Modelling the biphasic growth of non-starter lactic acid bacteria on starter-lysate as a substrate

Since cheese is poor in energy for bacterial growth, it is believed that non-starter lactic acid bacteria growth and flavour development are supported by the nutrients from lysis of the starter culture. This study was performed to investigate the dynamics of interaction between starter and non-starter strains from cheese. A starter culture lysate was prepared by enzymatic digestion and tested as a growth substrate for Lactobacillus sp. strains. The two starter culture strains of Lactococcus lactis were also tested on the starter-lysate. All seventeen strains were individually inoculated at the level of 5.0 log₁₀ cfu mL^-1 in M17 broth, with or without 10% starter-lysate, and incubated at 30 °C for 140 h. The optical density_{600 nm} was modelled with the primary log-transformed Logistic model with delay and lag phase duration, maximum specific growth rate as well as maximum population density obtained. Biphasic growth was mainly observed when the strains were able to utilize the starter-lysate as an energy source. To deal with the lack-of-fit related to the biphasic growth, the observed data points of the curve were divided after graphic evaluation and according to deviation of the residuals from the range ±0.05. Modelling was then performed in two phases by applying the same primary Logistic model in each of the two parts of the growth curve. Values of root-mean-square error and graphic evaluation indicated the good fitting of the data with the suggested approach. The growth of the two Lactococcus lactis strains was not affected by the starter-lysate. However, thirteen of the non-starter strains had their growth rates increased. The increase was greatest for Lactobacillus rhamnosus KU-LbR1, which reached maximum optical densities of 0.23 and 0.58 in the absence and the presence of starter-lysate, respectively. No effect of the starter-lysate was shown for the growth of Lactobacillus curvatus strains. The extend of the growth of non-starter strains on the starter-lysate was shown to be species and strain dependent.