Application of starter cultures to table olive fermentation

Table olives represent one of the most diffused fermented vegetable preparation in the Mediterranean area, but table olive elaboration is also spread in other parts of the world. A chemical (by NaOH)/biological (by microorganisms) de-bittering is needed to remove bitter compounds (e.g., oleuropein and other phenols) to obtain a stable and palatable preparation from a more or less ripe fruit. Even if different ways exist, the process mainly relies on a spontaneous fermentation of olives in brine (Greek style process for black olives), eventually starting after a lye treatment (Spanish style process for green olives) involving a balanced microbial association of lactic acid bacteria and yeasts. In recent years, the interest on nutritional and safety aspects related to table olives consumption is increasing, and many papers report on the importance of obtaining high quality products with enhanced bioavailability of antioxidant phenolic compounds, free from pathogens or not healthy compounds (e.g., biogenic amines) and eventually enriched with probiotic strains. All the above aims can be followed up by the correct management of the fermentation step. Actually, despite their importance and diffusion, table olives are traditionally produced both at artisan and industrial level by empirical processes based on indigenous microorganisms which, depending on the intrinsic condition of the brine (contaminating microflora, NaCl concentration, pH and aw value, presence of antimicrobial compounds, etc.) and extrinsic parameters (temperature of fermentation, oxygen availability, etc.) lead to a more or less high quality product, whose characteristics are often difficult to standardize. On the other hand, many experimental studies report on the possibility to select lactic acid bacteria and yeasts to be used as starter cultures to improve and standardize the product quality, lowering both the time and cost of the process. At present, even if many technological/functional characteristics of putative lactobacilli and yeast starter have been addressed (e.g., capability to hydrolyse oleuropein and resist to the inhibitory action of phenolic compounds, rapid and predominant growth at low temperature, salt tolerance, acidifying, β-glucosidase, esterase activities, etc.) the application of starter cultures in the field of table olives is quite far from reaching the diffusion it has in other sectors of food industry (e.g., dairy products and alcoholic beverages). From that point of view, it is of interest to share the main results of the scientific community in this field, to improve our knowledge on the technological and functional characteristic of putative starter strains, on the factors influencing the dominance and activity of individual or mixed starters in situ as well as on molecular mechanisms at the basis of strains adaptation and cell-cell communication in the complex brine ecosystem.