Effect of heating and probiotic fermentation on the phytochemical content and antioxidant potential of edible Irish brown seaweeds

Probiotic fermentation was conducted on two species of edible Irish brown seaweeds, Saccharina latissima and Laminaria digitata as a sole source of nutrition for the bacteria. Seaweeds were heat treated followed by fermentation with the probiotic Lactobacillus rhamnosus. Kinetics of cell growth, acid production, and sugar consumption were examined. The effects of heating and fermentation on the changes in total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity were also monitored. Lactobacillus rhamnosus grew well on both the seaweed species and reached nearly 10 log cfu ml ^-1 after 16-24 h of fermentation; however, the specific growth rate was 1.5 times higher on S. latissima. Lactic acid was the major product of the fermentation, followed by acetic acid. Heat treatment resulted in a 50-64.4% reduction in TPC and TFC for both the seaweeds concomitant with a leaching of phytochemicals into the water used for heating. Subsequently, fermentation resulted in 28% and 37.6% increases in the TPC and 1.18- and 2.04-fold increases in TFC for S. latissima and Lam. digitata, respectively, with a simultaneous increase in the antioxidant activity as well. Changes in the TPC and TFC were represented by zero-, first-, or second-order kinetic models. Fourier transform infrared analysis of the seaweeds as well as the broth revealed the presence of polyphenolic signals at different ranges.