**1. Introduction**

Table olives are the most important fermented vegetables because of their worldwide economic importance. The three main techniques for table olive production used in Italy concern 82% green olives, 16% black olives and 2% processed at the cherry ripened stage (UNAPROL, 2008). There are three main trade preparations of table olives: Spanish-style olives, Californian-style olives and naturally black or turning colour olives (Garrido-Fernández et al., 1997). The Spanish processing method includes treatment with sodium hydroxide solution, washing, brining, fermentation and packaging. The Greek-style method is milder and includes washing, natural fermentation in brine, air-oxidation for colour improvement, and packing. The Californian method includes lye treatment, washing, ironsalt treatment and air-oxidation, canning and heat treatment. This last method includes a final sterilization, so it is usually considered as a more safe production. Besides these most prominent preparations, there are many other traditional table olive elaboration recipes that are less known in the international market (Panagou et al., 2003) but very relevant at local market level, where they are frequently sold in in glass jars or plastic pouches.

In the South of Italy the main traditional process method for table olive production is the natural olive process, according which untreated, generally green, olives are washed, put into containers and then filled with freshly prepared brine. Both treated and natural (untreated) olives have to fermented, and in order to enhance their safety extent, the current practice requires the reduction of pH to a value of 4.5 or below. The fruits are maintained in the brine until they lose their natural bitterness, at least partially (Arroyo-López et al., 2008a), and where they undergo the fermentation process whose characteristics depend on the cultivar and on the applied conditions. At the end of the process the olives acquired typical characteristics of final products.

Olives contain a significant amount of oil, ranging from 12% to 30%, depending on the considered cultivar. The fermentable carbohydrates of flesh olives generally ranges from 2% to 6%, however, when the olives are washed or lye treated sugars are also lost along with

© 2012 Romeo, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Romeo, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

other soluble compounds. Olive fermentation is considered to be over when the sugars are totally consumed by microorganisms. The months necessary for this process might change depending on several factors, such as the variety and olive size, the salt concentration and temperature (Cardoso et al., 2010). Nowadays there are neither physico-chemical nor microbiological controls to objectively determine the end of fermentation and producers decide, according to personal criteria, when olives are ready to eat (Hurtado et al., 2008).

Microbiological Aspects of Table Olives 323

The control of salt, temperature, anaerobiosis (or low oxygen percentage) and process hygiene is necessary for successful fermentation. Under appropriate conditions, most nonlactic acid bacteria will grow slower than LAB that are less affected and that will grow and rapidly produce acid compounds, mainly lactic acid. These acids, along with CO2 that may also be produced, create an even more stringent environment for competitors. So the

In the past, fermentation was always considered to be an economical means for temporary preservation of different kind of foods. Nowadays, the consumption of fermented foods is also promoted because of their health benefits, nutritional value, sensorial properties and functionality. This last aspect has been studied for some foods, such as yogurt and fermented milk (Lavermicocca et al., 2005), but it must be improved with regards to vegetable fermentation. The challenge of the next years must be the enhancement of this

The positive role of yeasts in table olive fermentation has recently been reconsidered. Yeasts are especially relevant in directly brined green and black natural olive fermentations, where fruits are not treated with NaOH solutions. In these conditions, in the first fermentation step the LAB growth is slow because of the presence of phenolic compounds in brine. Growth of oxidative yeasts and molds may occur in brine surfaces if the tanks are open. To prevent this growth, the air layer between the liquid and the top of tank must be reduced as much as

The main roles of yeasts in the processing of fermented olives, are associated with the production of alcohols, ethyl acetate, acetaldehyde and organic acids, compounds that are relevant for the development of taste and aroma and for the preservation of the typical

Some yeast species seem to improve the growth of LAB. Yeasts are able to synthesize substances such as vitamins, amino acids and purines, or breakdown complex carbohydrates, which are essential for the growth of *Lactobacillus* species that request a nutritionally rich environment for optimal growth (Viljoen, 2006). However, in table olive processing yeasts can also produce spoilage such as off-flavour production, clouding of

Recently, molecular methods have been applied for the identification of yeast associated with table olives. These techniques confer a higher degree of accuracy in the final identification than classical biochemical methods. Deiana et al. (1992) observed that the species found depended on the degree of maturation of the olive fruits. The genera *Candida, Pichia, Rhodotorula, Saccharomyces, Debaryomyces, Kluyveromyces, Kloeckera, Torulopsis, Trichosporon* and *Cryptococcus* were found by several authors (Arroyo-López et al., 2008a; Rodríguez-Gómez et al., 2010). While the main frequently isolated species, in both naturally

competitors disappear while LAB overcome the lactic acid fermentation.

research field and the design of new functional foods.

**2. Microorganisms associated to table olives** 

characteristics of this fermented food (Alves et al., 2012).

brines and softening of fruits (Arroyo-López et al., 2008a).

**2.1. Role of yeasts** 

possible.

Other important components of olives are polyphenols. The most important classes of phenolic compounds in table olives are phenolic acids, phenolic alcohols, flavonoids, and secoiridoids (Sousa et al., 2006). The olive phenols are nutritionally interesting due to their antioxidant activities, moreover, these compounds are determinant in the shelf-life of olive oil and sensory qualities of both oil and table olives. Some of these, such as oleuropein and its hydrolysis derivatives, have antimicrobial activities against a wide variety of microorganisms, including lactic acid bacteria (LAB). The inhibiting effect of many polyphenols on LAB growth has been widely studied (Fleming et al., 1973; Ruiz-Barba et al., 1993). Moreover, the increase of oleuropein content in the growth medium reduces the activity of bacteria to hydrolyse this glycoside (Romeo & Poiana, 2007). Recently several studies on antimicrobial activity of olive products have been carried out, namely with olive leaves, olive fruits and their pure compounds, such as oleuropein, hydroxytyrosol and aliphatic aldehydes (Sousa et al., 2006) and it has been found that ferulic acid exhibits toxicity effects toward several microorganisms (Sayadi et al., 2000).

Compared to the relatively few microbial species employed in other fermented foods, microorganisms evolved in vegetable fermentations are many and different. In olive fruits the epiphytic microbial population consists of yeast, fungi, and both Gram positive and Gram negative bacteria but throughout the fermentation process, *Enterobacteriaceae*, LAB and yeasts are the most relevant microorganisms (Garrido-Fernández et al., 1997).

It has been generally established that LAB are responsible for the fermentation of treated olives. While LAB and yeasts compete for the fermentation of untreated olives, and in some cases yeasts can be exclusively responsible for fermentation on untreated olives.

LAB usually isolated from fermentation brines of treated olives include both heterofermentative and homofermentative species (Hutkins, 2006), and *Lactobacillus plantarum* is considered essential to produce the lactic acid needed for preservation and typical flavour. *L. plantarum* generally coexists with a yeast population until the end of the fermentation process and during storage, although other microorganisms may be involved depending on the applied parameters of the process. Organic acids, such as lactic acid, and sodium chloride are primary preservatives for table olives. Olives show a water activity greater than 0.85 and a final pH close to 4.6 or below, which represent the most important hygienic limit to avoid microbiological risks for consumers.

The control of temperature during the fermentation steps often led to beneficial effects, especially in those region where the fermentation temperature follows environmental fluctuations. Unfortunately, in most companies the temperature control is not applicable because it is an expensive procedure.

The control of salt, temperature, anaerobiosis (or low oxygen percentage) and process hygiene is necessary for successful fermentation. Under appropriate conditions, most nonlactic acid bacteria will grow slower than LAB that are less affected and that will grow and rapidly produce acid compounds, mainly lactic acid. These acids, along with CO2 that may also be produced, create an even more stringent environment for competitors. So the competitors disappear while LAB overcome the lactic acid fermentation.

In the past, fermentation was always considered to be an economical means for temporary preservation of different kind of foods. Nowadays, the consumption of fermented foods is also promoted because of their health benefits, nutritional value, sensorial properties and functionality. This last aspect has been studied for some foods, such as yogurt and fermented milk (Lavermicocca et al., 2005), but it must be improved with regards to vegetable fermentation. The challenge of the next years must be the enhancement of this research field and the design of new functional foods.
