The lactic acid bacterium as a cell factory for food ingredient production

Abstract

In this contribution, the homofermentative lactic acid bacterium as an efficient cell factory for different (food) ingredients will be presented. The emphasis will be on some successful examples of metabolic engineering and on the physiology of these bacteria, which makes them so suitable as a cell factory. One interesting conclusion of the metabolism of these bacteria is that they have clearly chosen for speed instead of efficiency, although some evolutionary results can still not be explained on mechanistic level.

Introduction

Natural fermentation has already been used for ages to increase the shelf-life of various food materials. This process has resulted in a number of traditional food products such as: the dairy products cheese, butter, buttermilk and yoghurt; fermented meat, plants and fruits such as sausages, silage, sauerkraut, olives and grapes; and finally fermented cereal products such as bread and beer (Caplice & Fitzgerald, 1999). With the exception of beer and wine, both involving alcoholic fermentation by yeast, all these food products result from bacterial acidification, leading to longer shelf-lives (Ross, Morgan, & Hill, 2002). The bacteria that are found in these fermented products are almost always lactic acid bacteria (LAB), named for the organic acid produced during fermentation. In most cases, a specific species of LAB will become dominant in a fermentation as a result of its extremely efficient conversion of the available sugars and the rapid formation of lactic acid that inhibits the growth of most other microorganisms present. Besides producing the (lactic) acid, the acidifying bacteria, also called starter bacteria, contribute to the flavour, the texture and the nutritional value of the fermented foods through production of aroma components, through production or modification of exopolysaccharides and proteins, and through the production of nutritional components such as vitamins.

The species, strain or variant that becomes dominant during a specific fermentation is determined by the food substrate that is used, the temperature of the process, and other environmental conditions. In Gouda cheese, which does not involve cooking of the curd, the lactic acid bacterium Lactococcus lactis prevails, a so-called mesophilic bacterium with a growth optimum at 30 °C and an inability to grow at temperatures above 35–38 °C. In yoghurt and Parmesan cheese, however, where heating to 50 °C is employed, so-called thermophilic LAB (Delcour, Ferain, & Hols, 2000) are found such as Streptococcus thermophilus and Lactobacillus helveticus.

Nowadays, dairy fermentations are often carried out at such large scale that there is a strong need to control the process. By adding a high dosage of active starter cultures the fermentation is no longer a randomly occurring process, but has become a carefully controlled process. The production of these starter cultures (Hansen, 2002) is now a worldwide business of approximately $500\mathrm{M}€$.

In this overview some characteristic features of homofermentative LAB will be described, and if possible, explained. These features are the “choice” of these bacteria for their specific metabolism, the use of these bacteria for metabolic engineering, and the use of these bacteria for delivery of flavour and nutritional components into fermented foods.