Determination of Variable Humidity Profile for Lactic Acid Maximization in Fungal Solid-State Fermentation

Solid-state fermentation (SSF) is the bioprocess where microorganisms are cultivated in the absence of free water under controlled conditions. Lactic acid can be produced by <i>Rhizopus oryzae</i> SSF of grape stalks. During the microorganism’s growth, the temperature and water content of the solid bed fluctuate, leading to areas of either dry or excessive moisture in the solid substrate. Therefore, it is crucial to control the water supply to the matrix. In this work, we obtain lactic acid through SSF of grape stalks using <i>Rhizopus oryzae</i> NCIM 1299. The SSF was conducted at a fixed temperature of 35 °C, with five constant relative humidity (<i>RH</i>) levels: 50, 57, 65, 72, and 80%<i>RH</i>. Mathematical models, including the Logistic and First-Order Plus Dead-Time models for fungal biomass growth and the Luedeking and Piret with Delay Time model for lactic acid production, were adjusted to kinetic curves. Growth kinetic parameters (<i>X_max</i>, <i>μ_max</i>, <i>T_p</i>, <i>T</i>₀, <i>Y_p/x</i>, and <i>t_d</i>) were determined for all conditions. These kinetic parameters were then correlated with relative humidity using a second-degree polynomial relationship. We observed a decrease in <i>X_max</i> with an increasing %<i>RH</i>, while the value of <i>Y_p/x</i> increased at a higher %<i>RH</i>. Finally, the optimal variable relative humidity profile was obtained by applying the dynamic optimization technique, resulting in a 16.63% increase in lactic acid production.