Browning in fruits, vegetables, and legumes is a common yet frustrating problem in food processing. It not only affects the appearance of food but can also degrade its nutritional value and shelf life.
A recent study published in the South African Journal of Botany (May 2025) has made significant strides in addressing this issue, particularly for Bambara groundnuts—an underappreciated but nutrient-rich African crop.
The research focuses on understanding and controlling the enzyme responsible for browning, polyphenol oxidase (PPO), using natural fruit extracts.
The Science Behind Browning and Its Impact on Bambara Groundnuts
When fruits, vegetables, or legumes like Bambara groundnuts are cut, bruised, or processed, they often turn brown due to a chemical reaction called enzymatic browning. This reaction is driven by an enzyme called polyphenol oxidase (PPO).
PPO interacts with oxygen and naturally occurring compounds in plants called polyphenols, converting them into dark-colored pigments called quinones. While this process is harmless, it makes food less appealing and can reduce its market value.
For Bambara groundnuts—a drought-resistant crop packed with protein, vitamins, and minerals—browning is a major post-harvest challenge. These legumes are a lifeline for rural communities in Africa, providing affordable nutrition and income.
However, their tendency to brown quickly limits their use in food products and commercial markets. Traditional methods to prevent browning often rely on synthetic chemicals, which can be expensive, unsafe, or environmentally harmful. This study explores a safer, natural alternative: fruit extracts.
Understanding the Enzyme: Polyphenol Oxidase (PPO)
The researchers isolated the specific enzyme responsible for browning in black-eyed Bambara groundnuts: a 37 kDa polyphenol oxidase (PPO).
Through advanced purification techniques, including ammonium sulfate precipitation and chromatography, they confirmed the enzyme’s structure and behavior.
The purified enzyme had a molecular weight of 37 kDa, a purification fold of 25.8, and a recovery rate of 19.7%.
The study found that PPO works best at 40°C and pH 6. However, its activity drops sharply at temperatures above 40°C or in highly acidic/alkaline environments.
For instance, at 90°C and pH 11, the enzyme retained only 36% of its activity after 2 hours. This suggests that high temperatures and extreme pH levels can effectively reduce browning.
Additionally, the enzyme showed maximum stability at 30°C and pH 6, retaining 96% and 76% of its activity after 2 and 6 hours, respectively. PPO also showed a strong preference for diphenols like catechol, which are abundant in Bambara groundnuts.
The kinetic parameters of the enzyme revealed a Km of 0.02 mM and a Vmax of 0.14 µmol/mL/min, indicating high catalytic efficiency. This specificity for diphenols makes PPO a key target for controlling browning in food processing.
Natural Fruit Extracts as Anti-Browning Agents
The study tested five natural fruit extracts—lemon, grape, tomato, onion, and pineapple—to see how well they could inhibit PPO activity. Among these, lemon extract was the most effective, reducing PPO activity by 77%.
Grape extract also showed significant inhibition at 52%, while tomato and onion extracts had minimal effects (18–20% inhibition).
Surprisingly, pineapple extract increased PPO activity by 53%, possibly due to compounds that activate the enzyme.
Both lemon and grape extracts worked by competitively binding to PPO, blocking its active site and preventing it from interacting with polyphenols.
This mechanism is similar to how a key fits into a lock—except here, the fruit extracts act as “fake keys” that stop the real reaction.
The study also found that lemon extract had a stronger inhibitory effect than grape extract, likely due to its higher content of citric and ascorbic acids, which are known to chelate metal ions and disrupt enzyme activity.
The Role of Temperature and pH in Controlling Browning
The study also explored how environmental conditions influence PPO activity. Browning slows down significantly at temperatures below 30°C or above 70°C.
For example, storing Bambara groundnuts at 30°C retained 96% of PPO activity after 2 hours, while heating them to 90°C reduced activity to 36%. This suggests that temperature control is a practical way to delay browning in food processing.
Interestingly, PPO showed peak activity at pH 2, which is highly acidic. This unusual behavior suggests that the enzyme is adapted to harsh environments, possibly explaining the crop’s resilience.
However, the enzyme’s activity dropped sharply at neutral and alkaline pH levels, indicating that adjusting pH during processing could also help control browning.
Why Natural Inhibitors Matter
Synthetic anti-browning agents, such as sulfites, are widely used but come with risks. They can trigger allergies, harm the environment, and leave residues on food.
Natural inhibitors like lemon and grape extracts offer a safer, sustainable alternative. They are non-toxic, cost-effective, and already part of human diets. Moreover, consumers increasingly prefer “clean-label” products free of artificial additives, making natural inhibitors a market-friendly solution.
For Bambara groundnuts, this could mean longer shelf life, better-quality products (e.g., flour, snacks, or plant-based milk), and increased profitability for farmers.
The study’s findings also have broader implications for other fruits and vegetables prone to browning, such as apples, potatoes, and bananas.
Practical Applications and Future Directions
The study’s findings have immediate real-world applications. For instance, adding lemon or grape juice during grinding or packaging could prevent browning in Bambara groundnut-based products.
Farmers could also use these extracts to treat seeds or leaves, reducing post-harvest losses. Beyond food processing, PPO inhibitors might have uses in medicine, such as treating skin disorders like vitiligo.
However, challenges remain. Extracting large quantities of natural inhibitors cost-effectively, ensuring taste compatibility, and maintaining long-term stability are areas that need further research.
Future studies could explore blending multiple extracts, optimizing extraction methods, or genetically modifying Bambara groundnuts to produce less PPO.
Conclusion
This groundbreaking study not only sheds light on the biochemistry of Bambara groundnut browning but also offers a practical, nature-based solution.
By harnessing the power of lemon and grape extracts, we can reduce food waste, improve nutrition, and support smallholder farmers in Africa.
As the demand for sustainable and safe food grows, such innovations bridge the gap between science and everyday life, proving that sometimes, the best solutions come straight from nature.
Reference: Lawal, O. T., Fasan, D., Ogundele, O. P., Olopoda, A. I., Omotoyinbo, O. V., & Sanni, D. M. (2025). Biochemical characterization of browning-induced enzyme in black-eyed Bambara groundnut (Vigna subterranea): Inhibition by natural fruit extracts. South African Journal of Botany, 180, 134–143. https://doi.org/10.1016/j.sajb.2025.02.034
