Microbial Processes in Traditional Table Olive Fermentation Explored

Summary

A new paper in Applied Microbiology dis­cusses the micro­bi­o­log­i­cal processes behind tra­di­tional table olive fer­men­ta­tion, high­light­ing the impor­tance of native micro­biota and local envi­ron­men­tal fac­tors in cre­at­ing unique fla­vors. Yeasts and lac­tic acid bac­te­ria play key roles in trans­form­ing raw olives, pro­duc­ing enzymes, volatile com­pounds, and metabo­lites that con­tribute to the fla­vor, aroma, and tex­ture of the final prod­uct, with fer­men­ta­tion meth­ods and olive cul­ti­var also affect­ing the micro­bial devel­op­ment and result­ing sen­sory pro­file.

A new paper, pub­lished in Applied Microbiology, exam­ines the micro­bi­o­log­i­cal processes under­ly­ing tra­di­tional table olive fer­men­ta­tion. 

While there are stan­dard­ized indus­trial olive fer­men­ta­tion processes, tra­di­tional meth­ods still dom­i­nate arti­sanal pro­duc­tion in many Mediterranean regions. These rely on native micro­biota and local envi­ron­men­tal fac­tors, giv­ing each batch a unique char­ac­ter.

Fermented table olives owe their dis­tinc­tive fla­vor, aroma and tex­ture to a suc­ces­sion of micro­bial processes. 

Yeasts and lac­tic acid bac­te­ria are the dom­i­nant organ­isms, work­ing in stages to trans­form raw olives into palat­able prod­ucts. The exact micro­bial com­po­si­tion and sequence of each stage are crit­i­cal to the out­come, as dif­fer­ent strains con­tribute uniquely to the ulti­mate sen­sory attrib­utes of the olives.

The authors iden­ti­fied a total of 97 yeast species and 45 lac­tic acid bac­te­ria species. Yeasts typ­i­cally dom­i­nate the early stages of fer­men­ta­tion. Species such as Candida, Pichia and Saccharomyces secrete enzymes that reduce bit­ter­ness by break­ing down oleu­ropein, a harsh phe­no­lic com­pound.

At the same time, yeasts gen­er­ate a range of volatile com­pounds and metabo­lites, such as ethanol, glyc­erol, carotenoids and toco­pherol, many of which con­tribute to the fla­vor pro­file, con­tribut­ing, for exam­ple, fruity or flo­ral notes. 

These com­pounds also affect tex­ture. In the case of glyc­erol, this is due to an increase in the vis­cos­ity of the brine, which can result in a smoother tex­ture for the olives.

As fer­men­ta­tion pro­gresses, lac­tic acid bac­te­ria become the dom­i­nant species. Others, such as Lactiplantibacillus plan­tarum, L. pen­to­sus, Leuconostoc and Pediococcus acid­ify the envi­ron­ment by con­vert­ing sug­ars into lac­tic acid, which low­ers pH, inhibits spoilage organ­isms, and intro­duces a tangy fla­vor. 

These bac­te­ria also con­tribute directly to aroma through the pro­duc­tion of com­pounds includ­ing diacetyl, which adds but­tery notes, and ethyl lac­tate, which intro­duces mild fruity tones.

They also play an essen­tial role in tex­ture. The enzymes they release soften the olive flesh over time by inter­act­ing with the cell walls. The grad­ual nature of this process soft­ens the flesh while main­tain­ing struc­tural integrity.

The rela­tion­ship between micro­bial com­po­si­tion and sen­sory pro­file also varies between fer­men­ta­tion meth­ods. 

Spanish-style green olive fer­men­ta­tion meth­ods use lye treat­ment and con­trolled brin­ing. This encour­ages rapid lac­tic acid bac­te­r­ial col­o­niza­tion and lim­ited yeast diver­sity, result­ing in cleaner, more sour pro­files.

Natural Greek-style meth­ods rely on spon­ta­neous micro­bial activ­ity with­out the use of lye. This results in greater diver­sity and longevity among both yeasts and bac­te­ria, which cor­re­sponds to richer, more com­plex fla­vor and aroma pro­files with greater vari­a­tion.

In addi­tion to the fer­men­ta­tion method, the olive cul­ti­var and har­vest time also affect micro­bial devel­op­ment and, con­se­quently, the char­ac­ter of the result­ing table olives. 

Olives from ear­lier har­vests con­tain higher phe­nol lev­els and lower sugar lev­els, result­ing in sharper, more acidic fla­vors. Those from later har­vests result in greater alde­hyde con­tent, which is asso­ci­ated with ​“green” fruity aro­mas.

The same processes that are respon­si­ble for the dis­tinc­tive sen­sory qual­i­ties of tra­di­tional table olives also enable their preser­va­tion. As with other ancient fer­mented food prod­ucts, tra­di­tional olive preser­va­tion is more a mat­ter of inher­ent micro­bial ecol­ogy than of design.

During spon­ta­neous fer­men­ta­tion, the micro­bial envi­ron­ment evolves in stages. In the ini­tial phase, the envi­ron­ment is vul­ner­a­ble to the growth of bac­te­ria such as Enterobacteriaceae, which thrive in the rel­a­tively high pH and nutri­ent-rich con­di­tions. 

As lac­tic acid bac­te­ria become estab­lished, the acid they pro­duce through sugar fer­men­ta­tion low­ers the pH to below 4.2, cre­at­ing con­di­tions under which most such pathogens and spoilage organ­isms are unable to sur­vive.

They also pro­duce a vari­ety of other sub­stances that inhibit the growth of com­pet­ing microbes. These include hydro­gen per­ox­ide and pep­tidic tox­ins called bac­te­ri­ocins. Combined with acid­i­fi­ca­tion, this antimi­cro­bial activ­ity pre­vents the pro­lif­er­a­tion of dan­ger­ous pathogens such as Clostridium bot­u­linum and Listeria mono­cy­to­genes.

Yeasts alter the envi­ron­ment in dif­fer­ent ways. Their meta­bolic activ­ity low­ers oxy­gen lev­els, inhibit­ing oxi­da­tion and lim­it­ing the sur­vival of aer­o­bic organ­isms. Some strains also pro­duce ethanol and other volatile com­pounds that dis­rupt the growth of spe­cific micro­bial species.

Over time, these nat­ural processes shift the micro­bial ecosys­tem toward a sta­ble, self-lim­it­ing state in which rel­a­tively few pathogens or spoilage organ­isms sur­vive. 

This is in sharp con­trast to ster­il­iza­tion-based preser­va­tion meth­ods, which rely on the erad­i­ca­tion of all liv­ing organ­isms, often accom­pa­nied by a con­cur­rent reduc­tion in nutri­tional value.

The authors note that the study of tra­di­tional olive fer­men­ta­tion, espe­cially Greek-style fer­men­ta­tion, has already led to advances in food pro­duc­tion and preser­va­tion tech­niques, as well as other fields of research.

• Swiss Agricultural Research