Essential oils against yeasts and moulds causing food fungi, yeasts and moulds ... In cereals, flours and bakery ... Fungal contamination of heat treated and packaged foods occur with airborne spores,

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  • Essential oils against yeasts and moulds causing food spoilage

    Judit Krisch1, Rentsenkhand Tserennadmid 2, Csaba Vgvlgyi3 1Institute of Food Engineering, Faculty of Engineering, University of Szeged, Mars tr 7., H-6724 Szeged, Hungary 2Institute of Biology, Mongolian Academy of Sciences, Ulaanbaatar-51, Mongolia 3Department of Microbiology, Faculty of Science and Informatics, University of Szeged, Kzp fasor 52., H-6726 Szeged,

    Hungary

    In foodstuffs with low pH, low water activity, or high carbohydrate content spoilage bacteria are, with some exception, usually not present because this environment is unfavourable for their growth. Food-spoilage yeasts and moulds, however, can grow under these circumstances and cause deterioration of various products, such as fruit and vegetable juices and purees, soft drinks, pickled vegetables, dairy products, bread, dried fruits, sausages. Heat treatment and antiseptic packaging exclude yeast and mould spoilage as long as the packaging is intact. Products that cannot be pasteurized are usually treated with weak acid preservatives: sorbic, propionic or benzoic acid or their salts. However, there is a strong consumers demand to avoid or diminish the use of artificial substances in their food. Chemical preservatives also present some problems: it was recently reported that benzene can be formed from benzoic acid in foods by decarboxylating action of some spoilage microorganisms. The use of plant-derived essential oils (EOs) or their components as natural preservatives can overcome these problems. Most EOs are regarded as safe (GRAS) and are accepted by consumers. EOs can be added directly to the food or can be applied in active packaging in vapour phase. Both our experiments and data from the literature showed that EOs and their components increase the lag phases and diminish the maximum cell count in the stationary phase of yeast growth. The colony forming ability of moulds was also reduced or stopped by the EOs. The strong aroma of the EOs can affect the organoleptic properties of the foods but the synergistic combinations of EOs with each other or with other hurdle techniques can reduce this effect. Essential oils represent a natural, effective, and consumer-accepted tool against food spoilage caused by yeast and moulds.

    Keywords yeasts; moulds; essential oils; food spoilage; antifungal

    1. Food spoilage by yeasts and moulds

    Spoilage fungi, yeasts and moulds can grow on raw and processed foods where the environmental conditions for most bacteria are unfavourable (low pH, low water activity, aw). The nutrients and oxygen available in the food are the main factors determining the kind of fungal spoilage. Moulds require oxygen for their growth, but dissolved oxygen in the foodstuffs is more important here than atmospheric oxygen tension. Fermentative yeasts are able to grow without oxygen. Moulds produce a vast number of enzymes: lipases, proteases, carbohydrases for the degradation of complex molecules, and can utilize nitrogen and carbon sources in many forms from nitrates to proteins and from simple sugars to complex carbohydrates. On the contrary, many types of yeast are unable to assimilate nitrate or complex carbohydrates such as starch, and require vitamins for their growth [1]. Ethanol fermenting yeasts, Saccharomyces, Schizosaccharomyces, Zygosaccharomyces strains, cause the deterioration of fruit juices, soft drinks, fruit purees and dairy products. The film-forming yeast Pichia anomala has been reported to cause spoilage in wines, fruit juices, soft drinks, pickled vegetables, yoghurts, and cream-filled cakes. The filamentous yeast Geotrichum candidum can be found in raw milk used for production of soft cheeses and other dairy products, and causes a bitter taste [1-3]. The yeast Endomyces fibuliger, called chalk mold, is an important spoiler of rye bread [4]. Mould growth on raw or processed foods leads to textural and sensorial changes: softening, off-odours and off-flavours. The most important aspect is, however, the formation of mycotoxins. Mycotoxins are secondary fungal metabolites and are toxic to humans and animals, causing severe disorders like cancer, immune suppression, or endocrine disruption. Since mycotoxins are very stable and mainly resistant against heat treatment and acidic environment, they remain in the food during processing and storage, causing a serious food safety problem [5]. The main mould spoilers in fruits and vegetables are Mucor and Rhizopus species from Zygomycetes, and Aspergillus and Penicillium species from Ascomycetes. Alternaria alternata and Botrytis cinerea are also very common causes of fungal rot in fruits. Aspergillus, Penicillium and Fusarium are the main associated fungi of wheat, rye and corn grains under field and storage conditions. P. commune and P. nalgiovense is associated with cheeses, fermented sausages and salamis. Mycotoxins found in juices made from pomaceous or stone fruits are patulin and citrinin. Patulin is a strong antibiotic but it is toxic to humans. In cereals, flours and bakery products ochratoxin A, aflatoxin and the Fusarium toxins: deoxynivalenol (DON), zearalenone and fumonisins can be found [5]. Yeast and mould spoilage results in considerable loss in food supply and enhances food safety problems.

    1135FORMATEX 2011

    Science against microbial pathogens: communicating current research and technological advances A. Mndez-Vilas (Ed.)_______________________________________________________________________________

  • 2. Classic methods for prevention of fungal growth in foodstuffs

    The prevention of fungal growth on crops means spraying fungicides over the fields. Most of these fungicides are synthetic chemicals with a directed site-specific effect on the pests; mainly by inhibiting an important metabolic pathway. Unfortunately, fungi can develop resistance against these fungicides and non-target organisms, insects, birds, mammals are also affected. Sometimes the degradation products of these chemicals are more toxic to humans than the parent molecules. Fungicide residues in foods present a food safety risk. For fruit and vegetable juices and purees, heat treatment, pasteurization, and antiseptic packaging are used to avoid microbiological deterioration. Breads and bakery products are baked at high temperatures but some heat resistant fungal spores can survive. Fungal contamination of heat treated and packaged foods occur with airborne spores, mainly after opening the packaging. In many processed foods where heat treatment is not recommended, but also in heat-treated commodities, preservatives are added to prevent microbial growth. These preservatives are mainly weak organic acids and their salts, or sulphites. Weak acids can penetrate the cell membrane in their undissociated form. In the cell, the acids dissociate and lower the cytoplasmic pH, but glycolysis and respiratory pathways are also affected. Foods with acidic pH are successfully treated with these preservatives because most weak acids are in undissociated form below pH 5 [6]. However, a growing number of consumers refuse the use of synthetic chemicals in their daily food, and there are also other problems with preservatives. Benzoic acid can be transformed by decarboxylation into benzene, one of the most carcinogenic substances. Yeasts and moulds are able to degrade sorbic acid to 1,3-pentadiene, causing a kerosene-like off-odour [5, 6]. S. pombe may produce off-flavours when sulphite is used as preservative in wines. There is growing interest to replace synthetic pesticides and preservatives, at least partly, by natural compounds possessing the same inhibitory effect.

    3. Essential oils

    Essential oils (EOs) are plant-derived volatiles with a hydrophobic character. They are extracted from various plant organs; leaves, fruits, flowers, bulbs, seeds, roots, wood and bark of aromatic plants. EOs are known to possess antiviral, antibacterial, antifungal and insecticide properties [7]. They can have more than 50 components, of which 1-3 are the main components representing 85-95% of the whole volume, while the others are minor components, sometimes below 1%. The chemical character of the compounds influences their antimicrobial efficacy and the mechanism of action on the target organism. The two main groups of EOs are terpenes and terpenoids, and aromatic and aliphatic constituents [8]. EOs have several targets in the cell. Degradation of the cell wall, and weakening the membrane causing enhanced permeability, lead to the loss of intracellular components. In Candia albicans yeast cells, tea tree oil increased the plasma membrane permeability that led to the loss of chemiosmotic control [9]. Lesion formation in the membrane and considerable reduction of ergosterol content (the major sterol component in fungal cell membrane) was reported for Thymus pulegoides (thyme) EO in Candida albicans [10]. Genes involved in ergosterol biosynthesis and sterol uptake were affected by -terpinene, a cyclic monoterpene in S. cerevisiae [11]. EOs can also react with important cell membrane proteins depleting their function [7]. The hydroxyl groups of phenolic and alcoholic EO components can form hydrogen bonds with amino acid residues in the active site of enzymes [12, 13]. Enzymes in the energy regulation can be involved: the monoterpenes -pinene and limonene inhibited the respiratory activity in intact yeast cells and also in isolated mitochondria [14]. Spore formation of Aspergillus species was reduced by lemongrass [15] and cassia, cinnamon or clove EO [16]. Spore germination of Aspergillus species, as well as B. cinerea, Cladosporium herbarum and Rhizopus stolonifer, was also inhibited by lemongrass and oregano EO [17, 18]. Hyphal morphology of Phytopht