Red Yeast Rice at a Glance
Microbiology and Food Chemistry | Nicol Naraysha
Red yeast rice (RYR) is produced by the fermentation of rice with the Monascus strain of mould. The azaphilone pigments formed during the fermentation process allow the product to have its bright red colour, contributing to its use in coloured food for centuries. RYR is also used in food preservation and as medicine for blood-related ailments. However, research has found that it produces citrin, a compound that may be harmful to humans. This review article gives insight into this unique fermentation product.
Red Yeast Rice, Superpower Since Ancient Times
Red yeast rice (RYR) has various names across Asia. In Southeast Asia, it is known as Angkak, in China it is called Hongqu, while in Japan it is called Red Koji [2], [3]. The term Koji in Japanese refers to the bean or grains that are processed with mould culture [4]. The oldest records about the ingredient date back to 1329 in Nichiyo-Honzo, a Chinese herbology book written by Duan Wu, which mentioned that RYR is a food component that could improve blood circulation [1].
When it comes to food additives, a 17th-century book called Tiangong Kaiwu mentioned that RYR is a powerful preservative, capable of keeping raw fish fresh in warm temperatures for 10 days without flies or maggots getting close [1]. Aside from its preservation power, RYR is also utilised as a food colourant. In the Okinawa region of Japan, RYR is used to make the red-coloured fermented bean curd known as Tofuyo, which has been famous since it was first introduced around the 1800s. In the 1832 book Edible Plant of Okinawa (Gyozen Honzo), Tofuyo was described to enhance digestive health and cure other ailments [5]. Tofuyo was brought to Japan in the 1800s from China, where the dish is known as Tofunyu, and has been consumed for over 1,500 years. In modern days, it is produced in Zhejiang, Jiangsu, and Sichuan [1].
RYR is still used similarly in the modern day. Various researchers have discovered the chemical behind each of the benefits mentioned above. The preservation power of RYR could be caused by the antibacterial power of the flavonoids and citrinin in it [6], [7]. The colouring power of RYR is caused by various Monascus pigments such as yellow (monascin and ankaflavin), orange (rubropunctatin and monascorubrin), and red (rubropunctamine and monascorubramine) [8]. Interestingly, these pigments also contribute to RYR’s ability to decrease lipids in the human body, along with its anti-cancer, anti-inflammatory, and anti-fatigue effects [9-12]. However, when talking about the medicinal power of RYR, monacolin, particularly monacolin K, is a superstar due to its cholesterol-lowering effect, which works by inhibiting the HMG-CoA (3-hydroxy-3-methyl-glutaryl-coenzyme A) reductase enzyme [13].
Figure 1: Red Yeast Rice [1].
How Is RYR Produced?
Before exploring the chemistry behind RYR’s properties, let us briefly explore how RYR is fermented. The process of RYR fermentation begins with the inoculation of the Monascus mould spore on steamed rice [4]. This fermentation process is known as solid-state fermentation, which has been the traditional way of producing RYR since ancient times [1]. Today, RYR can also be produced through liquid-state fermentation using fermenters and liquified rice [14]. In the modern process, the red yeast rice will be filtered and concentrated to obtain RYR with specific characteristics. The processes of both solid-state and liquid-state are shown in Figure 2.
Although M. purpureus is the most used strain, however, other Monascus strains, including M. anka, M. pilosus, and M.ruber, are also used to ferment RYR, as seen in Korea, Japan, India, and Thailand [15], [16]. In China, the choice of strain is specified for its intention; RYR produced for food colourant will be made by using M. purpureus, while those intended for monacolin K isolation will be made using M. pilosus [17].
Powerful Pigmentation of RYR
True to its name, RYR has a strong and attractive red colour, thanks to the Monascus azaphilone pigments produced as a secondary metabolite during the fermentation process [18]. One of the most popular traditional dishes that utilises RYR is the Char siu barbecue, which is known for its signature red colour. Just 1 teaspoon of RYR powder is needed for 1 kilogram of meat.
Azaphilone belongs to the polyketide class, which comprises other various natural products including the flavonoid pigment, macrolide antibiotic, and polyenes antibiotic, among others [18]. All of the compounds have something in common: they are produced by polyketide synthases with acyl CoA as the precursor [19]. Polyketides that are produced as a fungal secondary metabolite that feature a chiral quaternary center and have a pyrone-quinone structure are classified as azaphilones. The unique pigment of this group is due to azaphilone’s tendency to bond with amines such as amino acids, nucleic acids, and proteins, resulting in a red or purple colour [18]. The structure of RYR pigments is depicted in Figure 3, and some research has shown that these pigments are also contributors to other RYR functionalities.
Figure 3: Major Monascus pigments in RYR [8]
Figure 2: The traditional and modern RYR production [1].
Monascus Pigment Functionalities
One functionality of RYR is the hydrolipidemic power of the Monascus pigment. It achieves this by regulating the level of serum lipids and preventing lipid accumulation in the liver. This pigment also promotes the excretion of fecal bile acids, triacylglycerols, and cholesterol [9]. Aside from preventing lipid-related disease, researchers have also explored RYR’s anti-cancer properties. One of the orange pigments, rubropunctatin, has shown anti-cancer activity by its capacity to inhibit telomerase enzyme activity. Telomerase is an enzyme responsible for keeping a cell alive by adding DNA to the end of a chromosome, also known as a telomere. Tumor cells are known to produce telomerase, making it a key component of cancer diagnosis [10]. The azaphiloid pigment also has anti-inflammatory properties by preventing nitric oxide (NO) production triggered by lipopolysaccharide activity. NO is usually produced in the event of inflammation and may oxidise DNA, promoting the development of cancer cells [11].
RYR’s Antibacterial Properties
As explained before, RYR has been used as a food preservative since ancient times. Modern research has unlocked the reason behind this power - RYR’s antibacterial properties. A discovery in 2018 found that RYR contains two flavonoids, namely daidzein and genistein [20]. Flavonoids are classes of polyphenol compounds that are produced as plant secondary metabolites and are well known for their antioxidant and radical scavenging activities [21]. Daidzein and genistein themselves have been widely explored in the past 20 years as researchers discovered their anti-cancer capacity by halting cell-cycle progression [22]. Aside from anti-cancer properties, flavonoid compounds also have the potential to combat harmful bacteria, even drug-resistant bacteria. Flavonoids owe their antibacterial properties to the -OH group positioned at C5, C7, C3, and C4’ along with the prenyl group at C6 [6].
Another compound that might be responsible for RYR preservation properties is citrinin, a type of polyphenol that is produced during mould fermentation. Citrinin is not only found in Monascus but also in other filamentous moulds, such as Aspergillus and Penicillium. Citrinin primarily inhibits Gram-positive bacteria, but also has anti-fungal and anti-protozoa properties [7], [23]. Although it has antibacterial properties and belongs to the polyphenol group, which is famous for its antioxidant properties, citrinin is a double-edged sword. It is perceived as a mycotoxin due to its potential to damage the kidney [7]. As RYR supplements are widely available across Europe, the European Union limits the presence of citrinin in a product to 100 mg/kg [24]. Interestingly, research has found that the gene that encodes citrinin is absent in M. pilosus, but is present in M. purpureus and M. ruber, all of which are used to produce the commercially available RYR [25].
Conclusion
With its deep history in Asia’s ancient history, red yeast rice has been revered for both its medicinal and culinary use. It is a versatile ingredient that is utilised for food preservation and enhancing blood circulation. Scientific discoveries have revealed that chemical components from Monascus secondary metabolites bring the vibrant colour of RYR, their antibacterial properties, and their anti-cholesterol properties, all of which have been used since ancient times. Although rich in beneficial compounds, the discovery of citrinin, a potentially harmful substance, serves as a reminder to check the threshold and maximum usage of functional ingredients. Nonetheless, RYR will always be an interesting subject of study that bridges indigenous knowledge with modern scientific exploration.
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Nicol is a recent graduate from the Master of Food Science program (MFoodSci), a choice inspired by her love for everything about food - from its history to its function as a functional food. A coffee lover who loves everything chocolate, except the chocolate bar itself.