Biopreservation, an ecological approach to improve the safety and shelf-life of foods

1. Introduction
Modern technologies implemented in food processing and microbiological food-safety standards have
diminished, but not altogether eliminated, the likelihood of food-related illness and product spoilage in
industrialized countries. The increasing consumption of precooked food, prone to temperature abuse, and
the importation of raw foods from developing countries are among the main causes of this situation.
Hence, in Europe, morbidity from foodborne illnesses is second only to respiratory diseases, with
estimates of 50,000 to 300,000 cases of acute gastroenteritis per million population every year [1]. The
7th report (1993-1998) of WHO’s (World Health Organization) surveillance programme for the control
of foodborne infections and intoxications in Europe has documented 5517 of outbreaks of food
poisoning in Spain in that period, with 69553 people affected and 6820 hospitalized [2]. In the USA,
acute gastroenteritis affects 250 to 350 million people annually, and an estimated 22% to 30% of these
cases are thought to be foodborne diseases with the main foods implicated including meat, poultry, eggs,
seafood, and dairy products [3]. According to data from the Centres for Disease Control and Prevention,
it has been estimated that approximately one in four Americans may experience some form of foodborne
illness each year [4]. The bacterial pathogens that account for many of these cases include Salmonella,
Campylobacter jejuni, Escherichia coli 0157:H7, Listeria monocytogenes, Staphylococcus aureus, and
Clostridium botulinum [5]. Until now, approaches to seek improved food safety have relied on the search
for more efficient chemical preservatives or on the application of more drastic physical treatments (e.g.
high temperatures). Nevertheless, these types of solutions have many drawbacks: the proven toxicity of
many of the commonest chemical preservatives (e.g. nitrites), the alteration of the organoleptic and
nutritional properties of foods, and especially recent consumer trends in purchasing and consumption,
with demands for safe but minimally processed products without additives.
To harmonize consumer demands with the necessary safety standards, traditional means of controlling
microbial spoilage and safety hazards in foods are being replaced by combinations of innovative
technologies that include biological antimicrobial systems such as lactic acid bacteria (LAB) and/or their
bacteriocins. The use of LAB and/or their bacteriocins, either alone or in combination with mild physicochemical treatments and low concentrations of traditional and natural chemical preservatives,
may be an efficient way of extending shelf life and food safety through the inhibition of spoilage and
pathogenic bacteria without altering the nutritional quality of raw materials and food products [6-9].
Hence, the last two decades have seen intensive investigation on LAB and their antimicrobial products to
discover new bacteriocinogenic LAB strains that can be used in food preservation.
2. Biological methods for food preservation
Biopreservation, as commented above, can be defined as the extension of shelf life and food safety by
the use of natural or controlled microbiota and/or their antimicrobial compounds [10]. One of the most
common forms of food biopreservation is fermentation, a process based on the growth of
microorganisms in foods, whether natural or added. These organisms mainly comprise lactic acid
bacteria, which produce organic acids and other compounds that, in addition to antimicrobial properties,
also confer unique flavours and textures to food products. Traditionally, a great number of foods have
been protected against spoiling by natural processes of fermentation. Currently, fermented foods are
increasing in popularity (60% of the diet in industrialized countries) [11] and, to assure the homogeneity,
quality, and safety of products, they are produced by the intentional application in raw foods of different
microbial systems (starter/protective cultures). Moreover, because of the improved organoleptic qualities
of traditional fermented food, extensive research on its microbial biodiversity has been carried out with
the goal of reproducing these qualities, which are attributed to native microbiota, in a controlled
environment.
The starter cultures of fermented foods can be defined as preparations of one or several systems of
microorganisms that are applied to initiate the process of fermentation during food manufacture [12],
fundamentally in the dairy industry and, currently, extended to other fermented foods such as meat,
spirits, vegetable products, and juices. The bacteria used are selected depending on food type with the
aim of positively affecting the physical, chemical, and biological composition of foods, providing
attractive flavour properties for the consumer. To be used as starter cultures, microorganisms must fulfil
the standards of GRAS status (Generally Recognized As Safe by people and the scientific community)
and present no pathogenic nor toxigenic potential. In addition, use must be standardized and reproducible
[13]. The same cultures have been employed for different uses and under different conditions. For the
starter cultures, generally LAB, metabolic activity, such as acid production in cheese, is of great
technological importance, whereas antimicrobial activity is secondary. However, for the protective
culture, generally LAB also, the objectives are the opposite and must always take into account an
additional factor for safety as its implantation must reduce the risk of growth and survival of pathogenic
microorganisms [11]. An ideal strain would fulfil both the metabolic and antimicrobial traits.
2.1 Lactic Acid Bacteria
LAB include the genera Lactococcus, Streptococcus, Lactobacillus, Pediococcus, Leuconostoc,
Enterococcus, Carnobacterium, Aerococcus, Oenococcus, Tetragenococcus, Vagococcus, and Weisella
[14]. They form a natural group of Gram-positive, nonmotile, non-sporeforming, rod- and coccus-shaped
organisms that can ferment carbohydrates to form chiefly lactic acid; they also have low proportions of
G+C in their DNA (< 55%). LAB present attractive physiological properties and technological
applications (resistance to bacteriophages [12], proteolytic activity, lactose and citrate fermentation,
production of polysaccharides, high resistance to freezing and lyophilization, capacity for adhesion and
colonization of the digestive mucosa, and production of antimicrobial substances).
In general, LAB have GRAS status and play an essential role in food fermentation given that a wide
variety of strains are employed as starter cultures (or protective cultures) in the manufacture of dairy,
meat, and vegetable products. The most important contribution of these microorganisms is the
preservation of the nutritional qualities of the raw material through extended shelf life and the inhibition
of spoilage and pathogenic bacteria. This contribution is due to competition for nutrients and the
presence of inhibitor agents produced, including organic acids, hydrogen peroxide, and bacteriocins [15].