PACKAGING AND SHELF LIFE OF FERMENTED FOODS

Abstract

Food Packaging scientists are almost exclusively concerned with the negative roles that microorganisms play in foods. The threats for food safety and the risks of precocious spoilage are the main microbial issues they have to face daily, driving their efforts in selecting the best packaging material and good packaging technologies for specific foods and beverages. However, it cannot be neglected that there is an enormous variety of foods where microbial activities are essential features of their production, the main responsible of their sensorial and nutritional value, as well as the determinant of their potential durability. Fermented foods are known and used world widely but in this lecture the focus will be on some Mediterranean fermented foods only. For some of these fermented foods (red wine, fish and fresh cheese) the most recent achievements, obtained by our Department in this special field of research, are shortly presented. [PUBLICATION ABSTRACT]

Headnote

ABSTRACT

Food Packaging scientists are almost exclusively concerned with the negative roles that microorganisms play in foods. The threats for food safety and the risks of precocious spoilage are the main microbial issues they have to face daily, driving their efforts in selecting the best packaging material and good packaging technologies for specific foods and beverages. However, it cannot be neglected that there is an enormous variety of foods where microbial activities are essential features of their production, the main responsible of their sensorial and nutritional value, as well as the determinant of their potential durability. Fermented foods are known and used world widely but in this lecture the focus will be on some Mediterranean fermented foods only. For some of these fermented foods (red wine, fish and fresh cheese) the most recent achievements, obtained by our Department in this special field of research, are shortly presented.

Keywords: Fermented foods, shelf life, safety, packaging.

INTRODUCTION

Microbial activities are essential features in the production of a huge variety of foods, and food scientists call these foods Fermented or Microbial Foods. Nowadays the field of applied biology that uses living organisms to produce or modify food products is currently considered a relevant part of Biotechnology, and Packaging Science should paid much more attention than ever before on this field, for different reasons. One important motive, which here is not dealt with, is that the use of bio-resources and bio-processes in novel packaging materials production is gaining more and more interest; a second key reason, totally consistent with this lecture, is the special care that should be taken to these particular foods in order to preserve their outstanding value and to extend possibly their commercial lives. According to the most common definition, Fermented Foods (FF) are foods produced or preserved by the action of microorganisms. The definition, typically, refers to the fermentation of sugars to alcohol (by yeast) and to other fermentation processes which involve the use of bacteria (mainly lactic acid bacteria); several benefits are generally associated to FF consumption because it is known that they enrich the diet through the development of a diversity of aromas, they preserve the food through lactic acid, alcohol, and acetic acid production, they enrich the biological value of food with essential amino acids, fatty acids, and vitamins accumulation and eliminating, in special cases, possible present anti-nutrients; last but not least, they decrease cooking time and energy consumption also.

Actually what comes out from these general assumptions is not at all the truth, because fermentation can take place and proceed even without living cells and, actually, in 1907 Eduard Buchner from the University of Berlin, received the Nobel Prize in Chemistry for his biochemical researches and his discovery of cell-free fermentation. Moreover, not always all the advantages mentioned are effective and sometime even some risks have been noted in consuming fermented foods. Actually, risks of botulism for improperly fermented animal products, as well as accumulation of biogenic amines in fermented foods are well known. Alaska, for instance, has more cases of botulism than any other state in the United States of America, due to the traditional Eskimo practice of allowing whole fish to ferment for a long time before being consumed. This risk is even exacerbated when an air-tight container is used because the bacteria thrive in the anaerobic conditions created by the package.

HISTORICAL BACKGROUND

Fermented foods are produced and consumed everywhere, in all the countries of the planet. The FF consumption is probably higher in East and Southeast Asia, at least as far as vegetable fermented foods are concerned, but is also strong in Africa, Americas, Middle East, Europe and Oceania. The almost ubiquitary presence of FF in all the countries does not mean they are similarly constituted and/ or manufactured. On the contrary a broad diversity (we can really say biodiversity in this case) distinguishes these products for composition, manufacture characteristics and properties. FF are manufactured using meat or fish, like salami in south Europe or the fermented herring in the north Europe; a lot of fermented products origin from milk, especially in Mediterranean area but also in Eastern Europe where probably the Kefir and Yogurt production started in very ancient time. Largely spread all around the entire world are also several kinds of fermented beverages: beers and wines above all, at least in terms of worldwide production.

Investigating them and their peculiarities, we just write down in scientific language what has been discovered before mankind had any understanding of microbiology. They are an emblematic case of products that anticipated the knowledge: even several thousands of years ago (before any basic knowledge of fermentation process had been gained), it was gradually learned that the conversion of some food products by means of a natural transformation (fermentation) is an effective means of preservation. Really, we can conclude that fermentation is the most ancient way of shelf life extension, discovered and developed when the shelves were not been in use yet.

The earliest evidence of winemaking, which is one of the most ancient biotechnology the men ever used, dates from 8000 years ago, in Georgia, in the Caucasus area and 7000 year-old jars containing the remains of wine have been excavated in Iran. For sure we cannot know who first invented the first FF, but looking for a name to be cited about them we cannot neglect the name of Louis Pasteur. A worldwide well known French scientist (1822-1895), he was a chemist, even if his best achievements have been in the fields of Biology, Food Technology and Medicine.

We are in debt to Pasteur for several different things, many of them are well related to this topic. For instance, we learned from him to talk about microbiology in Food Science, instead of using the term bacteriology proposed and used by other scientists like Robert Koch (1843-1910). Pasteur's doctoral thesis on crystallography demonstrated for the first time the existence of chiral molecules and he did this investigating the grapes fermentation in wine making. We are in debt to him for new cures, both for animals and humans and very famous are his works against rabies and puerperal fever. However, he is best known to the general public for inventing a method to stop milk from causing sickness, a process that came to be called pasteurization and that is also applied after a fermentation process, in order to produce more stable products. Less known but definitely much more related to the topic of this lecture is the work he made on carbonic maceration a winemaking technique, in which whole grapes are fermented in a carbon dioxide rich environment prior to crushing. During carbonic maceration, an anaerobic environment is created by pumping carbon dioxide into a sealed container filled with whole grape clusters. The results of the carbonic maceration are a brilliant red wine production, the maintaining of the original sweetness of the fresh fruit and the selection of proper yeasts. The roles of the gases and the container in this wine making technology, well understood by Louis Pasteur, really represent an anticipation of Modified Atmosphere Packaging and Active Packaging technologies.

OLD AND NEW RESEARCH IN THE FIELD

It's easy to demonstrate that packaging has and always had a very important role in food preservation, even when fermented foods are concerned. However in its very long story, packaging changed its role and importance quite frequently. Starting from the most ancient time, the very first function of packaging (or better, the function of what we can consider a packaging prototype) may be assimilated to an hideaway: objects used to hide and protect foods, intended as essential and valuable products, against predators and enemies. Following the human beings evolution the functions of containing and transporting became more and more important and the need of protecting perishable products became a must when the distance of trading grew. In modern society the hedonistic role of packaging is well known and the definition of silent seller for packaging is worldwide used but, probably, the most intriguing change in packaging role and functions is the very recent shift from a passive to real active role in protecting and presenting foods. This fundamental change took place when it became clearly possible to manage and to control the properties of the packaging materials and the performance of the packages.

However this is not just a today achievement, because about 60 years ago, Dr. Charles Robert Oswin, a British scientist working in British Cellophane Ltd, wrote a fundamental book (nowadays almost lost or forgotten), focusing on the essential role of packaging materials performance (Oswin, 1954). The title of this book (Protective Wrappings) is something particular, being protective wrapping what we call today flexible packaging. Even if published more than 60 years ago, it was facing more or less the same packaging problems as we do today: e.g. high barrier and breathable materials were something to be achieved, and minimally processed vegetable was a big deal and a big potential business (Fig. 1).

Oswin has been the first packaging scientist who put oxygen permeability versus water vapor permeability, getting a single coordinate value to be used as a guide for shelf life optimization. He proposed this theoretical optimization through the so called Oswin's windows and in Fig. 2 we can see one of these windows, the one built up for a fermented meat product. The not masked areas of the diagram in the figure focus the only values of oxygen and water vapor permeability which should be compatible with the maximum shelf life of the product and the letters correspond to specific packaging materials; the window also emphasizes the risks of quality decay due to wrong choices of packaging materials. It's real a pity that Robert Oswin couldn't contribute to the modern packaging era and that he had to deal only with waxed papers, PVDC coated cellophanes and simple polyolefin films. Actually, in the 50s the flexible packaging was quite different from the one we know today and Oswin couldn't know the entire story; in particular the active packaging concept was totally unknown and, in fact, we must wait till the late 80s or better the 90s to read something really pertinent to the shift of the packaging role from passive to active. Packaging is defined as active when it performs some desired role other than to provide an inert barrier between the product and the outside environment (Yam, 2009).

But once again, what seems to be totally new is somehow a heritage, a legacy of the past. The reference is to very old containers, the earthenware potteries that can act as active packaging indeed (Park and Lee, 2010). Earthenware container that has not been fired to the point of vitrification is slightly porous and coarser than stoneware and porcelain; during the firing, the fine particles covering the surface fuse into an amorphous, glasslike layer, sealing the pores of the clay body but very likely also changing the surface properties of the materials and, maybe, inducing a weak infrared emission that can be relevant to some phenomena or process. These containers, whose permeability seems to be modulated by the manufacturing process, are thought to be about 9,000 years old and they are still widely used in the 21st century.

The short presentation of examples of research carried out on FF in the DeFENS (Department of Food, Environmental and Nutritional Sciences) starts from the use of earthenware containers in wine making and re-fining (Piergiovanni et al. , 20 1 0) .

The process of wine making is mainly based on the alcoholic fermentation of grape must by means of yeasts, especially Saccharomices cerevisiae, which transform glucose and fructose present in the fruits, in ethanol, glycerol and several minor, but very important, byproducts. The process of the red wine making is also characterized by a second fermentation which occurs later, reducing the acid level (by the transformation of malic acid in lactic acid), increasing the taste body by production of dextrans and glucans, increasing the flavor complexity (buttery, nutty, honey vanilla, leather, spices). At least three genera of Lactic acid bacteria [Lactobacillus, Pediococcus, Oenococcus) can be responsible for the Malo-Lactic fermentation. In the red winemaking process a third fundamental physical-chemical phenomenon is maceration, already investigated by L. Pasteur as we have seen before. This is the process where the phenolic materials of the grape (tannins, anthocyanins and flavor compounds) are leached from the grape skins, seeds and stems into the must. Maceration is also the process by which the red wine receives its peculiar color. In the production of white wines, grape is pressed, then all solid materials are removed before antocyanins and tannins are released in the must.

The very very ancient way of making, storing and delivering wines was always supported by earthenware containers. Therefore, we evaluated the chemical and sensory differences between a 2 years old red wine produced and refined in earthenware jars and the same wine obtained by means of the current technology in a concrete tank. Significant differences between the two wines are expressed by the color parameters and the levels of anthocyanin fractions as Table 1 shows. All of such data show a higher amount of anthocyanin-tannin polymers. The oxygen supplied to the wine by the earthenware jars certainly allowed proanthocyanidins to effectively polymerize both with themselves and with the anthocyanins as shown by the vanillin index to the proanthocyanidins ratio. Such a behavior can be explained only if the polymerization phenomena were promoted by an effective oxygen transfer in the jar. Both the sensorial evaluations and the analytical determinations performed on the two wines demonstrated that the ancient way of using earthenware jars for grapes fermentation and wine re-fining is still adequate to provide a marketable red wine. Moreover, noticeable differences were observed between the two wines that is reasonable to correlate to the specific performance of the clay tanks used, in terms of oxygen permeability, heat insulation and, possibly, of microorganisms selection: a selection very likely mediated by the specific surface properties of the earthenware.

At the DeFENS, fermented foods are also the object of microbiological researches, focused on fundamental issues like safety and nutrition, as the next examples shortly show. As just underlined, Malo-Lactic fermentation is a fundamental step in red wine making but it is problematic in cold regions. Genetic aspects and phenotypic traits of thirty six Oenococcus oeni strains, were investigated over three consecutive years. Microvinification experiments allowed the selection of strains with potential oenological performances and an interesting capability to grow in cold conditions was confirmed (Vigentini et al., 2009). The research also permitted to identify some Oenococcus oeni strains (see Fig. 3) able to form high level of biogenic amines phenylethylamine (up to 47 mg/ L) and tyramine (up to 36 mg/L), both in cultural broth and in wine, which for this reason pose some concerns in enological applications.

Again about possible presence of biogenic amines in fermented foods, the microbiologists of our Department developed quantitative PCR assay (qPCR) for the specific detection of Morganella morganí, a fish pathogen responsible for the Histamine Fish Poisoning (Ferrarlo et al., 2012). Specific PCR products were identified by melting curve analysis (see Fig. 4), and a reproducible distinct melting point (Tm) of 84°C.

Standard curves were constructed to investigate the linearity and the sensitivity of the method, leading to a highly specific and rapid assay for the detection of M. morgana in tuna fish samples. In the last example proposed (Cocolin et al 2007), two strains oí Enterococcus faecium, M241 and M249, isolated from goat milk, were studied for their capability to produce antibacterial compounds. Goat milk is used in the preparation of traditional fermented fresh cheeses. It was determined that the bacteriocins produced by both strains were active towards Listeria monocytogenes and Clostridium butyricum, and they did not have any activity with respect to other species of lactic acid bacteria. Enterocins A and B were targeted by polymerase chain reaction (PCR) and sequenced, after cloning, in both strains. The bacteriocins contained in the cell free supernatants were stable when subjected to treatments at high and low temperatures or with lipase, catalase and a-amylase.

Lastly, a co-culture experiment with L. monocytogenes in skimmed milk was also performed showing that in presence of the E. faecium strains, the pathogen showed a delay in the growth of about 6 hours and it reached a maximum counts of about two orders of magnitude lower with respect to the control as Fig. 5 shows.

CONCLUSION

These results suggest the possibility to use the strains studied as starter cultures to enhance food safety of dairy products and, more in general, all these examples witnesses how the research in shelf life, in packaging and in fermented foods can be strictly correlated and mutually beneficial.

References

REFERENCES

Centers for Disease Control and Prevention (U.S. federal agency). Why does Alaska have more botulism. http://www.phppo.cdc.gov/phtn/botulism/alaska/alaska.asp. Retrieved 18 July 201 1.

Cocolin, L., Foschino, R., Comi, G., Fortina, M.G. 2007. Description of the bacteriocins produced by two strains of Enterococcus faecium isolated from Italian goat milk. Food Microbiology 24: 752-758.

Ferrario, C1, Ricci, G., Borgo F., Fortina, M.G. 2012. Species-specific DNA probe and development of a quantitative PCR assay for the detection of Morganella morganii. Letters in Applied Microbiology 54: 292-298.

Oswin, R., 1954. Protective Wrappings. CAM Publications LTD. 311 Gray's Inn Road, W.C. London, UK.

Park, K.-Y., Lee, J.-H. 2010. Excellence of soybean paste fermented in Onggi. Proceedings of the International Academy Symposium and Lecture, 15 October 2010. Ulsan, Korea.

Piergiovanni, L., Gabrielli, M., Tirelli, A. 2010. Wine making and re-fining in earthenware containers. An ancient practice still alive and fruitful. Proceedings of the International Academy Symposium and Lecture, 15 October 2010, Ulsan, Korea.

Vigentini, L, Picozzi, C, Tirelli, A., Giugni, ?., Foschino, R. 2009. Survey on indigenous Oenococcus oeni strains isolated from red wines of Valtellina, a cold climate wine-growing Italian area International. Journal of Food Microbiology 136: 123-128.

Yam, K. L. (Ed.) 2009. The Wiley Encyclopedia of Packaging Technology. John Wiley & Sons.