Use of high O2 MAP

Information gathered by the author during 1993-1994 revealed that a few prepared produce companies had been experimenting with high O2 (e.g. 70-100%) MAP and had achieved some surprisingly beneficial results. High O2 MAP of prepared produce was not exploited commercially during that period, probably because of the inconsistent results obtained, a lack of understanding of the basic biological mechanisms involved and concerns about possible safety implications. Intrigued by the concept of high O2 MAP, the Campden and Chorleywood Research Association (CCFRA) carried out limited experimental trials on prepared iceberg lettuce and tropical fruits in early 1995. The results of these trials confirmed that high O2 MAP could overcome the many disadvantages of low O2 MAP. High O2 MAP was found to be particularly effective in inhibiting enzymic discolorations, preventing anaerobic fermentation reactions and inhibiting micro-bial growth. In addition, the high O2 MAP of prepared produce items within inexpensive hermetically sealed plastic films was found to be very effective in preventing undesirable moisture and odour losses and ingress of microorganisms during wet handling situations (Day, 1998).

The experimental finding that high O2 MAP is capable of inhibiting aerobic and anaerobic microbial growth can be explained by the growth profiles of aerobes and anaerobes (Fig. 15.1). It is hypothesised that active oxygen radical species damage vital cellular macromolecules and thereby inhibit microbial growth when oxidative stresses overwhelm cellular protection systems (Gonzalez Roncero and

% Oxygen

Fig. 15.1 Hypothesised inhibition of microbial growth by high O2 MAP.

% Oxygen

Fig. 15.1 Hypothesised inhibition of microbial growth by high O2 MAP.

Product inhibition

Colourless quinones

Substrate inhibition

Coloured melanins

Fig. 15.2 Hypothesised inhibition of enzymic discoloration by high O2 MAP.

Day, 1998; Amanatidou, 2001). Also intuitively, high O2 MAP inhibits undesirable anaerobic fermentation reactions (Day, 1998).

Polyphenol oxidase (PPO) is the enzyme primarily responsible for initiating discoloration on the cut surfaces of prepared produce. PPO catalyses the oxidation of natural phenolic substances to colourless quinones which subsequently polymerise to coloured melanin-type compounds. It is hypothesised that high O2 (and/or high Ar) levels may cause substrate inhibition of PPO or alternatively, high levels of colourless quinones subsequently formed (Fig. 15.2) may cause feedback product inhibition of PPO.

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