Use of Central Composite Design in food microbiology: a case study on the effects of secondary phenolis on lactic acid bacteria from olives

This paper focuses on the use of statistical Design of Experiments (DoE) to investigate the effects of two anti-lactic acid bacteria compounds on growth and metabolism of lactobacilli isolated from Italian table olives. As phenolic compounds were used p-coumaric and vanillic acids (0.0-0.4%) that were combined with salt (0.0-6.0%) and glucose (0.0-4.0%) through a Central Composite Design. As test organisms were used three strains of Lactobacillus plantarum (5 log cfu/ml); samples were stored at 37° C and cell counts and pH were evaluated periodically. The growth of lactobacilli was affected in a significant way by salt, p-coumaric and vanillic acids, being the salt the most significant factor after 24 h (short storage time), then replaced by p-coumaric acid. p-coumaric acid played also a significant role on the acidifying ability, expressed as decrease of pH of the medium: microbial metabolism, in fact, appeared as completely inhibited at 0.2% of p-coumaric acid.