Heated apple juice supplemented with onion has greatly improved nutritional quality and browning index

Highlights

• Heating reduces browning in apple juice independent of onion addition.

• Onion addition greatly improves nutritional quality and inhibits browning in apple juice.

• Maximal nutritional quality and anti-browning effect can be achieved in heated apple juice supplemented with onion.

Abstract

Although fruit juices are very popular, enzymatic browning occurs easily. Browning of fruit juice deteriorates nutrition value and product quality due to oxidation of polyphenol compounds. Therefore, development of natural food additives that reduce browning will be beneficial for improving quality of fruit juices. Onion has been reported to be a potent natural anti-browning agent. Here, we compared unheated and heated apple juices pre-supplemented with onion with respect to browning and nutritional quality. The unheated apple juice supplemented with onion showed reduced browning as well as increased total soluble solid, total phenol concentration, radical scavenging activities, and ferric reducing and copper chelating activities without any change in flavonoid concentration. On the other hand, heated juice supplemented with onion not only showed improved values for these parameters but also markedly increased flavonoid concentration. Thus, we conclude that application of heating and onion addition together may greatly improve quality of apple juice.

Introduction

Enzymatic browning can be initiated by enzymatic oxidation of phenols to colored quinines, which are then subjected to further reactions, leading to pigment formation (Nicolas, Richard-Forget, Goupy, Amiot, & Aubert, 1994). Browning is a major problem for various fruit products due to lowering of quality, safety, and nutritional value. Typically, apple juices are expected to show a yellowish color, which represents freshness. However, apple juices are highly susceptible to enzymatic browning since polyphenoloxidase in apple is highly active (Nicolas et al., 1994). Therefore, proper control of the browning reaction in apple juice has gained attention in the food industry.

Various studies have focused on identifying food additives from natural products that improve nutritional value and inhibit enzymatic browning (Espley et al., 2014, Kim et al., 2005, Sudha et al., 2011). Of these natural products, onion is well known for its antioxidant and health-promoting effects since it contains various functional compounds such as anthocyanins, kaempferol, quercetin, isorhamnetin, and alkyl cysteine sulphoxides (Griffiths, Trueman, Crowther, Thomas, & Smith, 2002). Furthermore, onion extracts were shown to efficiently inhibit polyphonoloxidase and thereby inhibit enzymatic browning of potato (Lee et al., 2002). In another study, onion extracts suppressed enzymatic browning of pear (Kim et al., 2005). Although onion addition suppresses enzymatic browning, the strong flavor and odor of uncooked onion may prevent its use as a food additive, especially for drinks. An efficient way to reduce the flavor and odor of onion is heat treatment. Furthermore, it has been reported that heating highly increases the functionality of onion, including antioxidant activity, metal chelating capacity, and polyphenol concentration (Kim et al., 2005, Woo et al., 2007).

Here, we examined the effects of onion addition on the browning, nutritional value, and especially antioxidant properties of apple juice. We specifically compared unheated and heated apple juices pre-supplemented with onion with respect to browning and nutritional quality.