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Lactic Acid Fermentation of Fruits and Vegetables: An Overview
Spiros Paramithiotis1, George Papoutsis2 and Eleftherios H. Drosinos1*
1. Introduction
Lactic acid fermentation has been carried out for thousands of years to preserve surplus and perishable foodstuff as well as to enhance them organoleptically. A wide range of fruits and vegetables have been traditionally used to serve as a substrate for lactic acid fermentation, resulting in a wealth of final products, some of which are characteristic of certain geographical areas.
Dominance of lactic acid bacteria in the developing microecosystem is of principal importance for a desirable outcome of the process. Rapid acidification that results from lactic acid bacteria metabolism suppresses growth of antagonistic microbiota. Salting is a processing step that assists in development of lactic acid bacteria during the beginning of fermentation; proper temperature as well as the quality of raw material also play a decisive role. Failure of acidity to develop quickly, due to the unsuccessful control of the aforementioned parameters, may lead to the dominance of antagonistic biota, mainly Enterobacteriaceae, the development of which is perceived as spoilage.
Another effective way to control the fermentation process is to develop starter cultures that have been the epicenter of intensive study, especially regarding the products that hold worldwide commercial significance, i.e. fermented olives, cucumbers, sauerkraut and kimchi. Several bacteriocinogenic strains, that enhance the safety as well as potential probiotic strains that improve the nutritional quality of the final products, have been examined and reported. At the same time, the microecosystem development of several other spontaneously-fermented fruits and vegetables is helping to improve our understanding of lactic acid bacteria in general and of specific products in particular.
The following paragraphs present an overview of lactic acid fermentation in fruits and vegetables. An historical overview, based on ancient Greek and Roman literature, the main attributes of the production procedure, the safety concerns and the nutritional properties, are also presented.
2. Historical Aspects
A huge variety of fruits and vegetables were cultivated and consumed in the East Mediterranean region during ancient times; references to which may be abundantly retrieved from the Odyssey of Homer and from subsequent Symposium of Xenophon, Deipnosophists of Athenaeus, etc. Additionally, certain types or varieties were highly appreciated, such as the pears from Kea, figs from Attica and Rome, turnips from Magnesia, pomegranates from Voiotia, almonds from Naxos, walnuts from Karystos, etc.
Preservation of fruits and vegetables was mostly practiced by salting, dehydration or acidification. Salt was applied as such or in the form of brine, with the latter being documented more often in ancient Greek literature. Plain brine was used for preservation of fish, meat and olives as well as other plant foods, such as the leaves and roots of oilseed rape (Brassica napus L.). Moreover, it was mixed with vinegar as well as other materials such as garlic, thyme, etc. Dehydration was a very common method applied mainly to preservation of fruits such as figs, grapes, dates, etc. Fermentation was often used for production of a series of products such as bread, wine, vinegar, dairy products and beer. More than 50 types of bread are documented in ancient Greek literature, depending on the raw materials, production procedure, and extent and type of fermentation. As for the latter, Atheneaus, in his Deipnosophists mentions the sour taste of some breads — a reference that probably implies the use of sourdough. Wine seemed to be a generic term that is referred to alcoholic fermentation in general. However, the term was used as such only when grapes were taken as substrate of fermentation. Use of other raw materials, either exclusively or mixed with grape, was indicated by an adjective added to the generic term. An extended variety of raw materials were used for that purpose, such as Cydonia oblonga Mill., Malus sp., Punica granatum L., Myrtus communis L., Pistacia lentiscus L., Phoenix dactylifera L., Ficus carica L., Laurus nobilis L., Pinus sp., Origanum dictamnus L., Thymus capitatus L., Satureja sp., Mentha pulegium L., Foeniculum vulgare Mill., Anethum graveolens L., Valeriana celtica L., Petroselinon hortense Hoffm., Daphne gnidium L., Daphne oleides L., Apium graveolens L., giving rise to a tremendous wealth of final products. There are two very special products worth mentioning separately. The first one was made from lotus fruit and the second from dates. The land of the lotus-eaters is mentioned in Odyssey of Homer; it is located in the Gabes Gulf in Tunis. Lotus was a name used to describe a plethora of fruits. However, the fruit mentioned by Homer seems to be Ziziphus lotus L. According to Herodotus and Polybius, lotus fruits were used for production of wine. However, due to their short shelf life of ten days, only small quantities were made each time. Dates have been used for wine and honey production since the 5th century B.C., according to Herodotus and Xenophon. Ancient Babylon was considered the hub of production and through trade practices, these products reached the Mediterranean basin. However, due to lack of mention in literature, it is believed that these products were either uncommon or not appreciated. Similarly, several vinegar types are documented on the basis of their origin or the raw materials used. The use of barley and wheat for production of alcoholic beverages was also known, but less appreciated in ancient Greece. On the contrary, Egypt, Thrace, Iberia and Britain were mentioned by Aristotle, Ekataios and Pedanius Dioscorides as centers where these products were widely consumed. Moreover, Pedanius Dioscorides, in his work entitled De Materia Medica, mentions that these drinks may harm the kidneys and the brain; are diuretic and cause flatulence. This was most probably the reason for their restricted acceptance; wine was much more appreciated as it was considered healthier and without any side-effects.
Regarding lactic acid fermentation of fruits and vegetables as perceived nowadays, it seems to be less widespread mainly due to (a) the availability of fresh fruits and vegetables throughout the year, as mentioned in the texts of the Homeric age, and (b) the preferred use of dehydration and smoking for preservation, as mentioned by Atheneaus in his Deipnosophists. However, there was a product, for which lactic acid fermentation may be inferred; it was called αβυρτάκη (abyrtáke) and the land of Medes (Northern Iran) was reported as its origin, from which it was spread throughout the Mediterranean basin. This product was more like sauce and made by mixing and grading a variety of raw materials, such as cardamom and mustard seeds, leek, garlic, raisins, pomegranate and leafy vegetables. Lactic acid fermentation may be inferred because of (a) the very intense and sour taste that characterized this product. Since the addition of vinegar was not mentioned in any Greek text but only in a few subsequent Roman ones, the existence of a variety of similar products may be concluded, and (b) the requirement of qualified personnel that have received training in the place of origin; ordinary cooks were unable to handle such a product.
Thus, in the east Mediterranean region, while Greek and Roman civilizations sequentially flourished until the first century A.D., fermentation was a common process that was adopted for the production of a range of products of everyday use. Lactic acid fermentation, was also applied and was equally appreciated as alcoholic fermentation, through a wide variety of dairy products and types of bread. Judging from the references, it seems that fruits and vegetables were used marginally as a substrate for lactic acid fermentation.
3. Production Procedure
Production of fermented fruits and vegetables essentially involves the following steps:
• Collection of the raw materials: Depending on the desired final product, the respective raw materials are collected, e.g. cabbage for sauerkraut.
• Primary processing: All essential steps, such as washing, coring, trimming, cutting, peeling, blanching, cooking or other treatments (e.g. lye treatment) are employed prior to fermentation, e.g. lye treatment to hydrolyze oleuropein, a phenolic glucoside that is largely responsible for bitterness in olives and release a proportion of the carbohydrate content of the fruit into the solution. Lye treatment is followed by washing to remove the excess lye solution, so that a suitable final pH value is obtained. In the case of sauerkraut fermentation, chopping of cabbage in an essential step in primary processing.
• Salting: During salt application, fermented fruits and vegetables may be divided into three categories, viz. dry-salted, brine-salted and nonsalted. In dry-salting, salt is applied on to the already chopped raw materials. Brine is formed upon osmotic extraction of water from the plant tissue. The amount of salt depends upon the type of raw material used but generally it ranges between 2.5–5 percent of vegetable weight. Brine-salting is alternatively used for vegetables from which extraction of sufficient amount of water is not possible. As soon as vegetab...
