Method of commercial sterility test for sterilized/UHT milk/flavored milk


UHT processing and aseptic filling are intended to achieve commercial sterility, microbial defects can occur at any stage of production due to insufficient heat treatment or high contamination of raw materials, resulting in food spoilage spore-forming   microorganisms. In commercial testing, the products are incubated in their final packaging for seven to thirteen days at 30 °C to allow surviving spores or contaminating microorganisms to recover and grow to detectable levels. When the products are intended to be stored at higher temperatures (> 40°C), additional incubation at 55 °C for five to seven days is also used. A second step focuses on detecting viable microorganisms that grew in the product using pH and acidity measurement

Chemicals and Reagents

  • Dilute Rosalina acetate solution
  • Phenolphthalein solution
  • Sodium hydroxide solution
  • pH indicator strips


Incubator- adjusted at 550 degree celcius

PH meter

  1. a) PH
  2. PH test is performed to assess the variation in pH levels during the incubation period of 0 and 7 days incubation.
  3. PH variation can be an indicator of microbial activity which may be leading to acid production.
  4. The determination of pH shall be done as per IS 1479 (Part 1) using indicator strips.
  5. Sample which does not show any physical alteration during intubation at 55±1 °C for 7 days and where the pH does not show a difference of more than 0.3 unit from the initial pH, is considered sterile.
  6. b) Titratable acidity


Acidity of Fresh Sample – Weigh 10.0 g of the sample into each of two white porcelain basins of approximately 60-ml capacity; add to both, 10 ml of water and stir to disperse the sample. Prepare from one dilution a color control by adding and stirring 2 ml dilute rosaniline acetate solution. Stir 2 ml phenolphthalein solution into the other dilution and while stirring vigorously, add as rapidly as possible sodium hydroxide solution from a IO-ml burette fitted with a soda-lime guard tube, until the color matches the pink color of the control. The titration shall be preferably done in north daylight or under illumination from a daylight lamp.

Acidity After Incubation – Incubate. Another 20 g of sample at 55±1 °C for 7 days. Examine the flask each day, then shake and replace it in the incubator. If any physical alteration of the content is observed (coagulation with or without exudation, grittiness, flocculation, formation of bubbles or scum, peptonisation or proteolysis) the results of the test shall be considered positive and the sample as non-sterile.  If no alteration takes place during 7 days incubation remove the sample from the incubator and cool to room temperature. Weigh 10 g of the incubated sample and determine acidity

Expression of Results

  1. a) Based on observation made on pH strip where pH differences not more than 0.3 from the initial pH is regarded as sterile
  2. b) Based on observation made on titratable acidity wherein the difference in TA not more than 0.02% lactic acid is considered as sterile