Statistical optimization of α-amylase production process parameters by RSM
The second-order polynomial response surface model RSM design was developed at three levels in the selection of each independent bioprocess parameter. Thirteen variables were found to be potent among the total parameters evaluated for their significance on α-amylase activity in the OFAT method (results are not shown here), namely pH, temperature, agitation, inoculum size, aeration, carbon source, nitrogen source, K2PO4, MgSO4, NaCl, incubation duration, fructose, and NaNO3. The response function was estimated by a second-degree polynomial with quadratic and interaction effects using the least squares approach (Rajulapati et al. 2011). The Definitive Screen (DSD) Design was used to assess pH (A), temperature (B), carbon source (C), nitrogen source (D), K2PO4 (E), MgSO4 (F), NaCl (G), fructose (H), and NaNO3 (I) ranges for experimental investigation. Table 1 shows the true ranges of coded factors based on the results of the OFAT technique (data is not provided). The statistical association between the ultimate goal (α-amylase activity) and each of the independent variables was determined using a second-order polynomial equation.
Y = 1085.35 + 1.15A + 0.25B + 1.0C + 1.5D + 17.5E + 6.0F + 17.5G + 1.0H + 5.0J-1.69A2-5.53C2-24.33D2-20.86E2-16.33F2-35.09G2-5.34H2-13.64J2 Eq. (1)
Where, Y is the level of α-amylase activity.
Use of RSM produced the ensuing quadratic regression equation for the final response of α-amylase activity [Eq. (1)]. The ultimate objective's optimised bioprocess parameter values were identified as 5.20, 35.92, 4.22, 2.06, 0.34, 0.14, 0.23, 1.48, and 0.53, respectively, for pH, temperature, carbon source, nitrogen source, K2PO4, MgSO4, fructose, and NaNO3 .
Table 1
Variables used in experimental design
Factor Code | Name | Lower limit (-1) | Upper limit (+ 1) |
A | pH | 4.0 | 6.0 |
B | Temp(oC) | 32.0 | 36.0 |
C | Carbon source (%) | 3.0 | 5.0 |
D | Nitrogen source (%) | 1.0 | 3.0 |
E | K2HPO4 (%) | 0.20 | 0.4 |
F | MgSO4 (%) | 0.05 | 0.2 |
G | NaCl (%) | 0.1 | 0.3 |
H | Fructose (%) | 1.0 | 2.0 |
J | NaNO3 (%) | 0.3 | 0.7 |
Evolutionary and swarm intelligence-based optimization
This study aims to optimize the nonlinear RSM model of α-amylase fermentation from Bacillus velezensis sp. using artificial intelligence-based GA and swarm intelligence-based PSO approaches.
Genetic algorithm (GA)
GA is an evolutionary algorithm that mimics natural evolution. It involves three operators: reproduction, crossover, and mutation. Reproduction selects good strings in a population and forms a mating pool. Crossover allows for new string formation by exchanging strings with another chromosome. Mutation perturbs the child vector, achieving local search and maintaining diversity. The process is repeated until a termination criterion is met. In this study, a binary-coded GA was used to optimize α-amylase extraction from fermented broth for enhanced α-amylase activity. The minimization problem is converted to a maximization problem using negative sign before enzyme activity
Particle swarm optimization (PSO)
The swarm intelligence-based optimization (PSO) approach, developed by Kennedy and Eberhart, overcomes drawbacks of generalized optimization (GA) such as convergence toward local optima and difficulty in dynamic sets. PSO involves a population of particles, each with a memory for its previous best position. It has advantages over GA, such as easier implementation, fewer parameters needed for adjustment, and higher memory capability. However, PSO can sometimes suffer from premature convergence, leading to suboptimal solutions. This study uses the multiobjective PSO-crowding distance (MOPSO-CD) approach to solve the optimization problem of α-amylase extraction.MOPSO-CD incorporates the crowding distance operator, which affects global best selection criteria by deleting non-dominated solutions. The algorithm's mutation operator is adapted for exploratory capability, initially performing mutations on the entire population and rapidly decreasing coverage over time to prevent premature convergence. The MOPSO-CD algorithm's working procedure is shown in a flow chart in Fig. 2.
To compute the new velocity, V[i]:
V[i] = W * V[i] + R1 * [Pbest (i) - P (i)] + R2 * [A(Gbest) - P (i)]
Where, W is the inertia weight, which is equal to 0.4, R1 and R2 are the random numbers in the range of (0–1), Pbest(i) is the best position reached by particle i and A(Gbest) is the global best guide for each dominated solution.
To calculate the new position of P[i]:
P[i] = P[i] + V[i]
The MOPSO-CD algorithm was used to optimize lipase extraction by exploring the nonlinear RSM model and particle size parameters.