Edible colorimetric label based on immobilized purple sweet potato anthocyanins onto edible film for packaged mushrooms freshness monitoring

Abstract

An edible colorimetric label has been developed to determine the freshness level of mushrooms, i.e. white oyster mushrooms (Pleurotus ostreatus). The edible indicator label has been fabricated based on purple sweet potato (Ipomoea batatas L.) anthocyanins (PSPA) immobilized onto an edible film made of chitosan and cornstarch with added PVA. The freshness parameters of the mushrooms were pH, weight loss, texture, and sensory evaluation. The results showed that the colorimetric label was dark purple when the mushroom was fresh, and turn to light purple when the mushroom was still fresh, and finally green when the mushroom was no longer fresh. The color value (mean Red) of the label was measured using the ImageJ program, where its color value (mean Red) increased with decreasing freshness level of the mushrooms. The edible label can distinguish fresh mushrooms from spoilage, making it suitable to be used in a packaged mushroom as a freshness indicator.

Introduction

Intelligent food packaging as part of smart packaging (Drago et al. 2020) uses materials that could detect packaged food quality or the packaged food's ambient condition and inform the users visually by a color change. Hence, intelligent food packaging can monitor the food packaging quality and communicate to the users regarding the food quality during transportation and storage (Almasi et al. 2022; Becerril et al. 2021; Kuswandi et al. 2011). One example of this intelligent packaging is the use of a colorimetric pH indicator as an intelligent or smart label, due to its compatibility to be used in food packaging at a low cost (Choi et al. 2017; Kuswandi 2017). Commonly, a colorimetric pH indicator is a color indicator, where its color changes in different pH conditions, such as acid–base solution (Xiao-wei et al. 2018; Yoshida et al. 2014).

In the case of intelligent food packaging, synthetic pH dyes are commonly used for perishable foods including fruits and vegetables (McAtee et al. 2013). However, the chemical synthetic pH dye used in pH indicators for perishable food freshness monitoring has effects harmful to human health and is not an ideal indicator used in food packaging (Dainelli et al. 2008). Therefore, natural dye or pigments as edible indicators are highly recommended in this case, particularly for on-package food freshness indicators. One of the potential alternatives as a natural indicator is anthocyanins which can be found in abundance in nature. They are edible, biodegradable, extractable pigment, and water-soluble (Roy and Rhim 2020; Zhang et al. 2014)). They can exhibit a color change in different pH conditions, from acid to basic conditions. The color changes in this natural indicator are due to the presence of phenolic or conjugated substances (e.g. cyanidin, delphinidin, pelargonidin, peonidin, and petunidin) that are made structural changes under interaction in different pH conditions (Shahid et al. 2013). Anthocyanin may have different colors at different pH, e.g., red at pH 1, purple at pH 7, blue at pH 10, green at pH 11, and yellow at pH 13. However, anthocyanin pigments are unstable at high pH values, since the yellow color represents the chalcones formation that is a degradation product of anthocyanin (Khoo et al. 2017). In this regard, the use of anthocyanins extracted from various plants as pH indicators have been reported in the literature (Roy and Rhim 2020; Yoshida et al. 2014).

Recently, there are many studies on colorimetric indicator development based on anthocyanins. For example, Roselle anthocyanins have been used as a natural dye for many foods (Rodriguez-Amaya 2016). Black carrot anthocyanins have also been developed as a calorimetric pH indicator for monitoring spoilage in pasteurized milk (Ebrahimi Tirtashi et al. 2019). Purple sweet potato anthocyanins have been used as pH indicator films for pork freshness monitoring (Choi et al. 2017). Other anthocyanins have also been used as intelligent pH indicators are extracted from the black soybean seed coat (Wang et al. 2019), grape (Yoshida et al. 2014), red cabbage, (Pourjavaher et al. 2017), Bauhinia blakeana Dunn (Zhang et al. 2014), and purple sweet potato (Yong et al. 2019).

The anthocyanins' color is corresponding to the solution pH. This is due to the anthocyanins' molecular structure having an ionic nature (Roy and Rhim 2020). In an acidic solution, the anthocyanins may appear red, while in a neutral solution, they have a purple color. In an alkali solution, the color changes to blue. The red color of anthocyanins is mostly in the form of flavylium cations (Sampaio et al. 2021). The flavylium cation allows the anthocyanin to be highly soluble in water at lower pH. The rate of deprotonation of the flavylium cation decreases when water concentration increases, thus reducing color stability (Li et al. 2019). Since these anthocyanins are more stable at a lower pH condition. Apart from the pH, anthocyanin-tannin polymerization could also increase color stability at a lower pH (Rogez et al. 2011). In the case of anthocyanins from purple-red potato extract (Ipomoea batatas L.) as one of the food natural dye sources, due to its ability to show different colors in a wide range of pH, from acid to basic conditions. Its color exhibits various colors toward various pH, where at low acid conditions, it is a dark purple, and changes to light purple as pH increases, and in alkali conditions, it is green. It has been developed as a colorimetric pH sensing film using an agar/potato starch for meat spoilage sensor (Choi et al. 2017) and as a pH-sensitive film based on starch/polyvinyl alcohol as a visual indicator of shrimp deterioration (Zhang et al. 2020), and indicator films based on carboxymethyl-cellulose/starch for monitoring of fish freshness (Jiang et al. 2020).

In this study, the edible film was made from a mixture of cornstarch and chitosan. The cornstarch selected is edible, cheap, and abundant. renewable, and biodegradable (Xu et al. 2005). Chitosan in addition to being able to form a thin layer can also be used as an antimicrobial agent in the edible film, so it can be said to be more environmentally friendly when compared to plastic packing materials (Rovina et al. 2020). The edible film with constituents in the form of polysaccharides tends to be more fragile when compared to others, so it is necessary to add a filtering substance (plasticizer) that can add flexibility to the edible film polysaccharide (Kuswandi and Asrofi 2020). Plasticizers on the market are many, such as phthalates, tripedal phosphate, polyethylene glycol, sorbitol, and glycerol. For its application in edible film, a plasticizer is needed with food-grade formula. The plasticizers used in this study were sorbitol and glycerol, to create low solubility in water, strong tenuous break, and good film permeability to water vapor as it will be used as a colorimetric indicator label (Bourtoom 2008).

The study aimed to develop an edible colorimetric label based on purple-sweet potato anthocyanins (PSPA) immobilized onto an edible film made of a mixture between chitosan and corn starch (CCS) with aid of PVA (1%) to monitor the mushroom freshness, i.e. white oyster mushroom (Pleurotus ostreatus). The oyster mushroom is widely cultivated in China, Japan, Thailand, and Indonesia. The freshness parameter of the mushroom was pH, texture, and sensory evaluation. The purple-sweet potato anthocyanins are water-soluble pigments that may appear pink (acidic), pink (neutral), and green (alkaline) (Choi et al. 2017). By using these parameters, we could correlate the edible indicator label color change toward mushroom quality in terms of freshness, medium, and spoilage, thus making it suitable to be used in a packaged mushroom as a freshness indicator.

Materials and methods

Materials

The materials used in the study were purple sweet potato (Ipomoea batatas L.), white oyster mushrooms (Pleurotus ostreatus), and cornstarch (Maizena) were purchased from a local supermarket in Jember, East Java, Indonesia. While chitosan, ethanol (96%), acetic acid (1%), sorbitol (80%), glycerol, polyvinyl alcohol (PVA), hydrochloric acid (37%), and sodium hydroxide were purchased from Sigma-Aldrich (UK). All other reagents are analytical grade and used as purchased without any further purification and treatment.

Preparation of edible film

The edible film was prepared according to Ishak et al. (2015) with some modifications as follow. Three grams of chitosan were mixed with 50 ml of 1% acetic acid in a glass beaker and then were heated at a temperature of 80 °C for 30 min. Seven grams of cornstarch were mixed with 100 mL of deionized water in a glass beaker while they were stirred with a magnetic stirrer for 22 min at 80 °C. The chitosan gel and cornstarch were mixed and then added with sorbitol and glycerol at 1 mL. Then the results obtained were stirred using a magnetic stirrer for 1 h. The mixture was poured on the surface of the clean glass plate. Then it was dried in the oven at 80 °C for 6 h. Once the film was dry, then it was cooled until it reached room temperature and then it was stored in a dry container for further use.

Extraction of anthocyanins

Anthocyanins from purple sweet potato (PSP) were obtained according to previous procedures (Jiang et al. 2021), with some modifications. After being washed and peeled, PSP was dried and powdered. Around 50 g powder of PSP was mixed with 500 mL ethanol–water solution (40%) and then stirred at 60 °C for 6 h. Afterward, the mixture was filtered through a Whatman filter paper (Whatman No. 1, USA) and the anthocyanin extract solution was obtained. After that, the extract solution was concentrated using a rotary evaporator at 50 °C and protected from light using aluminum foil. Finally, the solution was freeze-dried under a vacuum and the obtained purple sweet potato anthocyanin (PSPA) was as a concentrated extract solution. The content of PSPA extract was measured to be 35.07 mg/L by a pH differential method (Kuswandi et al. 2020).

The PSPA color change at different pH

The PSPA extract was diluted at various pH solutions (10 mL phosphate buffer solution from pH 3 to 11) to show that the PSA is water-soluble that may appear in a different color depending on the pH condition.

Preparation of colorimetric indicator label

The circle shape of the edible film (5 mm) was immersed in a glass container consisting of 3 mL PSPA extract and 1 mL PVA (1%) for 2 h. PVA (1%) was added to reduce the swelling and leaching of the indicator label (Kuswandi et al. 2020). Then it was dried at ambient conditions until the edible film was completely dry. After complete drying, the prepared indicator labels were stored in a sealed container and dry place before they are used.

Physical properties

The thickness of each PSPA indicator film was determined (20 measurements each) using a hand-held digital micrometer (Zhongtian Instrument, China). The swelling index (SI) and water solubility (WS) of a colorimetric indicator label were measured according to the method reported previously (Ebrahimi Tirtashi et al. 2019). These are including the moisture content (MC) after each film was conditioned for 48 h at 25 °C, and ∼(55 ± 1)% RH) was determined by thermal drying at 105 °C in an oven to a constant weight. The MC was calculated according to the reported previously (Jiang et al. 2020). The tensile strength (TS) of the edible films was measured using a universal texture meter (Super Technology Instrument, China), and TS was calculated according to reported previously (Jiang et al. 2020). The surface of indicator labels was visualized using a scanning electron microscope (Ultra, Carl Zeiss AG, Germany) operated at a voltage of 7 kV. The indicator label was stored under a dark container for 15 days at room temperature (25 °C), then the color stability was measured by observing the color, and measuring the means Red value.

Indicator response toward pH

A piece of the prepared calorimetric label (5 mm) was exposed to various pH levels using buffer solutions at pH 5.33 as fresh mushrooms pH, and pH 6.82 as spoilage mushrooms pH. The pH meter (Eutech Instruments, pH 700, Singapore) was used to validate the results of the pH values. The color change of the indicator to each buffer was taken by a flatbed scanner (CanoScan, LIDE 110, Japan) as JPEG files, and subsequently, color values were measured as mean Green, calculated using the ImageJ 1.5 for Windows (https://imagej.nih.gov/ij/). All of the measurement procedures in this experiment were performed in triplicate.

Mushroom pH measurement

The mushroom pH values were measured according to Kuswandi et al. (2017) 10 g mushroom sample was vigorously homogenized in 90 mL distilled water and pH values were measured by a digital pH meter (Eutech Instruments, pH 700, Singapore).

Freshness monitoring of mushroom

Firstly, fresh white oyster mushrooms (Pleurotus ostreatus) from Jember were tested for their pH using a pH electrode (Eutech, Singapore). To make a correlation between the color changes of the indicator label toward the mushroom's freshness, the mushroom was packaged in a transparent plastic container, where the indicator label was placed above the mushroom and attached to the plastic cover of the container. The packaged mushroom was kept at room temperature (25 ± 2 °C) and chiller (2 ± 2 °C) until it was spoiled. At different time intervals, the indicator label was removed and the color changes were recorded by a scanner, and the color value was analyzed using the ImageJ program. Other freshness parameters of mushrooms were also examined, such as weight loss, texture, and sensory evaluation.

The sensory evaluation was performed to evaluate the mushroom's freshness status, i.e., fresh, still fresh, and spoilage (Kuswandi et al. 2015). Firstly, each mushroom was washed with tap water. Afterward, they were stored in a packaged and attached inside the covered package with the edible indicator label for freshness monitoring during storage. During a period of study, every day the mushroom samples were taken from the package, tested, and scored by a panel consisting of ten people (4 males and 6 females with ages 19–22 years). The panelists were trained to have similar perception and evaluation regarding the level of freshness of mushroom, before they evaluate the mushroom samples (Ares et al. 2006; Wrona et al. 2015). The grading system used was based on scores from 1 to 3 scale in categories, i.e., color and odor. Then, the results were recorded in scores from 1 (least dominant) to 3 (most dominant). Furthermore, the subjective status of the mushrooms was evaluated from fresh, still fresh to spoilage. The samples were evaluated by individual testers independently, and the mean value of scores for each sample was calculated.

Statistical analysis

All measurements were performed in triplicate, and the data were analyzed using SPSS (Version 21, SPSS Inc., Chicago, IL), particularly to compare the physical properties of the edible membrane before and after immobilization with PSPA using Student t-test (Miller and Miller 2010). The results were presented as average values ± standard deviation (SD). Significant differences were defined with p ≤ 0.05.

Results and discussion

Physical properties

Physical properties of edible films, including thickness, swelling index (SI), water solubility (WS), moisture content (MC), and mechanical (TS) properties are shown in Table 1. The film thickness increased from 27.01 ± 1.25 to 37.55 ± 2.11 μm (p < 0.05) with the immobilized PSPA onto the edible film. This suggests that immobilized PSPA can create more complex matrices between anthocyanin and film materials, thereby resulting in the thickness of edible films being relatively higher.

Table 1.

The value of physical properties of edible film

Parameter tested	Edible film	Immobilized edible film	Calculated T-test
Thickness (μm)	27.01 ± 1.25	37.55 ± 2.11	2.408*
Swelling index (%)	75.85 ± 1.33	67.52 ± 1.23	5.592*
Water solubility (%)	21.45 ± 1.67	15.73 ± 1.33	3.765*
Moisture content (%)	22.51 ± 1.15	14.12 ± 1.56	9.288*
TS (MPa)	15.56 ± 0.28	22.69 ± 0.12	54.225*

 ± SD value; T-test Table 0.95 = 2.353

*Significantly different at 95% (p ≤ 0.05)

The swelling index (SI) reflects the film's ability to absorb water during the time of storage, which affects significantly the indicator response, due to higher SI causing a faster release of the dyes. The SI edible film reduced from 75.85% to 67.52% (p < 0.05) with immobilized PSPA onto the edible film. This is mainly due to the PVA(1%) addition during PSPA immobilization acting as strong binding for anthocyanin in the edible film, causing a reduction in intermolecular interactions between the film and water.

Some specifications of colorimetric indicators, e.g., physical barrier and water resistance are held by the film WS values. The WS of the edible reduced from 21.45% to 15.73% with the immobilization of PSPA. The edible film showed higher WS than the immobilized film, as immobilization of anthocyanine incorporated with PVA (1%) decreased the WS. This improvement in the film WS could be related to the binding property of the PVA network along with anthocyanine in the film.

With the immobilized PSPA extract onto the edible film, TS increased from 15.56 ± 0.28 to 22.69 ± 0.12 MPa (p < 0.05). The increase of TS of the immobilized edible films is associated with a stronger facial interaction via hydrogen bonds between PSPA (filler) and polymer film. The MC contents of the edible film and the immobilized edible films significantly reduced from 22.51% to 14.12 (p < 0.05). It is shown that anthocyanin contains an abundance of hydroxyl groups, which in turn, could create intermolecular hydrogen bonds with hydrophilic groups in chitosan and cornstarch.

The produced edible film was a yellow color with a smooth texture as depicted in Fig. 1a, Then, the edible film was cut using a hole puncher (i.d. 5 mm), to make a circular colorimetric film and then immobilized with PSPA extract as given in Fig. 1b. It can be shown that the edible film was yellow, while the immobilized edible film was purple, showing that anthocyanine (Fig. 1c) was already absorbed evenly in the edible film to produce a colorimetric indicator label. Furthermore, Fig. 1c shows the extract color change toward pH buffer at pH 3 to 11. The SEM images of the edible film are presented in Fig. 1(d and e). The SEM image shows the edible film before being immobilized with PSPA had a slightly smooth and porous surface (Fig. 1d), while the edible film that has been immobilized with PSPA extract shows a rough and porous surface area (Fig. 1e). This is due to the PSPA solution being spread evenly on the film by adding PVA so that the distribution of the matrix contained in the colored film is more apparent in this case.

Fig. 1.

The produced edible film (a), and edible fim after immobilized with PSPA extract (b), PSPA extract and its color change toward pH buffer (c), and the SEM image of edible film before immobilized (d), and after immobilized with PSPA extract (e) (color figure online)

In addition, it is also indicating that PSPA was successfully incorporated into the edible film, and had excellent compatibility with the edible film. Simultaneously, PSPA, PVA, chitosan, and cornstarch have extensive interactions, phenolic hydroxyl groups in anthocyanine could combine with hydroxyl groups in chitosan and cornstarch, thus the mechanical properties are improved (Zhai et al. 2017). Additionally, adding glycerol and sorbitol can result in better compatibility between cornstarch and chitosan by providing some intermolecular hydrogen bonds (formed by hydroxyl groups) between them, including the addition of PVA during PSPA immobilization (Zhang et al. 2012). Thus, the edible colorimetric film from cornstarch and chitosan immobilized with PSPA is an improved edible hydrocolloid film formulation, as it had improved the physical properties of the edible film. An excellent formulation for the edible film from cornstarch (7 g) and chitosan (3 g) with plasticizer sorbitol (1 mL) and glycerol (1 mL) that improved by PSAP immobilization added with PVA (1%) onto the film with SI value of 67.52%, WS value of 15.73%, MC value of 14.12%, and tensile strength of 22.69 Mpa. These physical properties are appropriate to be used as an edible colorimetric indicator label for intelligent packaging. Thus, overall the improvement of physical properties of the edible membrane has been achieved after immobilization with PSPA significantly at 95% using the Student t-test in terms of thickness, swelling index, water solubility, moisture content, and mechanical properties (Zhang et al. 2020) as shown at Table 1.

Indicator label characteristics

Before the edible colorimetric label is applied to monitor packaged mushroom freshness, some tests were carried out including the response of the indicator label toward the pH of fresh mushrooms at pH 5.33) dan spoilage mushrooms at pH 6.82). The reproducibility of measurements at these pH values and usable time of edible indicator was examined in this study. The indicator color change as a response to the pH of fresh mushroom at pH 5.33, and spoilage mushroom at pH 6.82 are presented in Table 2, where at pH 5.33 (fresh condition), the indicator color was purple and at pH 6.82 (spoilage condition) was green, and give similar color in three days. This can be used as evidence that the indicator color change can be used as a visual indicator to distinguish between pH 5.33 and pH 6.82, where fresh and spoilage mushrooms can be distinguished based on pH (Castellanos-Reyes et al. 2021), even if the pH change only at 1.5 degrees of pH. This pH range is also similar to other freshness indicators for fruits and vegetables (Choi et al. 2017; Kuswandi 2017).

Table 2.

The edible indicator color change toward the pH of fresh and spoilage mushroom

Day	pH 5.33 ± 0.12 (Fresh)			pH 6.82 ± 0.16 (Spoilage)
	1	2	3	1	2	3
Indicator calor
Mean Red	161.530 ± 0.565 (RSD = 0.024)			179.925 ± 1.128 (RSD = 0.029)

 ± SD value

Before the reproducibility of the indicator, label response was measured using the color value by ImageJ. Firstly, the highest color value was determined, wherein in this case mean red was selected as the highest color value (160.284) compare to others, i.e. mean green, blue, and RGB. The reproducibility of the indicator label toward pH 5.33 (fresh mushrooms) and pH 6.82 (spoilage mushrooms), where the RSD value is  < 1% (Table 2). Thus, the reproducibility of the indicator response toward pH was excellent as the RSD of the response in terms of mean red was less than 1% at both pH values, so this value was great to be used in this type of measurement.

The usable time of the edible indicator at both conditions (chiller and room temperature) was set up at the time when the indicator label gives a stable response to pH as analytes until its response time drastically decrease to about 15% of the earliest response. Herein, the indicator stored at chiller temperature can extend the usable time of the edible indicator to 18 compared to 14 days when it is stored at room temperature. Thus, this usable time indicator is suitable to be used as a colorimetric indicator label for mushroom freshness monitoring as its freshness will be around 3 days at room temperature and 7 days at chiller conditions (Castellanos-Reyes et al. 2021; Zhang et al. 2018).

Application as mushroom freshness monitoring

The application of the edible color indicator label toward packaged mushrooms' freshness at both temperatures (i.e. room and chiller) is given in Fig. 2. Herein, the packaged mushroom freshness stored at room temperature was up to 3 days, while at chiller was up to 7 days. Mushrooms (Agaricus bisporus) are perishable products that deteriorate in a short period after harvest, that affects by their storage and distribution as a fresh product(Castellanos-Reyes et al. 2021). High metabolic activity, respiration rate, and dehydration are responsible for the fast decay of mushrooms (Ares et al. 2006). It has a normal shelf-life of 1 to 3 days at ambient temperature during marketing (Xiao et al. 2011). Their consumer acceptance decreases are predominantly caused by the loss of white color, size reduction, weight loss, texture changes, and in some cases, spore formation (Ares et al. 2006). Hence, to prove that the edible indicator color change is associated with other freshness parameters, such as pH value, weight loss, texture, and sensory evaluation, therefore the indicator color change was related to these parameters as follows.

Fig. 2.

The color of edible indicator label (mean red) vs pH of packaged mushrooms stored at chiller (a) and room temperature (b) (color figure online)

Figure 2(a and b) shows the edible indicator label response (mean read) and the pH changes of the mushroom during the study at room and chiller conditions, respectively. Here, the mushroom pH values varied, starting from pH 5.50 at the fresh condition, and steadily increasing to pH 6.60 when it start to spoilage at both conditions. While at chiller condition, the mushroom spoilage was reached on day 7 (Fig. 2a), and at room temperature was reached on day 4 (Fig. 2b). According to Fig. 2, it can be stated that the edible indicator label follows a similar color change in response to a pH change, as it is shown by their color change when pH was increased at both temperatures. Moreover, the edible indicator label responds to the pH change inside the package as its color change in the range of the pH change inside the mushroom packaging for each freshness status, where the color developed from purple for fresh, pale purple for still fresh, and green for spoilage. Generally, the white mushroom is rich in acidic polysaccharides, dietary fiber, and antioxidants including vitamins C, B12, and D; folate; ergothioneine; and polyphenol (Mattila et al. 2001). Molds, bacteria, enzymatic activity, and biochemical changes can cause spoilage during storage along with pH change to have come higher, and even over neutral pH (Lagnika et al. 2014). In addition, the high tyrosinase and phenolic contents of mushrooms make them prone to rapid enzymatic browning (Lagnika et al. 2014; Mahajan et al. 2008). This phenomenon is causing increasing pH, which in turn, is the major cause of the overall quality loss of mushrooms accounting for a reduction in market value (Lagnika et al. 2014; Mukherjee et al. 2022).

Figure 3 shows the mushroom weight loss along with the edible indicator label response during storage at chiller (Fig. 3a) and room temperature (Fig. 3b). Based on Fig. 3, it can be stated that the edible indicator label response increase along with the increased weight loss (%) during storage at both conditions. Here, the mushroom freshness is reduced followed by its reduced weight. In this case, the edible indicator color change from purple, pale purple, and green during fresh, still fresh, and then finally spoilage. Commonly, mushroom weight loss is mainly due to the high metabolic rate of mushrooms, which caused short shelf-life and destruction of the mushroom cell membrane integrity (Wrona et al. 2015; Xiao et al. 2011).

Fig. 3.

The color of edible indicator label (mean red) vs weigh loss (%) of packaged mushrooms stored at chiller (a) and room temperature (b), the color of edible indicator label (mean red) vs texture (g/3 mm) of packaged mushrooms stored at chiller (c) and room temperature (d) (color figure online)

Figure 3(c and d) shows the edible indicator label response and the texture of mushrooms reduced during the study. The figure shows the average values of triplicate measurement of the mushroom texture at chiller (Fig. 3c) and room temperature (Fig. 3d). Thus, it can be stated that the edible indicator label response increased along with the texture value (g/3 mm) decreased to softer at both temperatures. This means that during storage, from fresh to spoilage, is followed by the mushroom texture hardness becomes lower. Generally, mushrooms can be physically evaluated by the texture, where the maximum stress, deformation degree, and power can be considered as the texture indices measures. Herein, the interior tissues have a hierarchical, loose spongy structure, and are not homogeneous, which in turn, will be reduced during freshness degradation and spoilage (Castellanos-Reyes et al. 2021; Xiao et al. 2011).

In the sensory evaluation (color and odor of the mushroom were evaluated), the evaluations were performed along with the edible indicator label visual detection. The sensory evaluation results were compared by the edible indicator color change. The evaluation was conducted in the laboratory setting without any special needs, as in the case of the label applications, such as at home, shop, restaurant, etc., as depicted in Fig. 4. This figure shows sensory evaluation toward mushroom freshness status, such as fresh, still fresh, and spoilage, where the edible indicator label shows a similar color change toward freshness status (purple for fresh, pale purple for still fresh, and green for spoilage) (Fig. 4c). Here, Fig. 4(a and b) shows the output score of the color and odor evaluation at both temperatures. Herein, color and odor were chosen over other sensory evaluation parameters due to the ease to see mushroom color via the naked eye and the ease to smell the odor via the panel's nose, where it can be used as freshness status and point of rejection (1.7) as well in both conditions. Commonly, the mushroom's freshness can be summarised as follows; when it is fresh, the white color of the mushroom is maintained, and changes to broken white when the freshness is reduced, and changes to slightly brown when it starts to spoil. The mushroom odor changed from fresh to mushy and smelly when it spoiled (Castellanos-Reyes et al. 2021; Xiao et al. 2011). Based on Fig. 4(a and b), the edible indicator label color change shows a similar trend to the color. and odor score, where the sensory score for the spoilage point was fit with the onset of spoilage of the indicator color change. This is indicated by the indicator color change from purple when it is fresh (Fig. 4d) to pale green when it is still fresh (Fig. 4e) and finally green for spoilage (Fig. 4f) at both temperatures. Thus, the edible indicator label can be applied as an intelligent label for real-time monitoring of mushroom freshness.

Fig. 4.

The color of the edible indicator label (mean red) vs sensory evaluation (color and odor) of packaged mushrooms stored at chiller (a) and room temperature (b), the label design with color comparator in Bahasa Indonesia, where segar = fresh; masih segar = still fresh; busuk = spoilage (c), the use of edible indicator label onto headspace of packaged mushrooms in fresh condition (d), still fresh (e), and spoilage conditions (f) (color figure online)

The white color loss of fresh mushrooms is caused by enzymatic browning (Nowak et al. 2018). Enzymatic browning is a biochemical process that involves the reaction of substrates, e.g. monophenols and polyphenols with atmospheric oxygen in the presence of enzymes, and the subsequent production of high-molecular-weight pigments (melanins) (Wrona et al. 2015). Polyphenols provide the color of many plant products, and the taste and flavor of beverages, and are also well-known antioxidants (Mukherjee et al. 2022). The most important phenolic compounds as substrates in mushroom browning reactions are tyrosine, L-DOPA, and pyrocatechol (Nowak et al. 2018).

Conclusions

An edible colorimetric label based on an edible film made of chitosan and cornstarch immobilized with PSPA has been developed as an on-package indicator label for mushroom freshness monitoring. The results show that the edible indicator label could be used for the mushroom freshness monitoring, since the indicator color change is in a similar trend with mushroom freshness, as freshness degradation could be visually detected, when the edible indicator color change from purple when it's fresh, to pale purple when it is still fresh, and finally to green when its spoilage. The edible color indicator change was caused by the increase in pH as a degradation process occurred in the mushrooms. Thus, the edible indicator label could be employed as an on-package smart label of the mushroom shelf-life over the “used by date” traditional label, where it can be used as a display of mushroom freshness status for consumers, while for the trader, it could also be employed in mushroom stock management and distribution so that it could reduce mushroom lost and waste.

Author contributions

BK designed and directed the project, collaborated in data analysis and edited the manuscript. MS carried out the work and wrote the manuscript; DKP supervised the work, corrected and edited the manuscript. All authors contributed to the final version of the manuscript.

Funding

This research was supported the LP2M, University of Jember, for supporting this work via the PRG grant to BK.

Data and materials availability

All data generated or analysed during this study are included in this published article.

Code availability

Not applicable.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

Not applicable.