Preservation of Phenols, Antioxidant Activity, and Cyclic Adenosine Monophosphate in Jujube (Ziziphus jujuba Mill.) Fruits with Different Drying Methods

Abstract

Jujube, commonly known as the Chinese date, is a nutritious fruit with medicinal importance. Fresh jujube fruits have a shelf life of about ten days in ambient conditions that can be extended by drying. However, nutrition preservation varies with the drying method and parameters selected. We studied total phenolic content (TPC), proanthocyanidins (PA), vitamin C, cyclic adenosine monophosphate (cAMP), and antioxidant activities in jujube fruits dried with freeze-drying (FD), convective oven drying (OD) at 50 °C, 60 °C, and 75 °C, and sun drying (SD) with FD as a control. The cultivars used for this study were 'Capri' and 'Xiang' from Las Cruces in 2019, and 'Sugarcane', 'Lang', and 'Sherwood' from Las Cruces and Los Lunas, New Mexico, in 2020. Freeze-drying had the highest of all nutrient components tested, the best estimates of mature jujube fruits' nutrient contents. Compared with FD, the majority of PA (96-99%) and vitamin C (90-93%) was lost during SD or OD processes. The retention rates of antioxidant activities: DPPH and FRAP were higher in OD at 50/60 °C than SD. SD retained a higher cAMP level than OD at 50/60 °C in both years. The increase in oven drying temperature from 60 °C to 75 °C significantly decreased TPC, PA, antioxidant activities, and cAMP.

Keywords: antioxidant activity; bioactive compounds; convective oven drying; cyclic adenosine monophosphate; freeze drying; proanthocyanidins; sun drying; total phenolic content; vitamin C.

Figure 1

Dried jujube fruits’ powder of cultivar Xiang from left to right: freeze drying, convective oven at 50 °C, 60 °C, 75 °C, and sun drying.

Figure 2

(a) Antioxidant activity with 2,2-diphenylpicrylhydrazyl (DPPH), (b) Ferric reducing antioxidant potential (FRAP), and (c) cyclic adenosine monophosphate (cAMP) in ‘Capri’ and ‘Xiang’ from Las Cruces New Mexico, U.S. in 2019 with different drying methods: FD-freeze drying, OD 60 °C -convective oven drying at 60 °C, OD 75 °C -convective oven drying at 75 °C, SD-sun drying. Bars that share the same letter in each drying method do not differ (p ≥ 0.05). Antioxidant activity with 2,2-diphenylpicrylhydrazyl (DPPH), ferric reducing antioxidant potential (FRAP), and cyclic adenosine monophosphate (cAMP) are expressed as mg Trolox/g, milligrams of ascorbic acid equivalent per gram, and microgram per gram, respectively, on a dry-weight basis.