Diffusion of lactic acid in a buffered gel system supporting growth of Lactobacillus curvatus

Abstract

Diffusion of lactic acid in a buffered gel system was investigated using fluorescent ratio imaging. The range of pH studied was from 5.5 to 3.8, which included the minimum pH of growth of Lactobacillus curvatus (4.26). Diffusion of lactic acid in this pH range was found to be Fickian and the estimated effective diffusion coefficient of lactic acid at 20°C was 2.81 Â 10 À10 m 2 s À1 with a standard deviation of 0.21 Â 10 À10 m 2 s À1 . This effective diffusion coefficient was estimated by fitting the one-dimensional solution of the Fickian diffusion equation for infinite systems to a transformed experimental data set. The original data set as recorded by the fluorescent microscope consisted of concentration-distance curves at specific time intervals. This was transformed by realigning the distance scale of all the concentration-distance curves.

Bacterial Colonies in Solid Media and Foods: A Review on Their Growth and Interactions with the Micro-Environment

“…In conclusion, all the studies on bacterial colony growth have suggested that the growth of colonies (growth rate, final size, and shape) was determined by local concentration of substrates and thus by possible limitations of the diffusion of substrates or end-products in solids (McKay et al, 1997 ; Walker et al, 1997 ; Malakar et al, 2002b ; Pipe and Grimson, 2008 ).…”

“…Furthermore, some of these studies initially suggested diffusion limitations of the substrates, but then concluded, in the case of numerous and small colonies in favorable growth conditions, that there were no mass transfer limitations of substrates and lactic acid (Stecchini et al, 1998 ; Malakar et al, 2002b ; Kabanova et al, 2012 , 2013 ). For instance, Malakar et al ( 2002b ) concluded after they measured the effective diffusion coefficient of lactic acid in gelatine medium that the diffusion of lactic acid was not limiting for growth, and that the growth rate was determined only by the generation time of L. curvatus , a LAB strain. They obtained a mean diffusion coefficient of 2.81 × 10 −10 m 2 /s in MRS with 10% gelatine at 20°C, which they compared with a diffusion coefficient of 1.74 × 10 −10 m 2 /s in water at 25°C previously measured by Cussler ( 1997 ).…”

“…They obtained a mean diffusion coefficient of 2.81 × 10 −10 m 2 /s in MRS with 10% gelatine at 20°C, which they compared with a diffusion coefficient of 1.74 × 10 −10 m 2 /s in water at 25°C previously measured by Cussler ( 1997 ). Therefore, Malakar et al ( 2002b ) concluded that because both values were of the same order of magnitude in the gel and in the aqueous solution, lactic acid produced by bacteria can easily diffuse through gels. This conclusion is questionable though, because others studies such as those conducted by Ribeiro et al ( 2005 ) and Øyaas et al ( 1995 ) reported values of diffusion coefficient of lactic acid in water much higher than that of Cussler ( 1997 ) of between 7 × 10 −10 and 10 × 10 −10 m 2 /s at 25°C, meaning that diffusion of lactic acid in the gel media was around 4–6 times lower than in the aqueous solution.…”

Exploring Bacterial Colonies in Solid Foods or Model Foods Using Non-Destructive Techniques

“…In conclusion, all the studies on bacterial colony growth have suggested that the growth of colonies (growth rate, final size, and shape) was determined by local concentration of substrates and thus by possible limitations of the diffusion of substrates or end-products in solids Malakar et al, 2002b;.…”

“…Even if microgradients of pH and O 2 have been measured, to our knowledge, microgradients of redox potential, inhibitors, or substrates have not, and their existence is still to be shown. Furthermore, some of these studies initially suggested diffusion limitations of the substrates, but then concluded, in the case of numerous and small colonies in favorable growth conditions, that there were no mass transfer limitations of substrates and lactic acid (Stecchini et al, 1998;Malakar et al, 2002b;. For instance, Malakar et al (2002b) concluded after they measured the effective diffusion coefficient of lactic acid in gelatine medium that the diffusion of lactic acid was not limiting for growth, and that the growth rate was determined only by the generation time of L. curvatus, a LAB strain.…”