphere inside the package head space. While the reference label for reading the label status was attached outside the covered package, just above of the indicator. So in this case, no direct contact with the fruits. However, if in the certain case, the label makes contact with the fruit unintentionally, the label will not change the color as long as no pH change, since it only changes when pH change inside package head space. Then, they stored at chilled and room temperature, in order to evaluate the applicability of the developed indicator label to monitor the ripeness process of grape. Besides, the color indicator can easily be viewed by nude eye in term of color change during the grape ripening. For quantification of color measurement of the indicator, a simple method was used by color analysis. In addition, we also add the three controls labels (i.e. label without fruit) for both (room and chilled temperature) in the experiment. A digital camera (Samsung, ES60, Seoul, Korea) was used to take the color of the indicator label with similar set-up and background for reproducible color measurements, and then the CorelDrawx4 as graphics software was used to analyze the color of the indicator as described previously [16]. Since, in this color measurement method, the required equipment and software costs are low, the experimental setup and operating are simple, and the measurements and analysis are adequate for this measurement as well as suitable for field application. Sensory evaluation In order to describe grape ripening state by sensory evaluation, an additional test was performed [19]. At the beginning, each grape was washed with water. Afterward, the berries were stored in a packaged and labeled with the indicator label for ripeness. During a period of investigation (20 days at room and 30 days at chiller condition), every 4 days for room temperature and 5 days at chiller temperature, the berry samples were taken from the package, tested and scored by a panel consisting of ten people (four male and six female with age 20–30 years). The grading system
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Fig. 2 Relationship of ripening degree days to soluble solids content (SSC) and titratable acidity (TA) (a), pH and berry weight lost (Wt) (b), and hardness and berry color (c) during berry ripening process at ambient temperature
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B. Kuswandi, E. A. Murdyaningsih
Simple on package indicator label for monitoring of grape ripening process using colorimetric… Fig. 3 Indicator label response towards ripening degree of grape berry at room (lower line) and chiller temperature (upper line)
was based on scores from 1 to 5 (scale) in categories color, hardness, flavor, juiciness, and aroma, and the results were presented in score from 1 for least dominant to 5 for most dominant. In addition, the prevailing subjective status of ripeness was assessed from ripe, peak maturity to overripe. The samples were tested by individual tester independently, and the mean value of scores was calculated. The color change of indicator labels and the evaluation of other parameters, such as SSC, TA, pH, hardness and weight loss total volatile acid for the berry sample were measured every 4 days (room temperature) and 5 days (chiller temperature) during the period of investigation. The day for measurements (4 days at room temperature and 5 days at chiller temperature) are selected based on the color change of indicator label along with the color change of white grape to become yellowish.
Results and discussion Range of grape ripeness Since our sensor label work based on pH, therefore the documentation and examination of the range of ripening grape profiles based on TA, pH, SSC, and berry weight for this study, are important. Based on this data, we have been able to determine the range of ripening state, i.e. ripening, peak maturity and over-ripening as given in Fig. 2. Based on this Fig. 2a, b the ripening was up to 8 days, then afterward until 12 days in peak maturity and starting at day 16 forward, the grape was over-ripe at room temperature. This was confirmed with the hardness of berry that significantly
decrease along with increasing of yellow color (b*) of berry as given in Fig. 2c and also with the grape taste using sensory evaluation that during ripening stage, the grape was crunchy, at the peak maturity was firm and at the over-ripe state was juicy at room and chiller temperature. The TA was slightly decreased during the period of investigation, while SSC (%brix) increased sharply during the period of investigation (Fig. 2a). A significant increase in the pH was observed, while significantly decreased in berry weight was observed during the period of investigation (Fig. 2b). At the beginning of the grape ripening (véraison) is characterized by softening and coloring of the berry. Many of the solutes that accumulated during the growth period of the grape development remain at harvest, yet due to the increase in berry volume, their concentration is reduced significantly [15]. Some organic acids produced during the growth period are reduced during the ripening period, among of these are malic and tartaric acid, where the malic concentrations decrease relative to tartrate. [13, 20, 21]. This is the reason why the TA was slightly decreased during the period of investigation, as consequent a significant increase in the pH was also observed during the period of investigation (Fig. 2a, b). Despite these major reduces in the organic acids produced, the important story during the ripening period is the tremendous increase in sugars, that occurs as a result of a total biochemical shift into fruit ripening mode. Since, sucrose produced from photosynthesis is imported into the grape berry during fruit ripening, where, it is hydrolyzed into its constituent sugars glucose and fructose [22]. Beyond sugar accumulation, the major determinants of a wine’s quality are the secondary
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B. Kuswandi, E. A. Murdyaningsih
Fig. 4 Relationship of indicator label change (RGB) toward pH of grape during berry ripening process at room (a) and chiller temperature (b), where the insets show the linear relationship
metabolites, such as aroma compounds that are distributed in the flesh and skin of the berry. In white grape varieties, the most volatile flavor components produced during fruit ripening are terpenoids, which are important to the pleasant aroma of many varieties, such as Riesling and Muscat, and fruity aroma precursors [23, 24]. This is the reason why SSC (%brix) increased sharply and consequently significantly decreased in berry weight was also observed during the period of investigation with different rate (Fig. 2a). Furthermore, it is also supported by the hardness of berry that significantly decreased (Fig. 2c). This trend will also be similar when the grape is stored in chiller temperature with a different rate of ripening process as described in the following section.
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Response of the indicator label All indicator labels were placed in close proximate (~1 cm) with the berry samples in order to respond to the slight increase in pH generated during ripening process with a very distinct color change from white to slightly yellow then to yellow. The indicator label was monitored periodically until no further color change was observed. Figure 3 shows the rate of color change of the indicator label (mean RGB value) towards ripeness process at room and chiller temperatures. Figure 3 shows the indicator response decrease steadily, as the indicator color change from white to beige, then finally to yellow within the period of investigation (20 days) at room temperature (lower line). Here, the indicator changed from white to beige at day 12 and then change to orange at day 16 onward at room temperature.
Simple on package indicator label for monitoring of grape ripening process using colorimetric… Fig. 5 Relationship of indicator label change (RGB) toward TA of grape during berry ripening process at room (a) and chiller temperature (b)
While upper line in Fig. 3 shows the indicator response decrease steadily as well within the period of investigation (30 days) at chiller temperature. The indicator slightly decreased within 15 days, and significant color change at day 20 onward as the indicator change from white to beige, then to yellow. In addition, visual inspection did not detect differences in color changes between these indicator labels of different batch samples. The onset of peak maturity was detected at day 12 and 20, while for over-ripe was detected at day 16 and 25 for room and chiller temperatures respectively. This indicated that the grape samples reduced, its volatile acid compounds at a relatively long time since its
ripeness lasted longer within 16 and 25 days for room and chiller temperatures respectively. This might be due to the fact that the most volatile flavor components produced during fruit ripening are terpenoids, and fruity aroma precursors [23, 24]. In addition, the control label results show that the set of the control (3 control labels) did not change their color over the same period of time in both conditions, due to no pH change during the period of investigation. This is due to the fact, when the indicator label was used as a control label over the same period of time in both conditions (chilled and room temperature), no color change of the label has been observed, due to no interaction between
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B. Kuswandi, E. A. Murdyaningsih
Fig. 6 Relationship of indicator label change (RGB) toward SSC of grape during berry ripening process at room (a) and chiller temperature (b)
the indicator label with the headspace inside packaging, since the atmospheric condition is similar to the atmospheric condition outside packaging or atmospheric condition before it use as a control label. In this atmospheric condition, no new equilibrium occurs between the indicator label and the headspace inside the control packaging, where at this atmospheric condition mainly contain air, so atmospheric condition relatively similar during the period of investigation. In the case of packaged grape, the interaction between the indicator label and the headspace inside packaging take place due to the pH change. Here, the pH increase as the volatile organic acids decreased gradually in the package headspace due to formation of sugars during berry ripening process, where new equilibrium achieved
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during each ripening state (ripe, mature and overripe), which in turn, change the pH of microenvironment inside the indicator label that resulted change the color of the indicator label. At the beginning (ripe state), no color change of the indicator label, since the pH of head space is approximately ~4.30 equal with fresh ripe grapes and caused no color change of the indicator label at this pH value. The indicator label changed to yellow when pH 5.28 at overripe state, as the microenvironment of the indicator label also changed as it achieved new equilibrium with the head space at this pH value, where the atmospheric condition is different with the beginning condition. At the beginning, there were more volatile organic acids produce, while at the late condition, there were more carbon dioxide and water vapor
Simple on package indicator label for monitoring of grape ripening process using colorimetric… Fig. 7 Relationship of indicator label change (RGB) toward hardness of grape during berry ripening process at room (a) and chiller temperature (b)
produced as a result of enzymatic oxidation of sugars in berry during the respiration processes. Thus, we could be stated that the color change of the indicator label solely due to the change in pH inside the head space of the package. Correlation of the indicator label towards pH, TA, SSC, hardness and weight loss Figure 4a, b show a trend in the pH changes of the berry samples along with the indicator label response (RGB value) at room and chiller respectively. The pH values of the berry samples were varied from pH 4.30 at the early ripening to pH 5.28 at overripe state at room temperature (Fig. 4a). The pH values of the berry samples were varied
from pH 4.21 at early ripening to pH 5.24 at over ripe state at chiller temperature (Fig. 4b). It can be seen from Fig. 4 that the indicator label also follow similar trend inversely as shown by pH response at both conditions. Furthermore, the indicator label also response to the increase in pH value in the package headspace, since the range of CPR membrane color change fits the levels of the pH change in head space of the berry samples, particularly the color change from white to beige then to yellow. By using regression analysis, the coefficient correlation (r) between the color of the indicator label (RGB) versus pH of the grape during berry ripening process at room (a) and chiller temperature (b) were found to be 0.981 and 0.966 for room and chiller respectively. The values show the linear relationship between the
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Fig. 8 Relationship of indicator label change (RGB) toward weight of grape during berry ripening process at room (a) and chiller temperature (b)
color of the indicator label and pH of grape during berry ripening process. The level of pH in head space increased due to the fact that organic volatile acid significantly decreases during the transition between the ripening state to the peak maturity and then to overripe state in different berry varieties [15, 25, 26]. This could be due to in the late ripening of grape; many important aroma and flavor compounds are produced. Some of these components are produced as precursors and are not actually volatile and their precursors are present as glycosides [15, 27]. These glycosides, which contribute to the fresh and fruity grape flavor, during ripening process [27]. Figure 5a, b show a trend in the TA changes of the berry samples along with the indicator label response (RGB
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value) at room and chiller respectively. The pH values of the berry samples were varied from TA 11.8 (g/l) at the early ripening to TA 4.8 (g/l) at overripe state at room temperature (Fig. 5a). The pH values of the berry samples were varied from TA 11.5 (g/l) at early ripening to 4.4 (g/l) at overripe state at chiller temperature (Fig. 5b). It can be seen from Fig. 5 that the indicator label also follows the similar trend as shown by TA response at both conditions. Similarly using regression analysis, the coefficient correlation (r) between the color of the indicator label (RGB) versus TA of the grape during berry ripening process at room (a) and chiller temperature (b) was calculated to be 0.959 and 0.975 for room and chiller respectively. The values show the linear relationship between the color of the indicator label and TA of grape during berry ripening process. The
Simple on package indicator label for monitoring of grape ripening process using colorimetric… Fig. 9 Relationship of indicator label change (RGB) toward sensory evaluation (juiciness score) of grape during berry ripening process at room (a) and chiller temperature (b)
reduction in TA during grape ripening is partly related to the respiration of malic acid in the berry [22]. The organic acid concentration contributed from tartaric, malic and, to a much lesser extent, citric acid. However, changes in TA could be useful in assessing maturity and the changes in the rate of maturity. During grape ripening process in this experiment (20 days in the room and 30 days in chiller conditions), soluble solid content (SSC) of berry samples has been increased as given in Fig. 6a, b. The coefficient correlation (r) between the color of the indicator label (RGB) versus SSC of the grape during berry ripening process at
the room (a) and chiller temperature (b) using regression analysis, were found to be 0.959 and 0.975 for room and chiller respectively. The values show the linear relationship between the color of the indicator label and SSC of the grape during berry ripening process. The SSC increased during berry ripening is similar to the results of others [26, 28]. The increase in SSC observed (Fig. 6) due to the tremendous increase in sucrose that occurs as a result of a total biochemical shift into fruit ripening mode. Sucrose produced from photosynthesis is imported into the grape berry during fruit ripening. Once transported into the berries, the sucrose is hydrolyzed into its constituent sugars glucose
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Table 1 The results of sensory evaluation of grape samples at room temperature Stored time (day)
Color
Flavor
Softness
Aroma
Juiciness
0 4 8 12 16 20
1.0 ± 0 1.3 ± 0.48 2.2 ± 0.42 3.2 ± 0.42 4.1 ± 0.31 5.0 ± 0
5.0 ± 0 4.7 ± 0.48 3.6 ± 0.70 3.0 ± 0.47 2.0 ± 0.47 1.1 ± 0.31
1.0 ± 0 1.4 ± 0.52 2.4 ± 0.52 3.4 ± 0.52 4.4 ± 0.52 5.0 ± 0
5.0 ± 0 4.6 ± 0.52 3.6 ± 0.52 2.6 ± 0.52 1.6 ± 0.52 1.0 ± 0
1.0 ± 0 1.3 ± 0.48 2.2 ± 0.42 3.3 ± 0.48 4.3 ± 0.48 5.0 ± 0
Mean (n = 10) sensory score for different attributes Score 1 = least dominant and 5 = most dominant Table 2 The results of sensory evaluation of grape samples at chiller temperature Stored time (day)
Color
Flavor
Softness
Aroma
Juiciness
0 4 8 12 16 20
1.0 ± 0 1.3 ± 0.48 2.2 ± 0.42 3.2 ± 0.42 4.1 ± 0.31 5.0 ± 0
5.0 ± 0 4.7 ± 0.48 3.6 ± 0.70 3.0 ± 0.47 2.0 ± 0.47 1.1 ± 0.31
1.0 ± 0 1.4 ± 0.52 2.4 ± 0.52 3.4 ± 0.52 4.4 ± 0.52 5.0 ± 0
5.0 ± 0 4.6 ± 0.52 3.6 ± 0.52 2.6 ± 0.52 1.6 ± 0.52 1.0 ± 0
1.0 ± 0 1.3 ± 0.48 2.2 ± 0.42 3.3 ± 0.48 4.3 ± 0.48 5.0 ± 0
Mean (n = 10) sensory score for different attributes Score 1 = least dominant and 5 = most dominant
and fructose [22]. Their eventual concentration is dictated partly by the length of time the grape berry is allowed to stay on the vine. Sugars typically account for 90% of SSC found in mature grape berries [29]. SSC expressed as °Brix are a proxy for sugar content that is based on the refractive index of the juice. SSC represents the relative sugar weight of a juice sample, for example, 1° Brix denotes 1% sugar by weight. In mature berries, SSC gives a fairly accurate account of sugar content and levels of soluble solids are within 1% of actual sugars (glucose and fructose) present [30]. The hardness of berry samples was conducted along with the indicator label response. Figure 7a, b are the average values of the hardness of grape samples at room and chiller temperature respectively. Each datum is the average of three measurements under identical conditions. It can be seen from Fig. 7 that the indicator label response decreased as the hardness value reduced. The ripeness process occurs along with the reduced hardness of the grape berry samples, as the indicator label change from white to beige, then to yellow. The coefficient correlation (r) between the color of the indicator label (RGB) versus hardness of the grape during berry ripening process at room (a) and chiller
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temperature (b) were calculated to be 0.994 and 0.972 of the room and chiller temperature respectively. The values show the linear relationship between the color of the indicator label and hardness of the grape during berry ripening process. Thus, the decrease in hardness of berries is caused by ripening process. During the onset of ripening when the berries are expanding the softening skin accumulates increased amounts of sugar and potassium [15]. Metabolism within the cells dictates physiological and biochemical changes throughout the ripening process that makes the berry softer. A good example of physiological and biochemical changes is the accumulation of the two most abundant phenolics, anthocyanins, and condensed tannins, which make natural coloration of grapes occurred [31]. The weight loss of grape berry samples was conducted along with the color indicator response as given in Fig. 8a, b. These are the average values of the reduced weight (weight loss) of berry samples at room and chiller temperature respectively. It can be seen from Fig. 8 that the indicator label response decreases along with decreased in weight (increased weight loss) of berries during storage. The ripeness process occurs along with the reduced weight of the grape samples, as the indicator label change from white to yellow. The coefficient correlation (r) between the color of the indicator label (RGB) versus reduced weight of the grape during berry ripening process at the room (a) and chiller temperature (b) was calculated to be 0.991 and 0.942 of the room and chiller temperature respectively. The values show the linear relationship between the color of the indicator label and reduced weight of the grape during berry ripening process. The largest components of the juice from ripe berries are water, sugar (glucose and fructose) and the organic acids (mostly malate and tartrate). Altogether these make up 99.5% of the mass of the juice [15]. Therefore, the loss of water, sugar, and acid during the ripening phase of the berry are the main reason. One of the most loss of weight is a reduction in water content in berries during ripening process [32]. This is due to variable contributions from xylem and phloem to berry water inflow, where water flow into the berry during ripening was proposed to occur predominantly via the phloem that it becomes non-functional resulting in water loss via transpiration exceeding water inflow [33]. Correlation of the indicator label towards sensory evaluation The color, hardness, flavor, juiciness, and aroma of grape samples were first evaluated by sensory evaluation and the measurements were conducted along with the indicator label response. The results of the indicator response were therefore confirmed by the sensory evaluation. The measurement was done in the laboratory conditions without any
Simple on package indicator label for monitoring of grape ripening process using colorimetric… Fig. 10 Polar plot for grape ripening at the various state of ripeness according to the sensory evaluation (score from 1 to 5) at room (a) and chiller (b) temperatures; just ripe/ripe (crunchy), mature/peak maturity (firm) and overripe (juicy)
special requirements considering the application at a shopping center, restaurant, storage room and others. Regarding the taste, the panel describes the taste in term of flavor and Juiciness scores of the grape tested, where score 1 for least dominant and 5 for most dominant. This score applied in sensory evaluation in order to simplify how the panel distinguishes the grape based on the level of ripeness, and this sensory evaluation is important as no standard available
regarding the level of ripeness, and solely depends on the sensory evaluation, particularly for the human taste. Figure 9a, b shows the output score of the juiciness measurement (juicy in this case was selected due to it tends to mushy) as the point of rejection (4) corresponding to Tables 1 and 2. From Fig. 9, it can be seen that the indicator label change show the similar tendency to sensory response (juiciness score), where the point of rejection of
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Table 3 The index of explicity in reading the label to distinguish the label color change in correlation with the reference label status by naked eyes Stored time (day)
Room
Stored time (day)
Chiller
0 4 8 12 16 20
5.0 4.7 3.3 4.2 3.7 4.8
0 5 10 15 20 25 30
5.0 4.7 2.6 4.3 4.5 3.6 4.7
Mean (n = 10) Score 5 = very easy and 1 = very difficult
sensory score was similar with the onset of detection of indicator response. Tables 1 and 2 list the results of the sensory evaluation in room and chiller temperature respectively. As expected, the estimated general assessment, the grapes ripeness indicator, color, flavor, softness, aroma, and juiciness were appraised. In general, the following tendencies can be deduced, during ripening green color of grape increase to yellow. The flavor/smell of berries changed from “fresh & aromatic” to “musty, old and sticky”. The softness of berry skin and juiciness increased during storage, whereas aroma content decreases. In general, the correlations between the changes of color indicator toward the results of classification of ripeness criteria in the sensory evaluation are given as polar plot in Fig. 10 at room (a) and Fig. 11 Application of indicator label for grape ripeness, a white for fresh (crunchy), b beige for maturity (firm) and c yellow for not fresh/overripe (juicy)
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chiller (b) temperatures, where the polar plot for grape ripening at the various state of ripeness according to the sensory evaluation (score from 1 to 5) at room (a) and chiller (b) temperatures shows similar plot similar ripening state, i.e. just ripe/ripe (crunchy), mature/peak maturity (firm) and overripe (juicy). The polar plot is similar As estimated for typical ripeness stage of white grape, the yellow color, softness of skin and juiciness increased, whereas aroma content decreased occurs in grape [3, 15, 22]. In addition, during the sensory evaluation session the experimentation regarding the index of explicitly of the label, in order to know how easier the panel to distinguish the color changes of the label in correlation with the reference labels status by naked eyes. The results show that the color changes are easy to distinguish with the help of the reference label status (Table 3). Some of the panels said that the difficulty only occurs to distinguish between ripe and earlier stage of maturity, another status is easy to distinguish, particularly between ripe and overripe (fresh and not fresh). Thus, the color indicator can be used as an effective tool for direct monitoring of berry ripeness (Fig. 11).
Conclusions A CPR membrane was used as on package indicator label and the correlation between the indicator response and grape ripeness was investigated. The results show that the indicator label could be used for monitoring grape ripeness since the color change of the label and the grape ripening
Simple on package indicator label for monitoring of grape ripening process using colorimetric…
process is in similar tendency and has a linear correlation with the berry decay that could be detected clearly when the indicator change from white to beige then finally to yellow. The indicator label has a response to the reduced of volatile organic acid as ripening process indication of berry, where the color change to yellow for the overripe indication. The color indicator may serve as on-package active shelf-life devices in conjunction with the “used-bydate” labeling, when attached to individual package product units as the display of ripeness state for consumers or traders, or could be used in fruit logistic chain and management, which in turn could reduce fruits lost. Acknowledgements The authors gratefully thank the DRPM, Higher Education, Ministry of Research, Technology and Higher Education, the Republic of Indonesia for supporting this work via the Competency Grand Program 2017 (Hibah Kompetensi 2017).
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