Edible biofilm based on pomegranate peel extract extends the shelf life of strawberries
An edible biofilm, obtained from agricultural and fishing waste and developed by researchers at the São Carlos Institute of Chemistry of the University of São Paulo (IQSC-USP) in Brazil, allows the shelf life of strawberries (Fragaria x ananassa Duch.) to be extended.
In laboratory tests, the researchers found that over 12 days of refrigerated storage, the fruit coated with the film lost 11% weight and took between 6 and 8 days to start becoming contaminated with fungi, compared to 4 days for fruit not covered with the material.
The results of the work, carried out in collaboration with researchers from EMBRAPA Instrumentation and the Federal University of São Carlos (UFSCar), are published in the journal Food Chemistry.
“By applying the coating, it was possible to double the shelf life of strawberries kept under refrigeration and delay the dehydration of the fruit, while preserving the taste, texture and volatile compounds that give the fruit its characteristic aroma,” Mirella Romanelli Vicente Bertolo, first author of the study and a postdoctoral researcher at EMBRAPA Instrumentation, says.
The work began during Bertolo’s doctoral studies at IQSC-USP under the supervision of Professor Stanislau Bogusz Junior. During their research, they developed a technique that allowed them to extract 84.2% more antioxidants—substances with preservative properties—from the peel of pomegranate (Punica granatum L.) using natural deep eutectic solvents (NADES).
“More than 40% of the pomegranate, depending on the variety, is made up of peel, which is wasted. Our idea was to use this waste to obtain extracts rich in phenolic compounds with antioxidant and antimicrobial activities,” says Bogusz.
With the success of developing the extraction method, the researchers decided to test the hypothesis of incorporating the antioxidants in pomegranate into coatings based on gelatin and chitosan—a polymer (natural polysaccharide) found in the skeletons of crustaceans such as shrimp—to develop a protective film for fruit.
“We chose to use chitosan extracted from squid glia [inner shells] through a process of deacetylation of the chitin found in this mollusk, because it doesn’t have the problem of allergenicity like that obtained from shrimp. And we combined this material with another polymer, in this case gelatin, to improve its mechanical properties,” explains Bogusz.
Highly perishable fruit
The strawberry was chosen as a model system to test the effectiveness of the biofilm because it is one of the items with the highest loss rates in Brazilian supermarkets due to its perishability and short shelf life of about less than seven days under refrigeration.
“The strawberry is a fruit that has very high respiratory activity and a very low pH [acidity]. It’s therefore very susceptible to microbial attack. It’s also very moist and the fruit is small. Based on this, we hypothesized that if the material we developed worked, it’d be effective on any other fruit,” says Bogusz.
To test this hypothesis, the researchers coated strawberries with the edible film by immersion and evaluated the effects of the material on the physicochemical, microbiological and volatile profile and sensory characteristics of the fruit over 12 days of refrigerated storage.
The results indicate that the material forms a film on the surface of the fruit that acts as a barrier to the passage of microorganisms, moisture loss and gas exchange, modifying the strawberry’s respiration. In this way, the coating slows down the metabolism of the fruit during the post-harvest period, thereby increasing its shelf life and preserving the color, firmness and bioactive compounds of the fruit.
“We found that the film made it possible to maintain the texture, delay contamination by microorganisms and reduce the loss of fruit mass, which is observed when the strawberry shrivels. This happens very often with uncoated fruit because it easily loses water and dehydrates,” says Bertolo.
According to the researcher, the film also made it possible to reduce the severity of fungal damage and improve the volatile profile of the fruit. “The material made it possible to preserve 40% more of the compounds responsible for the fruit’s aroma,” says Bertolo.
The biofilm also didn’t interfere with the sensory characteristics of the fruit, such as flavor, as confirmed by sensory analysis tests conducted with undergraduate chemistry students at IQSC-USP.
“The results of the tests showed that there were no differences in the taste, aroma or visual characteristics of strawberries coated with the material compared to strawberries without the film,” says Bertolo.
The researchers have filed a patent application for the formulation and intend to license the technology to interested companies.
Economic analyses indicate that the coating could cost an estimated BRL 0.15 per fruit.
“This is a cost that consumers may be willing to pay for fruit with a longer shelf life and greater usage,” Bertolo estimates.