Avaliação do poder antimicrobiano de emulsões de óleos vegetais em água

Mestrado em Engenharia Alimentar - Instituto Superior de Agronomia

Vegetables in their natural state are susceptible to spoilage by microorganisms at a rate that depends on some intrinsic and extrinsic factors. Consumers are increasingly demanding for convenient ready-to-eat vegetables and with fresh like quality, not only because their nutritional reasons but also due to the increasing participation of women in the labor market. Food contamination is a enormous public health problem, but it could be controlled by the use of natural preservatives, such as essential oils, instead of synthetic ones. Essential oils are the odorous, volatile products resulting from the secondary metabolism of herbs, usually produced in special cells or group of cells. The essential oil composition can vary due several factors such as climate, soil composition, plant organ, age, vegetative cycle stage and extraction method. The antimicrobial effect of essential oils is attributed to a small number of terpenoid and fenolic compounds like thymol, carvacrol and eugenol. This compounds act directly on the cytoplasmic membrane, causing changes in its structure and functions. Essential oils have a large antimicrobial spectrum against bacteria, yeasts and molds. However, Gram negative bacteria are less sensitive to the antimicrobial activity of essential oils due to the lipopolysaccharide outer membrane of this group, which restricts diffusion of hydrophobic compounds. Essential oil of oregano (Origanum vulgare) is widely used as a flavoring component in pizzas, lasagnas and sauces, and can be effective against pathogenic bacteria. Sage oil (Salvia lavandulaefolia) is used in food industry, and pharmaceutical receipts. Rosemary (Rosmarinus officinalis) has been used in food as a flavoring, antioxidant, antiseptic and preservative to prevent the attack of fungi and other microorganisms. An emulsion is a colloidal dispersion composed of two immiscible liquids (usually oil in water) where one liquid is dispersed in another in the form of small spherical droplets. To improve the quality of procedures using essential oils, we use solvents, detergents or emulsifiers agents such as Tween 80, in order to facilitate their dispersion on culture medium. The aim of this study was to evaluate the antimicrobial effect of three essential oils, already referred, against one yeast (Yarrowia lipolytica), one Gram negative bacteria (Pseudomonas aeruginosa) and one Gram positive bacteria (Listeria monocytogenes), in order to appreciate their possible application on salads to control the microbial flora. In this work the dilution method in liquid medium was used, and the antimicrobial activity was determined by counting the viable colonies in agar. V Yarrowia lipolytica showed the best results since its minimum bactericidal concentration was 0.235% (v/v) for Origanum vulgare and 6.336% (v/v) for Salvia lavandulaefolia. As the minimum inhibitory concentration of Rosmarinus officinalis was already relatively high (5.625% (v/v)), its minimum bacterium concentration was only tested until 22.5% (v/v). Pseudomonas aeruginosa proved to be the less sensitive microorganism to the antimicrobial effect of essential oils. Its minimum inhibitory concentration was too high to be used in salads and it was only checked for the Origanum vulgare oil, but just to 0.12% (v/v) with no antimicrobial effect shown. Listeria monocytogenes showed good results with Rosmarinus officinalis oil, and its minimum bactericidal concentration was 3.75% (v/v). Values about 0.051% (v/v) for Origanum vulgare showed no antimicrobial effect against this microorganism