Thermal stability of periplasmic BfrA in free and entrapped cells. Samples were incubated in 50 mM sodium acetate buffer (pH 5.5) without substrate for 15 h at different temperatures. The remaining invertase activity was quantified by adding sucrose to 120 mM and reacting for 15 min at 60°C. Percentage values are relative to the activity of the free and immobilized cells kept at 4°C before the reaction. Data are the means of triplicate measurements ± standard deviation.