Effect of high pressure carbon dioxide on the browning inhibition of sugar-preserved orange peel

Highlights

• HPCD pretreatment at 9 Mpa could effectively inhibit the browning of processed orange peel.

• PAL activity significantly decreased with the pressure of HPCD treatment.

• HPCD treatment effectively reduced Maillard reaction between reducing sugars and amino acids.

• HPCD application has practical implications for the inhibition of browning.

Abstract

In the present study, the effect of high pressure CO₂ (HPCD) pretreatment on the browning of reducing sugar-preserved orange peel was evaluated. Fresh orange peel cubes were subjected to different pressures (4, 7.4 and 9 MPa; 31 °C; 20 min). The changes in the browning-related components during the processing (pretreatment, sugar curing, and thermal drying) of preserved orange peel were studied. The results shown HPCD treatment could slow down the enzymatic browning and Maillard reaction in orange peel. Specifically, A significant decrease in PAL activity (p < 0.05) was found in all HPCD-treated samples. The total phenolic compound, free amino acid, and reducing sugar content in the HPCD-treated samples were higher than in the untreated ones. HPCD treatment inhibits the activity of PAL due to the acidic effect of CO₂, and reduces the dissolved oxygen inside the tissue during the decompression process, thereby slowing down the enzymatic browning in orange peel; while the interaction of CO₂ and basic amino acids protects the alkali amino acids, thereby slowing Maillard reaction during orange peel sugar curing stage. HPCD pretreatment at 9 MPa could effectively inhibit the browning of processed orange peel (p < 0.05). HPCD application under appropriate treatment conditions has practical implications for the inhibition of browning and provides theoretical support for other preserved fruits.

Introduction

Citrus peel is a valuable and considerable by-product of the citrus industry, which was widely utilized in pharmaceutical, foods, and cosmetic industries because of its abundant nutrients including phenolic compounds, carotenoids, and ascorbic acid [1]. Recently, there has been an increase in interest in promoting the additional value of citrus peel via the sugar-preserved process and stuffing for baked product [2]. Although sugar-preserved processing could improve the flavor and increase the storage period of citrus products, the browning reaction during the sugar-preserved processing could produce undesirable properties, such as reducing sensory quality and lowing the nutrition (i.e. the loss of amino acids).

Browning reaction, which can be divided into two aspects (non-enzymatic and/or enzymatic browning), is a main factor to limit fruit qualities [3]. On one hand, the non-enzymatic browning of citrus products occurred through Maillard reaction during the processing or storage step, which involves the condensation of reducing sugar with free amino acids to form N-substituted glycosilamine and the formation of Amadori and Heyn's products contributing to browning pigments [4]. Paravisini and Peterson found that the Maillard reaction was supported as a primary mechanism of orange juice browning, which affected brown color generation [3]. On the other hand, enzymatic browning is also vital to the citrus qualities. Enzymatic browning is due to the oxidation of phenols to quinones by oxidoreductive enzymes (PPO and POD) in the presence of oxygen [5,6]. In our previous study, we found high polyphenol oxidase activity during fresh orange peel leading to the occurrence of enzymatic browning. We also demonstrated that the cutting process damages fresh plant tissues, leading to direct contact between enzymes and substrates [7,8]. Furthermore, Murtaza et al. reported that polyphenol oxidase (PPO) and peroxidase (POD) remained residual activity or activation at high temperature (80 °C), which may lead to enzymatic browning [9]. Meanwhile Cantos's group demonstarted that phenylalanine ammonia-lyase (PAL) was also responsible for browning reactions with the generation of different phenolic substances, flavonoids, lignins and various secondary metabolites in phenylalanine pathway [10]. The browning reactions during the sugar-preserved process of preserved fruit include fresh cutting during the early stage, sugar curing, and thermal drying in the later stage, which could contribute to the loss of flavor and color. Therefore, the browning reaction is the bottleneck in the effective utilization of citrus peel.

To date, researchers have attempted to find new strategies to prevent and/or inhibit browning reaction, which can effectively improve product quality. For example, various anti-browning agents and heated treatment have been utilized to avoid the browning reactions in fruits and vegetables [[11], [12], [13]]. However, a high dosage of anti-browning agents could cause the residue of additives that could be harmful to health. Meanwhile, prolonged heating could lead to the loss of heat-sensitive compounds and decrease in commercial value. Recently, non-thermal processing technology, which involves relatively low processing temperature, is a promising new technology [14]. An emerging alternative to traditional processing to control browning is the utilization of high pressure CO₂ (HPCD) technique. Compared with anti-browning agents and heated treatment, HPCD has gained much attraction, because CO₂ is considered to be non-toxic, non-flammable, chemically inert, physiologically safe and readily available gas [15]. Many researches have demonstrated that high HPCD technique could control and prevent the browning reactions in many different fruits and vegetables [[16], [17], [18], [19]]. Meanwhile, this technology can also be applied in treating solid foods [15], including shrimps and other sea food [20,21]. To our knowledge, the HPCD treatment has little been studied in preventing the browning reaction of sugar-preserved fruits.

The causes of citrus products browning are multifaceted and complex, which are not fully demonstrated at present. In this study, we tried to reveal the browning mechanism of sugar-preserved orange peel and reduce browning by HPCD pretreatment. The effects of inhibition by HPCD were evaluated by the activity of PPO, POD, and PAL, the contents of total phenol, free amino acids (FAAs), reducing sugar, and 5-hydroxymethylfurfural (5-HMF) in orange peel during preserved orange peel fruit processing. This work could provide a novel insight into the browning inhibition of sugar-preserved fruit.