Producing Lactic Acid Using Cull Potatoes 

Problem Addressed

More than 170,000 acres in Washington State are planted in potatoes annually. With harvesting averages of 60,000 pounds per acre the estimated total production is 10,200 million pounds, 10 percent of which are culls. At $10 per ton the cull potato is an underutilized agricultural product that provides minimal return to the grower. Potato growers in Washington want to develop a new product, lactic acid, from the currently under-utilized cull potatoes.

Lactic acid is a colorless compound commonly used as a food additive for flavor and preservation. It is of high value and importance in both international and domestic markets. Presently, over half of all lactic acid is produced chemically from coal, petroleum, and natural gas, and the rest biologically from the bioconversion of carbohydrates, and plant biomass. Cull potatoes, with their rich composition in starch and nutrients, are an ideal raw material for lactic acid production.

Goal

The goal of this applied research is to assist the potato growers of Washington with the development of lactic acid from cull potatoes.

Implications

With 15.44 percent potato starch content and a yield of 60 percent of glucose to lactic acid, cull potatoes can be converted to approximately 94.5 million pounds of lactic acid. At the current wholesale price of $0.50 per pound of lactic acid, the estimated total value would be $47 million. The cost of raw material of lactic acid per pound produced using cull potatoes is less than $0.10 which will allow potato growers to offer a competitive price for Washington produced lactic acid. In the near future this competitively priced product can replace the imported products and will allow export to international markets in the long run.

Procedures

The steps to producing lactic acid from cull potatoes include selecting a pure or mixed species of microorganisms for fermentation, optimizing the fermentation conditions and reactor design, determining the most effective process for product separation, and developing a pilot system for demonstration and process optimization. Numerous fungal strains, all of which produced lactic acid from cull potatoes, were screened to find the highest lactic acid yield. The team found that when amylolytic enzyme was used first to hydrolyze the potato and then inoculated with the bacterial strains, the lactic acid yield reached 80 percent. During the project researchers found that Chitin can be extracted from fungal biomass. Therefore, the fungal fermentation process can produce lactic acid and chitin, making the whole process more competitive while increasing the potential for commercialization.

Techniques and Technologies Developed

When using amylolytic enzyme, first hydrolyzing the potato and then inoculating bacterial strain, the yield can reach to 80 percent. The key reason the fungal culture has lower lactic acid yield is because it usually forms a cotton like mycelia which makes the mass transfer more difficult. In bioreactors, clump mycelia increase the viscosity of the medium, which wrap around the inside elements of the reactor, which leads to low yield and productivity of lactic acids in fungi fermentation. The team recently solved this morphology problem so the strain can form pellet biomass, increasing lactic acid yield to 66 percent and lactate concentrations to 90-100g/L. Forming pellet morphology influences broth rheology which is responsible for successful momentum, mass and heat transfer during the fermentation process in a bioreactor.

Publications/Journal Articles From Project

Liu, Y., Z. Wen, W. Liao, C. Liu, S. Chen. “Optimization of the L (+) –Lactic Acid Production Process from Cull Potato by Rhizopus oryzae.” Applied Biochemistry Biotechnology (in press).

Patents

An Invention Disclosure was submitted to WSURF on December 20, 2004.
Liu, Y., Liao Wei, and S. Chen. A fermentation process for co-production of lactic acid and chitin and chitin derivatives using the pellet form of Rhizopus oryzae.

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