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Production of starter concentrates
If the pH is maintained in the region 6.0 - 6.3 by neutralisation of the lactic acid produced by the starter bacteria then the cell population can be increased about 10-100 fold depending on the neutraliser used. Both sodium hydroxide and ammonium hydroxide have been used, use of the latter results in higher cell densities.
The cessation of growth of starters grown in fermentation media under pH control is due to several factors including the accumulation of inhibitory concentrations of:
• hydrogen peroxide
Higher cell densities (greater than 1010 CFU/g) can be obtained by harvesting the cells from the fermentor medium by centrifugation, to give a starter population of 1011 - 1012 CFU/ml. Even higher cell densities can be obtained by freeze drying the 'sludge' obtained by centrifugation. Unfortunately, the increase in cell population for some strains does not necessarily parallel the increase in the ability of the concentrated culture to produce acid. These strains are susceptible to damage during the fermentation, centrifugation and freeze-drying/freezing and storage stages.
An alternative method, which is not being used commercially for producing concentrates and does not require centrifugation, diffusion culture, has been described (Osborne, 1977). The culture medium, containing starter bacteria, is pumped across a suitable membrane. Membranes are now available that can withstand heat and chemical sterilisation. The metabolic by products diffuse across the membrane into fresh medium that is pumped across the other side of the membrane. The major factor determining the effectiveness of this method is the provision of a sufficiently large membrane area to permit adequate diffusion of the rapidly produced metabolic by products. Osborne, (1977) has indicated that 5 cm2 of membrane per ml of fermentor medium are required, together with a fermentor to culture reservoir volume ratio of 1:15. This method is capable of giving very high cell densities.