- The objectives of this study were to evaluate the physicochemical properties and bioactive compounds from four different types of fruit pomace, including apple, blueberry, raspberry, and cranberry, and further to investigate the feasibility of creating apple pomace based biocomposite boards using thermal compression molding and to optimize the biocomposite formulations possessing high mechanical strength, low water sensitivity, and slow biodegradation.
Fruit pomace were collected per batch at the end of juice processing, frozen at -18 °C, and further thawed for 30 min at room temperature for analysis. Apple pomace had pectin as high as 18.87% for acid extractable pectin and 8.66% for total soluble pectin. Blueberry pomace was significantly higher in anthocyanins among other pomace (256.18 mg cyanidin-3-glucoside/100 g dry weight). Raspberry pomace contained insoluble dietary fibers (73.30%) that was predominated by Klason lignin. The total phenolic content and radical scavenging activity in cranberry pomace were significantly higher than other pomace, being 6.77 mg gallic acid equivalent/g dry weight and 2.68 mg ascorbic acid equivalent/g dry weight, respectively. Cranberry pomace also had high amount of acid extractable pectin (10.58%) following apple pomace. In addition, blueberry and cranberry pomace were also high in insoluble dietary fibers (63.85% and 67.20%, respectively).
Since apple pomace is rich in dietary fibers and the amount generated from apple juice industry is huge, apple pomace was chosen to create biocomposite boards through thermal compression molding at 30 MPa and 160 °C for 20 min. The biocomposite formulations were optimized through L₉ orthogonal array of Taguchi design aimed for high mechanical strength, low water sensitivity, and slow biodegradation. In the pomace biocomposite formulations, soy protein isolate (SPI) and poly(methyl methacrylate) (PMMA) were added as binders, stearic acid as hydrophobic agent, span 80 as surfactant, and 5% methylene diphenyl diisocyanate (MDI) as crosslinking agent. The ratio of SPI and PMMA and the concentration of span 80 did not affect any of the tested parameters significantly (P > 0.05). On the other hand, stearic acid significantly influenced breaking strength (P < 0.05), while pomace-to-binder (P/B) ratio gave significant effect on modulus of elasticity, water solubility, moisture content, and biodegradability (P < 0.05). The optimum formula to create biocomposite board from apple pomace was SPI:PMMA = 2:1 at P/B ratio of 7:3 with 5% MDI and without the addition of stearic acid and span 80.
The results from this study provided new information on the utilizations of different types of fruit pomace. Dietary fibers in apple and cranberry pomace have potential to create packaging materials, whereas bioactive compounds in blueberry and raspberry pomace are good candidates to fortify different types of food products. Moreover, the developed biocomposite boards from apple pomace can be applied to make ecofriendly packaging materials.