TRADITIONAL AND NATURAL FOOD ANTIMICROBIAL AGENTS

Antimicrobial agents have been investigated for their effectiveness to kill or inhibit growth of microorganisms in the matrix and on the surface of foods. This has been done with the aim to increase food safety for the consumers, as well as to increase the shelf life of food products. Based on their origins, they can be simply classified as traditional chemical preservatives and natural antimicrobials.

Traditional Chemical Preservative Agents

According to definition of Davidson et al. (2013b), antimicrobials are classified as traditional ones when they (1) have been used by many years, (2) are approved by many countries for inclusion as preservatives in foods, or (3) are produced in large scale by synthetic processes. Commonly used traditional chemical preservatives include acetates, benzoates, lactates, sulfites, nitrites, fatty acids and esters, etc. (Jay et al., 2005). Among these traditional chemical preservatives, one of the most widely studied antimicrobials are organic acids, such as lactic, acetic, and sorbic acids, which will be discussed in this following part (Table 5.1).

As a group of chemicals, organic acids are considered to be any organic carboxylic acid of the general structure R-COOH, including fatty acids and amino acids. Initially, organic acids are used as food additives, and these organic acids have a pKa, the pH at which the acid is half dissociated, between 3 and 5, are usually found to have antimicrobial activity.

The mode of action of organic acids and their derivatives is mainly due to the consumption of ATPase. As depicted in Fig. 5.1, at pH lower than their pKa, more organic acids in the undissociated form exist, which is better able to penetrate the microbial membrane due to the lipophilic property. This undissociated form is prior to dissociate into the free proton and acid anion,

TABLE 5.1 The Structures and Properties of Commonly Used Organic Acids (Davidson and Zivanovic, 2003)

Compound

Structure

pKa

Microbial

Target

Limitations in Food

Acetic acid

4.71

Yeasts,

bacteria

Not exceed GMPa (21CFR184.1005)

Lactic acid

3.79

Bacteria

Not exceed GMP (21CFR184.1061)

Benzoic

acid

4.19

Yeasts, molds

  • 0.1%
  • (21CFR184.1733)

Sorbic acid

4.75

Yeasts, molds

Not limited (21CFR182.3089)

aGMP: Good manufacturing practice.

Mode of action of organic acids and their derivatives. Scheme was modified from Davidson et al. (2013b)

FIGURE 5.1 Mode of action of organic acids and their derivatives. Scheme was modified from Davidson et al. (2013b).

since the interior cytoplasmic pH of bacteria cell is usually neutral. This dissociation causes the acidification of the cell interior. In order to maintain the cytoplasmic pH level to protect compounds such as structural proteins, enzymes, nucleic acids, and phospholipid, bacteria cells have to consume their ATP to extrude the liberated protons to the outer environment. The bacteria cells are finally too exhausted to proliferate, resulting in some degree of bacteriostasis. Derivatives of some weak organic acids likely have similar mechanisms of microbial inhibition.

Acetic acid is the primary component of vinegar. Acetic acid and its salts are considered as some of the oldest food antimicrobials. In general, they are more effective against yeasts and bacteria than against molds. Their activity in foods varies in different studies. The application of 150 mM acetic acid to raw chicken breast meat resulted in 1—2log (CFU/ml) reduction of S. typhi- murium and E. coli O157:H7, while there was no significant effect on the levels of L. monocytogenes (Over et al., 2009) for 12 days storage at 4°C. While Carpenter et al. (2011) stated that 2% acetic acid washes only lowered recoverable numbers of pathogens by 0.6 to 1log/cm2 for E. coli O157:H7 on beef plate, Salmonella on chicken skin and pork belly, and L. monocytogenes on turkey roll. In another study, populations of E. coli O157:H7 and Salmonella on beef trim exposed to 2% or 4% acetic acid were 2.0—2.5log (CFU/g) less than populations on controls after acid application (Harris et al., 2006). Sodium acetate and sodium diacetate (SDA), the salt forms of acetic acid, are approved at levels not to exceed 0.25% of the product formulation (9 CFR 424.21). Fresh salmon slices treated by dipping in 2.5% (w/v) aqueous solution of sodium acetate have 4—7 days more shelf life than that of the control (Ibrahim Sallam, 2007). Frankfurters with 0.25% SDA inhibited L. monocytogenes on surfaces over 40 days of storage at 10° C and the levels of the pathogen were 2.5 log (CFU/cm2) less than controls at the end of storage (Barmpalia et al., 2004).

Lactic acid is synthesized naturally by lactic acid bacteria during fermentation. Similar to acetic acid, lactates are the salt forms of lactic acid, both of which show similar activities. It was found that dipping of salmon slices in aqueous solutions (2.5%) of the sodium lactate (SL) was efficient against the proliferation of various categories of spoilage microorganisms (Ibrahim Sallam, 2007). Combination of SL (2.5%) and SDA (0.2%) was bacteriostatic to L. monocytogenes and bactericidal to S. enteritidis after 20 days at 5°C and 10°C, which was more effective than the salts applied alone in beef emulsions samples of 79% moisture (Mbandi and Shelef, 2001). These authors also reported the enhanced antimicrobial effect of combination of SL (2.5%) and SDA (0.2%) on ready-to-eat meat (Mbandi and Shelef, 2002).

Benzoic acid occurs naturally in cranberries, plums, prunes, cinnamon, cloves, and most berries. Benzoates, the sodium salt of benzoic acid, is much more soluble (66.0g/100ml at 20°C) in water than benzoic acid (0.27% at 18°C) and is much more preferred to use in many cases (Chipley, 2005). It is a commonly used food additive preservative that is listed among the “generally recognized as safe” (GRAS) compounds by the United States Food and Drug Administration (FDA), and can be present in foods at a concentration up to 0.1%. Both of them exhibited inhibitory activity against a wide range of microorganisms, especially fungi, yeasts, and molds (Chipley, 2005). E. coli 0157:H7 was reduced from 5.2log CGU/ml to 0.3log CFU/ml and 1.4log CFU/ml by 0.1% sodium benzoate and potassium benzoate, respectively, at 8°C after 14 days of storage. At 25°C, the population of E. coli 0157:H7 was reduced by 4.8 CFU/ml and 4.0 CFU/ml by 0.1% sodium benzoate and potassium benzoate, respectively, at the end of storage (Ceylan et al., 2004). Incorporation of 0.1% sodium benzoate in mango juice reduced the time to half compared to the control group to achieve 3-log reduction of heat-resistant mold Neosartorya fischeri ATCC 200957 (Rajashekhara et al., 1998).

Sorbic acid is a trans-trans unsaturated monocarboxylic fatty acid that is slightly soluble in water (0.15 g per 100 ml at 20°C), while its salt form is significantly more water soluble (potassium sorbate as 58.2 g per 100 ml at 20°C) (Stopforth et al., 2005). Sorbic acid and sorbates inhibit a wide range of microorganisms, including Acinetobacter, Aeromonas, Alicyclobacillus acido terrestris, Bacillus, Campylobacter, E. coli O157:H7, Lactobacillus, L. innocua, Pseudomonas, Salmonella, Staphylococcus, Vibrio, and Y. enter- ocolitica (Davidson et al., 2013b).

 
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