The study was conducted to develop a simple and cheap pH-sensitive color changing quality indicator by using crude extracts of purple leaf sandcherry (Prenus cistena) for chicken meat stored at 10±1 ºC temperature. Ethanol-acidic extracts of leaf containing Anthocyanins, changes its color due to structural change induced by alkaline pH. Developed indicator when attached inside the chicken meat packets during storage at 10±1° C, changed its color from pink-red to bluish-violet due to change of its pH. The efficiency of developed acid-base indicator as quality monitoring tool was judged by correlating its color change with changes in meat quality attributes viz., Total volatile basic nitrogen (TVBN) and pH during storage period of 8 days. Total volatile basic nitrogen (TVBN) content of meat increased significantly (P<0.05) where as the pH increased (P<0.05) from 5.4 to 6.2 at the end of the storage. It was found that, the change in indicator color was highly correlating with the deteriorative changes in quality attributes of meat. The developed quality indicator was expected to provide a convenient mean to monitor quality of meat during storage at 10±1° C.
In the present world, supply of quality food to the consumer in its safest form is the biggest challenge to the food processors. Quality food packaging plays a vital role to protect the product against the effects of the external environment, communicate with the consumer as a marketing tool, provide the consumer with greater ease of use and time-saving convenience, and contain products of various sizes and shapes (Yam et al., 2005). Therefore the demand for strategies to increase the food products shelf-lives and real time quality monitoring is increased by consumers and manufacturers. Maintenance and monitoring the food quality during transport and storage by application of intelligent packaging system may assure the packaged food quality and safety. Intelligent food quality indicator sensors are increasingly used cost efficiently, quickly and non-destructively to determine the physiological status and quality of perishable products as well as to evaluate technical procedures on the basis of physical measurements. Meat and its products are highly subject to spoilage and contamination due to bacterial effects (Gram et al., 2002). Intelligent packaging applications have been used for monitoring spoilage of meat and to predict its remaining shelf life. Intelligent packaging based quality indicators in meat packaging can effectively monitor the volatile organic compounds and gas molecules produced during meat spoilage (Vanderroost et al., 2014).
Color based pH indicators offer a potential use as indicators of microbial metabolites (Kerry et al., 2006), as a microbiological growth could induce a pH change (Smolander, 2003). Mills (2005) suggested that an ideal indicator for intelligent packaging should be inexpensive or should not require an expensive piece of analytical instrumentation. The employment of natural pigments in colorimetric sensors is advantageous, because such sensors do not have chemical effect on the packaged meat. Both petrochemical and bio-based materials can be used to develop quality indicator although the bio-based material is generally more compatible for food-contact applications since it has eco-friendly attributes and higher consumer acceptance (Botrel et al.,2007). Some natural pigments from fruits and vegetable sources, anthocyanins for example, have great potential as indicators in intelligent packaging systems. Color expression of anthocyanins is strongly influenced by its structure and pH. This color instability of anthocyanins makes these pigments especially useful to monitor food quality and therefore can be used as an indicator of food spoilage in intelligent packaging systems.
On the basis of the color changing principle of anthocyanins in crude extracts, the present study was conducted to evaluate the potentiality of purple leaf sandcherry extracts as quality indicator for quality monitoring of chicken meat during storage at 10±1° C.
Materials and Methods
Purple leaf sandcherry was collected from the gardens of IVRI, Izatnagar campus and adjoining area. Crude extracts of leaf was made by the method standardized by Metivier et al. (1980) with suitable modification. At the rate of, 15 grams of fruits were finely minced in pastel and mortar by adding 10 ml of 1% HCl-ethanol. The mixture slurry was transferred in a centrifuge tube and kept overnight in refrigerator wrapping in aluminium foil for proper extraction. Next day, it was centrifuged for 15 min at 5000 rpm. Supernatant crude solution thus obtained was transferred to an amber color bottle and kept in refrigerator for further use. Filter paper (Whatman® No. 42) strip of 2 cm x 2 cm square shape was prepared as base material for the sensor. The filter paper strips were dipped into the crude extracts and put for 15 sec. The developed indicators were conditioned over night at refrigeration temperature (4±1ºC) in covered petridish. The changes in color of indicator was determined by exposing conditioned quality indicators to the chicken meat (100 g) by sticking inside the lid of sterile petri dishes (90 mm diameter) and closing them tightly with parafilm® and stored for the period of 8 days at 10±1 ºC. One control indicator as reference color was also attached outside the petridish for comparison purpose. The changes in the color of the sensors were recorded by digital camera (SX160 IS, Canon, India).
The pH of stored meat was estimated by following the method of Trout et al. (1992) whereas TVBN concentration was determined by micro-diffusion technique according of Pearson (1968). The experiment was replicated three times and the data generated were analyzed by statistical methods of one way ANOVA, Mean± S.E and paired t-test using SPSS software package developed as per the procedure of Snedecor and Cochran (1995) and means were compared by using Dunkan’s multiple range test (Dunkan, 1955).
Results and Discussion
Changes in the Color of Indicator
Inside the packet of chicken meat volatile basic gases produced during its storage, which might have caused the color change of indicator from pink-red to bluish-violet at the end of the storage period (Fig. 1) as compared to the control indicator which were attached outside the packet remained with unchanged red color.
The bright red color of indicator [1. 0 day storage, 2. 1st day storage, 3. 2nd day storage, 4. 3rd day storage, 5. 4th day storage, 6. 5th day storage, 7. 6th day storage, 8. 7th day storage, 9. 8th day storage]
Fig. 1: Changes in color of purple leaf sand cherry based indicator in chicken meat packets during storage at 10±1º C
The pink-red color of indicator changed to bluish-red on the 4th day of storage. With the progress of storage periods the intensity of indicator color increased. On 5th day onwards the prominent bluish tint started to appear in the indicator. On 6th day of storage the original color of indicator totally changed to bluish-violet, which intensifies on the progression of storage periods up to 8thday of storage. Similar findings were also quoted by different researchers. Anthocyanins can be found in different chemical forms which depend on the pH of the solution (Castaneda-Ovando et al., 2009). At pH-3 or lower, the flavylium cation is the predominant species and contributes to purple, orange and red colors. As the pH increase, kinetic and thermodynamic competition occurs between the hydration reaction of the flavylium cation and the proton transfer reactions related to its acidic hydroxyl groups. While the first reaction gives colorless carbinol pseudo-bases (pH-5) which can undergo ring opening to yellow retro-chalcones (pH-6), the latter reactions give rise to more violet quinonoidal bases (pH-4). Further deprotonation of the quinonoidal bases can take place at pH between 6 and 7 with the formation of more bluish resonance-stabilised quinonoid anions. At pH values higher than 7, the anthocyanins are degraded depending on their substituent groups. It is well known that anthocyanins properties, including color expression, are highly influenced by anthocyanin structure and pH. According to Torskangerpoll and Andersen (2005), the parameters employed for describing color variation of anthocyanins solutions have mainly been shifts of the visible λ-max as a measure for hue variations and absorptivity changes for variations of color intensity.
Correlation of Color Changes in Indicator with pH Change in Stored Chicken Meat
The change in pH chicken meat was determined every day during 8 days storage to correlate with the color change in sensor (Fig. 1).
Fig. 2: Changes in pH value of stored chicken meat during storage at (10±1º C)
On the 0 day the pH of fresh chicken was 5.4 which decreased to 5.3 on second day of storage thereafter an increasing trend of pH has been observed which reached finally to 6.2 on the 8th day of storage (Fig. 2). A similar observation for chicken meat has also been reported by Debut et al.(2003), Duclos et al. (2007) and Baston et al. (2008), during its refrigerated storage conditions. At the beginning of the storage the pH of the chicken meat decreased, and then it increased until alkaline at the end of time storage. The decrease in the pH was might be due to the amount of lactic acid produced in meat, thereafter the increase of pH might be attributed to the accumulation of alkaline compounds such as ammonia and amino sugar complex and lipid (Jay and Shelef, 1978) due to microbial and enzymatic changes in meat.
Correlation of Color Change in With Changes in TVBN Value of Stored Chicken Meat
Plant extracts containing crude anthocyanins; a natural dye has obtained from purple leaf sandcherry. Anthocyanins were described as indicator for volatile bases generated on decomposition of meat (William et al., 2006). Anthocyanins bound to a suitable carrier can be utilized in intelligent packaging as volatile basic compounds due to its sensitivity to volatile bases and change their color on coming in contact with volatile bases and when trapped into a suitable polymer matrix can be introduced inside package and monitored. The pH based structural change in anthocyanins resulted in to visible change in color. The pattern of color change in quality indicator attached inside the packaged fresh chicken was correlated with the changes in total volatile basic nitrogen (TVBN) concentration of chicken meat during storage. At the first day of storage the TVBN found to be 8.4 mg/100g meat, which increased to 27.53 mg/100g meat on the 8th day of storage (Fig. 3). It was observed that the color changing pattern of quality indicator from red to violet, correlated significantly (P<0.05) with the increasing levels of TVBN concentration.
Fig. 3: Changes in TVBN concentration (mg/100g) of stored chicken meat during storage at 10±1º C
A remarkable visual color difference observed in sensor between 6th and 7th day of storage, which is highly correlating with the significantly (P<0.05) higher concentration of TVBN, on 7thday (27.1 mg/100g) as compared to 6th day (20.53 mg/100g). The increase in TVBN content might be due to break down of protein and deamination of amino acids leading to production of ammonia and other volatile bases. Similarly increasing level of TVBN content of beef was observed by Byun et al. (2003) during its refrigerated storage.
Anthocyanins from purple leaf sandcherry extracts, used for development of pH-sensitive quality indicator by using biodegradable and environment friendly filter paper as base material for application in meat packaging. The response of developed quality indicator found reliable and it was confirmed by the significant correlation with deteriorative changes in quality attributes of stored chicken meat. The color change in indicator represents a simple and visual method to detect quality degradation of flesh foods. Therefore, it is concluded the developed quality indicator has a positive potential as a diagnostic measure to assess flesh food safety and quality.