Profiling of Phenolic Compounds in Citrus Flowers and Their Biological Activities ^†

Abstract

Citrus, a genus of flowering plants in the Rutaceae family, holds substantial market importance as a crop worldwide. After prolonged periods of breeding and extensive hybridization, numerous species have emerged, each possessing a unique metabolism that produces a diverse array of secondary metabolites. It is important to note that phenolic compounds, in particular phenols and flavonoids, are among the most important secondary metabolites in Citrus flowers. These chemical compositions of Citrus flowers differ depending on various factors, such as variety, fruit maturity, environmental conditions, storage conditions, and extraction methods. Nevertheless, phenolic compounds extracted from Citrus flowers are well-recognized for their bioavailability characteristics and exhibit numerous health-promoting effects, including antioxidant, anti-inflammatory, anti-cancer, and antibacterial activities in humans. The information contained in this document provides a comprehensive summary of the latest investigations conducted on the subject matter. The aim is to thoroughly comprehend the biological functions of the bioactive compounds in the Citrus flowers and their potential impacts on various biological systems, shedding light on their potential therapeutic applications.

1. Introduction

Citrus, including fresh fruit and their derived products, hold considerable significance within the global market and the agricultural section. Their pivotal role is underscored by their widespread cultivation across over 140 countries worldwide, contributing to annual consumption and trade [1]. In 2021, the total output of fresh citrus fruit reached a staggering 143 thousand tonnes (Figure 1) [2]. The attraction of citrus fruits resides not only in their delectable flavor but also in their captivating colorings and aromas. Moreover, these fruits have a combination of health-enhancing constituents, comprising vitamins, minerals, essential macronutrients like carbohydrates, dietary fibers, crude proteins, lipids, and critical phenolic compounds [3]. Remarkably, the utilization of citrus fruits extends beyond their consumption in their natural form. Approximately one-third of the global citrus output is allocated to various industries, further attesting to their extensive utility [4].

The citrus genus demonstrates classification into four principal groupings: oranges, encompassing both sweet orange (Citrus sinensis) and sour orange (Citrus aurantium); tangerines (Citrus reticulata); grapefruits (Citrus paradisi); and lemons (Citrus limon) or limes (Citrus aurantiifolia). Amongst the extensively cultivated citrus varieties, oranges are the foremost contributors, comprising over fifty percent of the global citrus yield. Notably, they also dominate the international citrus trade, followed in succession by tangerines, lemons, and grapefruits. Also, in industry, approximately 80% of processed citrus fruits belong to the orange category, constituting the predominant share within the total processed citrus volume [1,2,5].

In recent times, there has been a surge in research focusing on the constituents of citrus fruits. This encompasses various parts, such as fruit juice, flowers, seeds, leaves, and peels, primarily emphasizing the composition of bioactive compounds [4]. It is worth noting, however, that despite the extensive exploration of these components, there remains a scarcity of comprehensive information, particularly concerning citrus flowers. Consequently, a thorough investigation is imperative to attain a complete comprehension of the biological functionalities inherent to the bioactive compounds within citrus flowers and their potential ramifications across diverse biological systems. This study aims to unravel the latent therapeutic utilities that these compounds might offer.

2. Phenolic Compounds and Their Biological Activities in Citrus Flowers

The bioactive phytochemicals in citrus flowers exhibit a remarkable diversity, encompassing various subclasses, most notably polyphenols, which include phenolic acids and flavonoids. In relation to phenolic compounds, recent results have highlighted the feasibility of distinguishing citrus species through an analysis of their distinctive phenolic compound profiles.

The flowers of Citrus aurantium L. (Bitter orange) are considered a promising natural source of ingredients in the development and formulation of functional foods and pharmaceutical products. A study investigated the potential in vitro antioxidant activity of C. aurantium L. flowers, and the polyphenols content, total phenolic content (TPC), and total flavonoid content (TFC) were 18.22 ± 1.44 mg GAE/g dw and 111.44 ± 20.60 µg QE/g dw, respectively, thus confirming the efficiency of the extraction procedure in extracting bioactive compounds [6]. One research represented the investigation of the potential anti-aging and anti-wrinkle effects attributed to the flowers of Citrus aurantium. Conducting this investigation involved examining the physiological activity of a 50% ethanolic extract derived from citrus flowers prior to and subsequent to the fermentation process. These mentioned compounds had the highest amount: phenolic (pyrogallol, syringic acid, ferulic acid, and caffeic acid) and flavonoids (neohesperidin, rutin, and naringin) [7]. In another study, Zhao et el. aimed to isolate constituents from the flowers of Citrus aurantium L. var. amara Engl (a specific type of Citrus aurantium L.) in order to assess their potential activities in terms of antitumor, antioxidant, antivirus, and anti-inflammatory effects. The main compound identified was 5-hydroxy-6,7,3,4-tetramethoxyflavone (HTF) [8]. Nabavi et al. compared four flowers of Citrus in terms of antioxidant activities. Citrus aurantium, Citrus sinensis, Citrus nobilis, and Citrus limon have TPC contents of 78.76, 78.47, 62.10, and 60.01 mg gallic acid equivalent/g of extract, respectively. Furthermore, Citrus limon and Citrus aurantium exhibited the highest value in TFC [9]. Anthocyanins are a subgroup within the extensive category of phenolic compounds, collectively classified under the group of flavonoids. The presence of numerous types of anthocyanins in flowers of lemon (Citrus limon (L.) Burm. f.), citron (Citrus medica L.), Piretto lemon (Citrus limoni medica Lush.), Rangpur lime (Citrus limonia Osbeck), and Meyer lemon (Citrus meyeri Y. Tan.) has been confirmed [10]. Moreover, coumarins, a subset of polyphenolic compounds, have demonstrated noteworthy anti-cancer attributes by engaging multiple mechanisms of action [11]. Pellizzeri et al. found coumarins in flowers of two citrus species, namely lemon (Citrus limon (L.) Osbeck) and grapefruit (Citrus paradise Macfad.), and they were determined as valuable resources for the perfumery and cosmetic industries [12].

3. Concluding Remarks

Citrus flowers emerge as valuable reservoirs of bioactive compounds, particularly phenolics and flavonoids, renowned for their pivotal role as natural antioxidants. In regards to abundant production and notable scientific interest, it is evident that the benefits offered by Citrus aurantium flowers surpass those of other citrus flower variants. Numerous comparative investigations have consistently demonstrated that C. aurantium flowers boast the highest levels of total phenolic content (TPC) and total flavonoid content (TFC). These secondary metabolites yield a host of advantageous biological and medicinal effects. These encompass a wide array of benefits, ranging from antioxidant and antimicrobial properties to potential anti-cancer, anti-diabetic, anti-obesity, and anti-anxiety effects. Moreover, their application extends to the cosmetic and food industries. In light of these findings, it is evident that C. aurantium flowers, among other citrus species, hold significant promise for future research endeavors.