Depending on the size and shape, firm to hard cheese should be sampled using a cheese trier (at least 100 g sample) or by taking a sector sample. Soft cheese can be blended for sampling or sector sampled depending on its texture. Cheese samples are stored in opaque air tight containers and fragmented using a grater or other device before analysis. It is important to grind and mix the sample well before subsampling for analysis.
If the analytical procedure requires less than a 1 gm sample it is desirable to prepare a liquid cheese homogenate and a subsample from the homogenate. An homogenate suitable for most purposes can be prepared as follows.
- Weigh 40 g cheese into a blender container
- Add about 100 g of 7% sodium citrate solution
- Blend until homogenous (high speed blender such as Polytron is most suitable).
- Rinse blender shaft into container and make up to final weight of about 200g.
Note that cheese is notorious for inhomogeneous composition. Brine salted cheese have pronounced salt and moisture gradients, namely, higher salt and lower moisture near the surface. Large blocks or wheels of pressed cheese, will have moisture and pH gradients, namely, increasing moisture and decreasing pH towards the interior. In addition to moisture and salt gradients, surface ripened cheese also has pH gradients, namely, pH increases at the surface during curing. These difficulties greatly complicate the matter of obtaining accurate composition and mass balance (yield) data. A useful approach to improve yield control of large blocks is to set aside small blocks (eg., 20 kg blocks of Cheddar) for early composition and quality testing, and subsequently, conduct representative sampling of the large blocks (eg., 240 kg blocks of Cheddar) during the cut/wrap process.
Obtain samples as described above for chemical analysis. Triers or knives used for sampling must be flame sterilized. Samples should be stored in sterile bags such as Whirl Pack bags, stored at 0-4C and analysed within 24 hours.
- Balance, 1,000 g capacity
- Blender container autoclaved or sanitized with 200 ppm chlorine solution for 5 min.
- Break the cheese into small pieces while still in the bag. Use a pestle or similar device if necessary.
- Heat dilution blanks of sterile aqueous 2% sodium citrate to 40C. Transfer 30 g of cheese to sterile blender container, add 270 ml diluent and mix for 2 min. at speed sufficient to emulsify the cheese properly. If temperature exceeds 40C during blending, use a shorter mixing time or decrease initial temperature of citrate solution. This 1:10 dilution should be plated or further diluted immediately.
- Further dilutions can be prepared as required. Pipette 11 ml of the 10-1 dilution of the homogenate, avoiding foam, into 99 ml dilution blank (0.1% peptone) or 10 ml into 90 ml dilution blank. Shake this and all subsequent dilutions vigorously 25 times in a one foot arc. Prepare 10-1, 10-2, and 10-3 dilutions.