Optimization of lactic acid production from dairy whey using indigenous lactic acid bacteria

Every time cheese is made, a vast amount of liquid is left behind. For every kilogram of cheese produced, roughly ten liters of this watery byproduct, known as sweet whey, are generated. While it looks like water, it is actually rich in sugars, proteins, and vitamins that the cheese-making process did not capture. In many places, this nutrient-dense liquid is treated as waste. When dumped into rivers or left to rot, it consumes oxygen in the water and creates a heavy environmental burden. However, this same liquid holds a hidden potential: the sugars within it can be fed to tiny, natural microorganisms to create lactic acid. This substance is a versatile building block used to make everything from food preservatives and cosmetics to biodegradable plastics. Turning a polluting waste product into a valuable material represents a shift toward a circular economy, where nothing is wasted and every output becomes an input for something new.

In the northern highlands of Rwanda, where the dairy industry has grown rapidly, researchers faced the challenge of managing the large volumes of whey produced by local cheese makers. A team at INES-Ruhengeri set out to see if they could transform this waste into lactic acid using bacteria that already lived in the region. Instead of importing specialized cultures from abroad, they looked to local yogurt, a traditional fermented food, to find the right microorganisms. They collected whey samples from three different cheese processors and yogurt from a local market to begin their search. The whey they tested was confirmed to be a rich soup of lactose, the sugar found in milk, along with small amounts of protein and minerals, making it an ideal food source for fermentation.

The researchers first isolated several strains of lactic acid bacteria from the local yogurt. These are the same types of friendly microbes responsible for turning milk into yogurt. After testing them in sterile whey, they selected one specific strain, which they named LAB-4, because it proved to be the most efficient at converting the whey sugar into acid. This strain was able to lower the pH of the whey significantly within three days, a sign that it was working vigorously. With this native bacterium in hand, the team moved to the next phase: finding the perfect environment for it to work. They knew that temperature, the starting acidity of the liquid, and the amount of bacteria added would all change the outcome, so they used a systematic method to test hundreds of combinations to find the sweet spot.

Through this careful testing, the scientists discovered that the bacteria performed best when kept at a warm temperature of 37 degrees Celsius, with a starting pH of 6.0, and when the mixture contained a specific amount of the bacterial culture. Under these optimized conditions, the process was remarkably effective. In just 48 hours, the bacteria converted the lactose in the whey into lactic acid, reaching a concentration of 43.2 grams per liter. This result meant that for every gram of sugar consumed, the bacteria produced nearly 0.87 grams of lactic acid, a high efficiency that rivals methods using imported commercial bacteria. The process also cleaned the waste water; the chemical oxygen demand, a measure of how much pollution was in the liquid, dropped by more than two-thirds, proving that the fermentation was simultaneously treating the waste and creating a product.

Once the fermentation was complete, the team needed to separate the lactic acid from the liquid. They used a straightforward method involving calcium hydroxide to turn the acid into a solid salt, which was then filtered out and treated with acid to release the pure lactic acid. This recovery step was successful, yielding about 75 percent of the available acid with a purity of 82.5 percent. While this purity is lower than what advanced industrial machines can achieve, it is sufficient for many food and technical uses. The researchers also looked at the economics of the process on a small scale, estimating that producing a kilogram of this lactic acid would cost about 2.10 US dollars. This price point suggests that the method is financially viable for local dairy processors, offering a way to turn an environmental liability into a revenue stream.

The study demonstrates that local solutions can address global problems. By using bacteria native to the region and optimizing their growth on local waste, the researchers showed that Rwandan dairy processors could manage their whey more sustainably. The work confirms that sweet whey is a suitable substrate for producing lactic acid and that indigenous bacteria are capable of doing the job efficiently without the need for expensive, imported cultures. While the current experiments were conducted in flasks and the recovery method was basic, the results provide a strong foundation for future development. The process offers a practical, low-cost route for waste valorization that aligns with the country's goals for environmental protection and economic growth, turning a potential pollutant into a valuable resource.