Professor Lars Angenent, based at the Center of Applied Geosciences at the University of Tübingen in Germany, along with researchers from Cornell University in the US, have developed a bioprocess that relies on microbiome cultures for conversion of that waste stream into MCCA oil for feed or fuel use.
“We showed that acid whey was converted into valuable medium-chain carboxylic acids (MCCAs), such as n-caproic acid (n-hexanoic acid) and n-caprylic acid (n-octanoic acid), without addition of external electron acceptors.
“MCCAs can be precursors for biofuels or chemicals or can be used as green antimicrobials or livestock feed additives,” noted Angenent and colleagues writing in the journal, Joule.
Due to biological chain elongation, the oil-like MCCAs were extracted with relatively low energy consumption, said the researchers.
The innovation is that the process does not need any other carbon-rich chemicals and only needs the wastewater, wrote Angenent in a note on the findings.
Sweet or sour
Acid whey is a byproduct produced during the making of acid types of dairy products, such as cottage cheese or strained yogurt. It contains lower amounts of proteins compared to sweet whey and, therefore, is not as valued in terms of a raw material for the food industry.
Data from the USDA shows that in 2015 in the US, some 771,000 metric tons of Greek yogurt was produced – that amount represented nearly 40% of the US yogurt market, a sector that has been steadily growing since 2004.
However, every liter of milk that goes into Greek yogurt produces another two liters of acid whey waste. Therefore, manufacturers are increasingly looking for ways to dispose of it.
Some companies now treat their whey in anaerobic digesters to generate methane for electricity or are looking to extract the lactose contained in acid whey.
The Greek yogurt waste by-product can also be transported to farms to be used as a fertilizer as it is rich in organic material. Though, farmers can use only limited amounts of acid whey as fertilizer. If they overuse it, there is risk of run off into nearby waterways, leading to algal blooms, and, eventually, fish kills.
In addition, the acid-whey supply is relatively large compared with the demand, said Angenent.
Acid whey cannot be fed to animals in large quantities due to its acidity, he said. Moreover, the use of acid whey directly as a feed additive has been limited by problems of lactose fermentation in rumen.
However, the process he and his colleagues have developed means these large volumes of wastewater can now be turned into valuable feed or fuel products, he said.
The scientists said they converted acid whey into MCCA oil with an open-culture carboxylate platform.
“We developed a temperature-phased bioprocess with different anaerobic reactor microbiomes, performing thermophilic lactic acid production and mesophilic chain elongation, to produce MCCAs (C6–C9) from acid whey via lactic acid as an intermediate.”
The team kept two microbiomes under different temperatures and placed them in series. The first hot microbiome (50°C) converts all the sugars into an intermediate acid. The second warm microbiome (30°C) performs chain elongation until a product is formed with six to nine carbons in a row.
The product from the bioreactor microbiome has six to nine carbons in a row, so it becomes more oil-like and can be separated from the water it was produced in, said the scientists. The resulting bio-oil must still be purified and further refined.
The researchers said the results of their study show a real organic waste stream can now be converted into primarily MCCAs without the addition of external electron donors that would have increased the operating cost and environmental impact.
“No expensive yeast extract needed to be added to the substrate to sustain chain elongation.”
The researchers said MCCAs could be used in their raw form as an additive for livestock feed (JK Mills et al.) or they can also be processed into longer carbon-chain biofuels and chemicals by abiotic post-processing such as ketonization and Kolbe electrolysis.
“Because the carboxylate platform does not require sterile fermentation conditions, this bioprocess can be implemented on a large scale to address the waste treatment needs of the Greek-yogurt industry. Waste mitigation costs could be outdone by the value of carboxylic acid oil when the capital costs for the product extraction system is economical.
“A full economic analysis is required before scale-up commences to fully understand the commercial viability.”
Title: Temperature-Phased Conversion of Acid Whey Waste Into Medium-Chain Carboxylic Acids via Lactic Acid: No External e-Donor
Authors: J Xu, J Hao, J JL Guzman, C M Spirito, L A Harroff, L T Angenent