1. Vacuum-packed products are most often spoiled by anaerobes or facultative anaerobes, for example, through acidification by acid-forming Lactobacillus spp. Certain isolates of Lactobacillus have also been shown to produce hydrogen sulphide bacteria.

Yersinia enterocolitica is a facultative anaerobic bacterium that can survive well in vacuum-packed meat stored at 1 °C; however, in processed meats the addition of potassium or sodium nitrate acts as bactericidal towards Y. enterocolitica.

2. Some bacterial problems that can occur in vacuum-packaged products are:

- gas formation, resulting in an inflated pack

- greening due to:

-sulphmyoglobin formation from reaction with hydrogen sulphide; sulphmyoglobin greening can be associated with growth of Pseudomonas mephitica or Brochothrix thermosphacta, and with poor barrier films which allow small amounts of oxygen into the pack

-the interaction between myoglobin and hydrogen peroxide; hydrogen peroxide greening has been associated with cooked cured meats under aerobic conditions

-'off' odours on opening the bag, such as trace amounts of ammonia from proteolysis

- spots on cured and fermented meats caused by undesirable moulds such as Cladosporium, which causes unsightly, deep-seated black spots on cured hams, or Scopulariopsis, which causes white spots on the skin of hams.

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