Enhancing the taste of an unsung fruit

A distinctive feature of the mountains of Guizhou, a province in southwest China, is the Guizhou cactus pear or Rosa roxburghii Tratt. Covering 140,000 hectares of the province, and with a bright yellow fruit coated with dense prickles, the Guizhou cactus pear is as unique to China as the giant panda, though far less well known.

That needs to change, according to food scientist Zhuhong Ding of the School of Liquor and Food Engineering at Guizhou University in Guiyang.

“Guizhou cactus pear isn’t just some fruit,” she says. “It’s a remarkable plant with an outstanding nutritional value.” It contains ten times more vitamin C than kiwi, a 100 times more than lemon, and an astounding 500 times more than apples, says Ding. It is also rich in antioxidant molecules that neutralize harmful free radicals. Diets that contain fruit and vegetables, which are abundant in antioxidants, protect against cardiovascular disease and certain cancers, say researchers¹.

Despite its nutritional offerings, consumption of Guizhou cactus pear — which is distinct from the invasive Mexican prickly pear of the genus Opuntia — is limited by its bitter taste. Now, Ding’s research team have shown that fermentation might be the solution to improving the flavour.

“We found that microbial fermentation not only enhances the antioxidant properties of Guizhou cactus pears, but contributes to the release and enrichment of flavour compounds, elevating the overall sensory experience,” Ding explains.

Her team hope to boost the Guizhou cactus pear industry, which already contributes to Guizhou’s rural economy.

For 400 years, wild Guizhou cactus pears have been used for food and tea, and in traditional Chinese medicine. But it was the discovery in the 1950s of their high vitamin C content that led to research into new varieties and processing techniques, allowing Guizhou’s cactus pear industry to grow into a 15-billion-yuan powerhouse, including more than 80 companies.

“Guizhou cactus pear is evolving from a regional specialty to a key ingredient in various products, including juices, fermented wine, snacks, and pharmaceuticals,“ says Yichun Sun, director pharmacist of Sinopharm Group Tongjitang (Guizhou) Pharmaceutical Co., Ltd., a traditional Chinese medicine manufacturer that helps fund Ding’s research. “It’s nutritional and functional values make it stand out, especially as consumers increasingly focus on high-quality, functional foods.”

To accelerate the growth of the Guizhou cactus pear industry, Ding’s team ran studies designed to develop processing techniques that will make the fruit more palatable without reducing its nutritional value. They tested a variety of processes, including freeze drying and grinding. One process — fermentation — stood out.

During fermentation microorganisms break down organic compounds such as carbohydrates into simpler molecules. Fermentation is used in food-preserving processes to change food’s taste and texture. It has been used for centuries, including to produce beer, bread and cheese. Now, food scientists are finding ways to refine and repurpose the technique to create new types of food.

Ding’s team has used a strain of the bacteria Lactobacillus acidophilus to ferment Guizhou cactus pear². A comprehensive analysis of 1,635 metabolites using chromatography and mass spectrometry techniques revealed that the concentrations of 205 were changed by fermentation. This included an increase in two types of antioxidant molecules — phenolic acids and flavonoids — contributing to a 35% increase in Guizhou cactus pear’s total antioxidant activity. The hypoglycemic activity of the pear pulp — its potential ability to reduce blood glucose levels — also increased, as judged by its ability to inhibit enzymes that break carbohydrates into glucose in a test tube. Foods with hypoglycaemic properties are of research interest because of their potential role in managing diabetes.

Fruitful future

Critically, fermentation also changed the profile of several taste-related metabolites in the Guizhou cactus pear pulp. For example, amino acids, which modulate taste perception, fell, while most organic acids and sugars, increased. Testing with an electronic nose and an electronic tongue — arrays of electronic sensors that detect chemicals which create taste and smell — found that fermentation with Lactobacillus acidophilus reduced bitterness and astringency of the pulp, and improved its aroma.

In a second study, the team compared Guizhou cactus pear pulps that had been fermented using three different Lactobacillus species³. Fermentation with Lactobacillus acidophilus best improved antioxidant activity. It also most improved the flavour of the pulp, according to a panel of 10 trained tasters, who rated the ferment on appearance, smell, colour and lustre and taste, using a 25-point scale.

Ding hopes that Guizhou cactus pear ferments could one day be used to modify the nutritional properties of processed foods. With that goal in mind, Ding and her team used a slew of techniques to explore the interaction between rutin and isoquercitrin — two antioxidant molecules in the Guizhou cactus pear — and the enzyme β-glucosidase, which helps break down sugars in our body⁴. They hope to use this understanding of the biochemistry to develop processing techniques that will enhance the antioxidant and hypoglycaemic properties of Guizhou cactus pear ferments. “Our next steps in the field of biotechnology involves exploring additional microbial strains, and combining other technologies such as enzyme technology⁵,“ says Ding.

Looking ahead, Sun sees the need for more collaboration between research and industry to grow the fruit’s visibility. “There’s a lack of widespread recognition, especially internationally,“ Sun says. “We need to promote Guizhou cactus pears as the giant panda of fruits.”