Film properties produced using grapefruit peel and determination of shelf life and quality of film-coated cakes

In this study, grapefruit peels were dried and ground into powder, and pectin extraction was performed using citric acid and sulfuric acid based on a hot acid method. The obtained pectin samples were evaluated for yield, moisture, ash, equivalent weight (EW), degree of esterification (DE), methoxyl content (MC), and anhydrogalacturonic acid (AGA) content, and compared with commercial pectin. The analysis results were supported by FTIR spectra. Pectins extracted with citric acid and sulfuric acid from grapefruit peels, as well as commercial pectin, were incorporated into a film matrix along with 2% grapefruit peel extract. The films were examined for moisture, water solubility, color, antioxidant capacity, water vapor permeability, mechanical properties, opacity and transparency characteristics, surface and cross-sectional morphology, and FTIR spectra. To evaluate the effects of the films on a food matrix, cake samples were prepared and coated with each of the three film types. A shelf-life test was conducted for 7 days. The moisture, ash, fat, and protein content of the cake samples were determined. During the shelf-life test, color, textural properties, water activity, and microbiological analyses were performed. FTIR spectra confirmed that both extraction methods yielded pectins with a typical pectin backbone, verifying the success of the extraction. The type of acid had no effect on pectin yield. In pectin analysis, pectin extracted with sulfuric acid stood out with a degree of esterification (DE) of 87.22 ± 1.9%, methoxyl content (MC) of 14.37 ± 0.29%, and anhydrogalacturonic acid (AGA) content of 94.55 ± 0.16%. In terms of film performance, films made with commercial pectin (CPF) showed superior results in color, opacity and transparency, water vapor permeability, and morphological characteristics. However, the highest antioxidant capacity was observed in films containing pectin extracted with citric acid (CGPF), while films containing pectin extracted with sulfuric acid (SGPF) had the lowest antioxidant capacity. Regarding mechanical properties, SGPF performed best in bursting and tensile tests. In the shelf-life test, the best color retention, textural stability, and water activity values were observed in cakes coated with CGPF. No microbial growth was detected in any of the three film groups. However, by the 7th day, water activity in all groups dropped below 0.60, and significant textural deterioration was observed, leading to the termination of the test.