Acid Inhibition on Polyphenol Oxidase and Peroxidase in Processing of Anthocyanin-Rich Juice and Co-product Recovery from Purple-Fleshed Sweetpotatoes

Abstract

With high phytochemical and starch contents, purple-fleshed sweetpotatoes (PFSP) have been processed into various functional ingredients and food products including juices and natural colorants. For juice processing, PFSP are usually subjected to heat treatment for inactivation of pigment-degrading enzymes. However, heating of sweetpotatoes gelatinizes starch and produces thick slurry with cooked flavor, which are the drawbacks. Development of alternative processes to overcome the stated problems will be beneficial to sweetpotato processors. This study demonstrated that acidified water (≥3% w/v citric acid) was effective in inhibiting polyphenol oxidase and peroxidase in raw PFSP resulting in an attractive reddish juice. About 93% total phenolics (TP) and 83% total monomeric anthocyanins (TMA) in PFSP were extracted by two repeated extractions. The combined PFSP juice (3.2 L/kg PFSP) had high levels of TP (1,850 mg/L) and TMA (475 mg/L). With the developed process, 167 g dried starch, and 140 g dried high-fiber pomace were obtained for each kg raw PFSP, besides the highly pigmented juice. Pasteurization of the PFSP juice samples (pH 3.2) at 80 °C for 12 s resulted in 15% loss in TMA and had no effect on TP. The results indicated an efficient process to produce sweetpotato juice with high bioactive compounds and recovery of starch and high dietary fiber pomace as co-products. PRACTICAL APPLICATION: Purple-fleshed sweetpotatoes (PFSP) are rich in polyphenolics and antioxidant activities. In PFSP juice extraction, heat treatment to inactivate the pigment-degrading enzymes results in starch gelatinization and cooked flavor. A nonthermal process using acidified water was developed for producing anthocyanin-rich juice from PFSP and concurrently recovering native starch and dried pomace, which would increase the economic feasibility of the developed process. The results demonstrate an efficient process for the sweetpotato industry in producing PFSP pigmented juice and co-products for various food applications.

Keywords: Ipomoea batatas; dietary fiber; polyphenolics; starch; sweetpotato juice processing.