In 2015, about 282,000 tons of pomegranate fruit were grown in California, with an economic value of $115.4 million . Pomegranate consists of three major parts , namely pomegranate peel , pomegranate seed , and pomegranate aril that is the flesh part for pomegranate juice production. Teh studied the different fruit part distribution of 5 pomegranate cultivars grown in California, observed a range of 38.33~50.38% PJ, 38.77~53.01% PP, and 7.71~12.10% PS. The study also determined the components of PP and PS through proximate analysis. The peel portion consisted of 90.6~91.9% carbohydrate as the principal constituent, followed by 3.1~3.9% protein, 1.3~2.3% fat, and 3.3~4.3% ash or minerals. The PS contained higher proportions of protein and fat and included 60.5~71.8% carbohydrate and 1.6~2.5% minerals. PJ consisted of 85% water, 10% total sugars, organic acids,amino acids, and phenolics. Pomegranate exhibits a sweet, sweet-sour, or sour taste, which color ranges from white to pink and red . Traditional processes in the pomegranate juice industry squeezed the whole pomegranate, which had low yield, impurity, and bitter taste due to non-edible parts. Nowadays, novel techniques introduced a deseeding step at the beginning. Then the seed, aril, and juice are separated from the peel and squeezed, while the peel and the remaining pulp are discarded as wastes in landfills or used as animal feed. The juice stream continues to go through the processes of pasteurization, centrifugation, membrane process, storage,25 liter square pot and quality assessment. Other juice extraction methods and their features were also discussed by as shown in Figure 1.2. PJ processing generates two types of solid by-products: peel and seeds. PP is non-edible and comprised mainly of bio-active compounds, such as hydrolyzable tannins at concentrations ranging from 27 to 172 g kg-1 , flavonoids , and complex polysaccharides .
Therefore, PP is an excellent source of phenolic compounds, tannins, flavonoids, sterols, fatty acids, dietary fiber, minerals, and vitamins. Fruit and vegetable pomace has a long history of simply being disposed of in landfills or underutilized as fertilizers and soil conditioners. Modaresi et al., added 12% pomegranate seed pulp into the goat diets and observed increased polyunsaturated and conjugated fatty acids in goat milk. A feed with 1-2% PP lowered cholesterol levels and improved oxidative stability in the harvested boiler chicken meats . Shabtay et al. supplemented PP in calves’ diets and observed a significant increase in weight gain and blood antioxidant contents in the ruminants. Therefore, pomegranate pomace demonstrated great potentials for nutritional feed with improved health benefits in ruminant and chicken feed. Due to the massive quantity of pomace, the convenience of disposal, and low realizable revenue from current waste utilization practices, processors and farmers have a low incentive to apply alternative waste management methods. With the pressure of climate change, energy shortage, and increasing nutritional needs, creating value-added products from the by-products would be an outstanding solution to incentivize novel waste practices. With an enormous amount and variety of polyphenols within the pomace, pomegranate by-products demonstrated great antioxidant and anti-microbial properties, which contributed to diverse application potentials as food additives and packaging materials. Lipid oxidation is the principal deteriorative reaction during food processing and storage. It sharply reduces the product shelf life, destroys essential nutritional components, and generates toxic compounds which pose hazards to human health .
Synthetic antioxidants have been dominantly applied in the food industry to prevent oxidation, such as butylated hydroxytoluene and butylated hydroxyanisole . Natural antioxidants are gaining attention as consumers prefer safe and natural ingredients . Topuz et al. incorporated alcoholic extract of PP into anchovy fish oil and observed a dose-dependent inhibitory effect on lipid oxidation, especially at a concentration of 500–1000 ppm. The antioxidant capacity of 500 ppm of PP extract was comparable with that of 100 ppm of BHT, indicating that PPE could be applied as a potent antioxidant. Turgut et al., infused PPE into freshly minced beef at 5000/10000 ppm and compared it with 100 ppm BHT. Their results demonstrated a lower thiobarbituric acid reactive substances value, peroxide formation, and other parameters, suggesting promising oxidation retarding effect of lipid and protein in pomegranate extract. However, they also observed a potential negative change of sensory value after extract addition, which required further research to quantitatively investigate the effects. Similar research also proved the preservative effects of pomegranate extract in burgers and cheese . A more comprehensive review on the practical use of PP in meat products was reported by Smaoui et al., . PP may also prevent food borne illness, which has been a worldwide safety concern. In the USA, there were millions of cases related to food contamination and food borne outbreak each year, posing a severe threat to public health . Traditional synthetic antimicrobial agents may have potential side effects, are expensive, and could induce drug resistance of microorganisms as their indiscriminate killing effects . Natural anti-microbial agents are needed. Pomegranate is rich in polyphenols, which can inhibit bacterial growth by interacting with the sulfhydryl groups of bacterial cell wall proteins and forming complexes, and then induce lysis . More applications of pomegranate by-products as anti-microbial agents could be found in Singh et al. .
Biotechnological products Biofuel is considered an ideal alternative to fossil fuel, as fossil fuels are experiencing a rapid depletion, uncertainty in the price, and contributing to significant environmental pollution . Bio-fuel is produced from biomass via thermochemical processes, including gasification, carbonization, pyrolysis, and direct combustion. Among all the methods, pyrolysis is considered the most viable due to its simplicity, cost-effectiveness, and wide range of final products . Siddiqui et al. studied the optimized process parameters for biochar production from PP. Results showed that, at a temperature of 300 °C, the pyrolysis reaction time of 20 min and the particle size of 3 mm, biochar could be produced at a yield of 54.9% with an improved higher heating value at 23.5 from 14.61 MJ/kg of parent biomass. Besides energy supply, PP-based biochar also demonstrated a desirable ability in eliminating inorganic compounds from water and CO2 capture/storage . As for bio-fuel, Demiray et al. optimized bioethanol production from PP by Saccharomyces cerevisiae and Pichia stipites. They successfully increased the ethanol yield produced by S. cerevisiae up to 44.9%. Ethanol productivity and Ethanol yield of S. cerevisiae increased to 0.46 g/L/h and 0.43 g/g, respectively. These findings along with other similar research have demonstrated that PP is a promising bio-fuel source. Polyphenols, a family of molecules, are commonly found in fruits, vegetables, nuts, seeds, flowers, and tree bark. These components fundamentally are plant metabolites to attract pollinators, but research concluded multiple pharmaceutical effects, including antioxidant, antimicrobial, anti-cancer, etc . This family derives from a fundamental polyphenol group . The structure ranges from simple elementary substances to complex polymerized molecules , contributing to a diversified classification profile . Furthermore, sugar residues can conjugate with the hydroxyl groups of natural polyphenols through direct linkage of the sugar unit to an aromatic carbon,gallon pot further diversifying the polyphenol structure . Based on the number of phenol rings and the binding components, polyphenols can be classified into the following 5 types of sub-groups: hydroxybenzoic acid, hydroxycinnamic acid, flavonoids, stilbenes, and lignans , as illustrated in Figure 1.3. The synthesis pathway of PU is shown in Figure 1.4. Phenolic acids including two gallic acids and ellagic acid are combined to form gallagic acid. With glucose addition, gallagic acid can form punicalin and be further transformed into PU by adding an EA. The difference in the glucose carbon-1 induces two isomers of PU . The complexity synthesis of PU results in a large molecular weight and rigid structure . These features provided higher functionality with a large number of hydroxyl groups and a lower chance of degradation. A typical phenolic composition distribution in HPLC analysis is shown in Figure 1.5 . Compared to CUAE, PUAE has an intermediate working process known as the duty cycle. Therefore, it can reduce energy use, depreciation of the equipment, and erosion of the tip. Kazemi, Karim, Mirhosseini, & Hamid optimized the PUAE process for higher TPC in pomegranate of Malas variety. At 105 W/cm2 intensity level with 50% duty cycle for 10 min extraction, highest PU content was achieved within 20.66 mg/g EA and 0.053 mg/g GA, accounting for 87.6%. Extraction time and duty cycle were the most and least significant ultrasound variables for PU retention, respectively. PUAE required lower energy use and degradation at the same time as CUAE, indicating a lower degree of oxidation induced by the ultrasound and contacting with open air. On the other side, the conditions of PUAE were more material-dependent and required more time to optimize the conditions. Sumere et al. developed an EXTRACT-US system using ultrasound-assisted pressurized liquids extraction , which consisted of a 10 mL stainless steel extraction cell coupled to the ultrasonic horn .
The extraction process was evaluated based on 5 parameters: consecutive cycles, particle sizes, temperature, solvent, and power of ultrasounds. The highest EY was achieved using water at 70 ⁰C, with 43.27 ± 0.76 mg/g TPC, including 11.48 ± 0.21 mg/g α-PU and 24.55 ± 0.61 mg/g β-PU. Under pressurized liquids, UAE did not influence the extraction of TPC for small particles , but 480 and 640 W powers improved the yields at large particle sizes . This research demonstrated a promising effect on improving UAPLE on an industrial scale. At the same time, several challenging factors need to be considered in the future, including high pressure and high capital investment. Santos et al. further developed the UAPLE system using expansion gas initial pressure , a different combination of ultrasound power , system pressure . The optimal extraction was achieved with 100 bar system pressure, 5 bar N2-Pi and 400 W, with 14.87 ± 0.36 mg/g α-PU, 37.13 ± 1.44 mg/g β-PU, and 1.11 ± 0.05 EA. N2-Pi influenced the process by facilitating the bubble formation and size expansion at the initial stage. However, 5 bar and 10 bar N2-Pi didn’t make significant differences in phenolic yield and composition. It is worth noting that the conditions required a complex setup but less TPC compared to other techniques. Moreover, the increase in system pressure hampered the cavitation because the bubbles were required to reach a higher critical pressure for breakage. MAE is another common extraction method with reducing extraction time and solvent consumption. The heat and mass transfer processes take place at the same time and accelerate the overall extraction. Kaderides, Papaoikonomou, Serafim, & Goula compared the extraction performance of microwave-assisted extraction and the UAE. Results showed that the optimal MAE condition was using 60mL 50% aqueous ethanol per gram at 600 W power for 4 mins, with an extract yield of 199.4 mg/g GAE, PU yield of 143.64 mg/g, and radical scavenging activity of 94.91%. In contrast to 10 mins of UAE at 52 W power using 32.2mL water per gram peel at 35⁰C, MAE achieved 1.7 times higher extract yield in a shorter processing time , which was due to higher cell destruction but similar PU yield and scavenging activity. Limited research has focused on this topic so far. Researchers suggested a focus on the MAE-assisted process to achieve a high yield of polyphenols as well as extract with great polyphenol profile and quality. PU is sensitive to temperature and pH. Therefore, conventional hot acidic processing methods might increase the hydrolysis of PU and reduce its bio-activity. Alexandre et al. proposed that cellulase and pectinase degraded the cell wall and released the intact PU along with pectin. They compared extraction under 300/600 MPa high pressure with that using 4% cellulase and pectinase. As result, enzymatic extraction yielded 1481.29 μg/g TPC with 62.9% PU, significantly higher than extraction at 300MPa and slightly lower than 600 MPa . Talekar, Patti, Vijayraghavan, & Arora, applied 55 U/g cellulases at a solvent ratio of 15 mL/g for 4h with 20 min ultrasound treatment. As the results, 84.8 mg/g PU was recovered, accounting for 71.2% of its TPC and was superior to the ones recovered by a conventional method . This result demonstrated reduced hydrolysis of PU under mild enzymatic extraction. Since enzymes could be costly due to low recovery and reusability, biocatalyst became a more sustainable option. Biocatalyst immobilizes enzymes onto solid phase for selective enzyme recovery after the reaction.