Culture Activity Test
This simple test is useful to ensure that cheese cultures have adequate activity before inoculating the cheese vat. For most cheese a general rule of thumb is that the activity and amount of inoculum should be sufficient to produce a titratable acidity of about .34% lactic acid, in 10% reconstituted skim milk, after 4 h of incubation at 37C. The test is also useful to compare types of cultures or bulk cultures prepared under different conditions. For these purposes a pH versus time chart is quite useful . A further application is to check sensitivity of the culture to bacteriophage in the plant (see next section Detection of Bacteriophage and Figure 3.1).
- Mix 10 g of low-heat, antibiotic-free skim milk powder in 90 ml of distilled water in a 100 ml Erlenmeyer flask.
- Sterilize at 15 lb pressure (1.05 kPa.) for 10 min.
- Cool to 37C.
- Inoculate with 3.0 ml starter or other amount as appropriate. Rinse pipette twice by drawing the sterile milk into it.
- Incubate at 37C for at least 4 h. Longer if desired for pH versus time profile.
- Check pH at 30 min. intervals.
- Titrate 17.6 ml with N/10 sodium hydroxide (NaOH) using 1 ml phenolphthalein. Divide the required ml of NaOH by 2 the obtain titratable acidity in units of percent lactic acid.
- Record starter activity as follows:
Active, over 0.34%
Slow 0.26 to 0.30%
Figure 3.1 Culture Activity Test