1. What causes vacuum leaks in processed meat packages (even before end of shelf life; all raw materials used are in specification)
Very occasionally, meat from a pack that is dominated by lactic acid bacteria is characterised by an aroma of hydrogen sulphide that rapidly dissipates when the meat is removed from the pack. Certain isolates of Lactobacillus have been shown to produce hydrogen sulphide. If vacuum-packed meat has a pH greater than 6.0, if the storage temperature is 5-10°C, or if there is residual oxygen in the pack due to using a packing film with a high oxygen transmission rate, there may be an increased growth of spoilage bacteria such as Brochothrix thermosphacta, Shewanella putrefaciens, and psychrotrophic enterobacteria. In high pH meat, off odours may be detected when the bacterial count is just over 1 million per g at the surface. These bacteria will cause a range of off odours and off flavours, and in the case of Shewanella putrefaciens,
spoilage is indicated by a greening of the meat surface and a strong hydrogen sulphide odour (like rotten eggs). When Brochothrix is a major component of the bacterial population, the aroma is variously described as ‘cheesy’, ‘dairy like’ or ’bready’. Odours and flavours in high pH meat from spoiled vacuum packs have been described as ‘faecal’ or ‘sulphury’. Hydrogen sulphide and various other sulphur containing compounds have been detected in such meat. Enterobacter and other enterobacteria have been shown to produce these. If off odours are evident when vacuum packs are opened, selective counts of these organisms can be useful in identifying why storage problems have arisen. With the combination of low temperature, low oxygen availability and low pH, lactic acid bacteria will dominate the bacterial flora on the meat, as stated earlier. Other bacteria cannot grow or grow very poorly in these conditions and do not spoil the meat.