The bio-industrial application of microalgae has gained much attention in recent years. One of the challenges in this field is to lower the cultivation and harvest costs and to achieve the steady productivity. To address this, new systems have been proposed, in which the products are extracted without killing the algal cells. These non-destructive extraction systems are called “milking.” Some of the milking systems reported so far are continuous processes where culturing and milking occur simultaneously, and the others are a periodic process where cells are cyclically cultured and milked. These systems are based on the organic solvent extraction of non-polar products, such as lipids, terpenes, and carotenoids. However, a special facility required for handling organic solvents increases the costs and solvents lost during milking need to be recouped. In this study, we examined a solvent-free method, based on mechanical milking using a shearing disperser (glass homogenizer). We cultured the N2-fixing filamentous cyanobacteria, Tolypothrix sp. PCC7601 in non-sterile agricultural water and performed long-term (87 days) milking cycles to harvest the extracellular carbohydrates and phycobiliproteins. As a result, the productivity of extracellular carbohydrates and the cell densities remained constant throughout the milking cycle, yielding 90–140 mg/L of extracellular carbohydrates every 3 weeks. Our results demonstrated that mechanical milking is a practical and effective method that can be used to harvest products from algae steadily.
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