Nanostructured Encapsulation Systems: Food Antimicrobials
431
another reason the search for alternative antimicrobials is gaining momentum (Institute of Food Technologists, 2006). II.B.2. Naturally occurring antimicrobials Naturally occurring antimicrobials can be obtained from animal, plant, microbial, and mineral sources. Highly active naturally occurring antimicrobials can be found in spices, herbs, or their essential oils, and include phenolic compounds and their subclasses such as terpenes, coumarins, flavonoids (Gaysinsky and Weiss, 2007). These compounds are thought to constitute an essential part of various biological systems’ defense mechanisms against predators (Hill et al., 1997), parasites (Acamovic and Brooker, 2005), and microorganisms (Hill et al., 1997; Acamovic and Brooker, 2005; Kong et al., 2008). Of particular interest have been plant-derived antimicrobials, e.g. in herbs and spices. An astonishingly high number (> 1340) of plants are known to be sources of antimicrobial compounds, including lemongrass, oregano, clove, cinnamon, palmarose, and others (Chao and Young, 2000; Velluti et al., 2003). For example, studies in the 1980s confirmed the growth inhibition of Gram-negative and Gram-positive bacteria, yeasts, and molds by garlic, onion, cinnamon, cloves, thyme, savory, sage, and others (Shelef, 1983; Conner and Beuchat, 1984; Deans and Svoboda, 1989). The antimicrobial compounds in plants appear to be mostly present in their essential oils. These lipophilic fractions of oils can be extracted from leaves (rosemary and sage), flowers and flower buds (clove), bulbs (garlic, onion), rhizomes (asafetida), fruits (pepper, cardamom) or other parts of the plant by expression, fermentation, extraction, or steam distillation (Lo´pez-Malo et al., 2000; Burt, 2004). Essential oils are composed of a heterogenic mixture of organic compounds such as phenylpropanes, terpenes, and aliphatic compounds of low molecular weight such as alkanes, alcohols, aldehydes, ketones, esters, and acids (Oka, 1964; Brul and Coote, 1999). Generally, essential oils possessing the strongest antibacterial properties are those that contain phenolic compounds such as carvacrol, eugenol, and thymol (Hirasa and Takemasa, 1998; Rota et al., 2004). Essential oils from oregano and thyme, for example, exhibit antimicrobial activity against several foodborne bacteria (Paster et al., 1990). Friedman et al. (2002) reported that the concentration level needed in some spices such as clove, oregano, cinnamon and others, to impart bactericidal activity against E. coli, ranged from 0.046 to 0.14%. The concentration needed to exhibit bactericidal activity with oregano, thyme, clove bud, and others ranged from 0.057 to 0.092%. Clove oil restricted the growth of Listeria monocytogenes in meat and cheese (Menon and Garg, 2001). Thyme essential oils inhibited mycelial growth of fungi