Starter cultures

Starter cultures

Gluten is a protein substance found in wheat, which consists of Gluteninins and Gliadins. These two proteins from a cohesive complex in aqueous solution. Hydrogen bondings between the amide side chains as well as intra- and intermolecular disulfide bondings between the cystine side chains are mainly responsible for the typical network formation associated with these proteins.

The objective of this section of the research programme is to (a) evaluate the impact of Lactic-acid bacteria (LAB)-associated proteolytic enzymes on gluten structure and (b) to determine the impact that LAB-associated metabolic end-products (such as lactic acid) have on endogenous cereal enzymes. The approach for objective (a) has been to screen for LAB, which express biopolymer-degrading enzymes. The preliminary screening is carried out using classical agar-based methods and substrates. To determine their effect on the cereal-associated macromolecules it was necessary to develop model systems containing the pure biopolymer components. The efficiency of the LAB in degrading these substrates has been elucidated by a combination of complimentary methods adapted to the specific biopolymers. These include spectroscopy, HPLC and electrophoresis. Such approaches have generated considerable fundamental information on the different biopolymer degradation processes. Subsequently, selected bacteria are incorporated into model dough systems and their effect on fundamental physical (rheological) properties evaluated using specialised apparatus. When applied to dough systems, the investigation has determined that the LAB essentially decrease the elasticity (therefore increasing the so-called complex modulus of the material). 
The mechanism by which the elasticity of the material is decreased by the extracellular activities of these bacteria was then elucidated. To achieve this, an in-vitro model system based on pure macromolecules (i.e. the gluten proteins) was developed. These macromolecules were exposed to metabolic end products (principaly lactic acid ). One of the findings from these studies was that acidic conditions increase the complex modulus and also increased the water absorption of gluten. The laser microscopic pictures taken of the material revealed that this was due to a more "open structure". The finding from the above in-vitro study was confirmed in dough, a complex system where a large number of interactions between the various components need to be taken into account. Selected lactic acid bacteria were incorporated into the dough material, the various macromolecules were isolated and the impact of the LAB determined. One of the results of this study was, that the acid conditions created by the LAB lead to a degradation of gluten due to activating indigenous proteolytic enzymes which are naturally present in the cereals. The proteolytic enzymes of the lactic acid bacteria seem to have a relatively minor effect in comparison to these endogenous enzymes. The model system confirmed that the complex modulus of the gluten was increased. Specific so-called "creep tests" performed using a controlled stress rheometer revealed that the relative recovery of gluten was also reduced. This was confirmed by electronmicroscopy.

The question as to how do the above results obtained in the dough material transfer into bread were also addressed. Thus a measuring system was developed by one of the PhD students to quantify the ultrastructural changes in a food materials. This is an image-analysis software programme and it permitted the research group to objectively determine the changes in the foam-like structure of bread. This system has been used extensively and has permitted us to also determine that the shape and the number of air-cells change with the addition of LAB. It was also possible to correlate the image characteristics with the textural and sensory properties of bread. The results also agreed with data from laser-microscopic analysis.

The group has thus shown that LAB change the viscosity of doughs and decrease the rate of staling essentially as a result of released LAB-associated lactic acid increasing the activity of the endogenous enzymes of the cereals. In turn, this leads to a partial breakdown of the gluten in the cereal material.

From the above studies, it is now possible to select specific LAB strains which are highly effective as well as modify processing accordingly. To modify processing conditions most effectively, mathematical modelling has been an essential tool.

Cereal and Beverage Science Research Group

School of Food & Nutritional Sciences, University College Cork, College Road, Cork Ireland