Culture Activity Test

Purpose

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).

Procedure

  1. Mix 10 g of low-heat, antibiotic-free skim milk powder in 90 ml of distilled water in a 100 ml Erlenmeyer flask.
  2. Sterilize at 15 lb pressure (1.05 kPa.) for 10 min.
  3. Cool to 37C.
  4. Inoculate with 3.0 ml starter or other amount as appropriate. Rinse pipette twice by drawing the sterile milk into it.
  5. Incubate at 37C for at least 4 h. Longer if desired for pH versus time profile.
  6. Check pH at 30 min. intervals.
  7. 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.
  8. Record starter activity as follows:
    Active, over 0.34%
    Slow 0.26 to 0.30%

Figure 3.1 Culture Activity Test