Measuring Lactose Concentrations in Cheese

Measuring Lactose Concentrations in Cheese

Image Credit: YSI

The YSI Model 2900 Series Biochemistry Analyzer can be used to measure lactose concentrations directly and quickly in complex matrices such as cheese. PH, color, turbidity, density, or the presence of reducing substances virtually has no effect on measurements.

Within the sample chamber, a sample is injected, and the lactose diffuses into the membrane containing galactose oxidase. The lactose is immediately oxidized to hydrogen peroxide and a derivative of galactose dialdehyde. At the platinum electrode surface, the hydrogen peroxide is distinguished amperometrically. The current flow identified at the electrode is directly proportional to the hydrogen peroxide concentration, and also to lactose concentration.

I. Materials and Setup

  1. Instrument used: YSI Series Biochemistry Analyzer with a 2702 Galactose Oxidase Membrane and 2705 Buffer.
  2. Lactose standards: 5.00 g/L, 25.0 g/L.
  3. The 2900 Series instrument is connected to an appropriate power source.
  4. Daily checks are performed on the instruments and membrane, as described in the Operations Manual (Section 5).
  5. Volumetric glassware: Class A recommended.
  6. Sample size: 25 μL: the recommended instrument setup.

Probe A Parameters

  • Chemistry - Lactose
  • Unit - g/L
  • Calibrator - 5.00 g/L
  • End Point - 30 Sec

Autocal Parameters

  • Temperature - 1 °C
  • Time - 30 Min
  • Sample - 5 Samples
  • Cal Shift - 2%

II. Limitations

The presence of galactose and other galactosides (raffinose, stachyose) will interfere with the analysis because they are also substrates of galactose oxidase.

III. Methodology

  1. An appropriate dilution factor is determined, and a 1 to 10 g/L lactose sample is injected.
  2. The cheese sample to be analyzed must be accurately weighed, and the sample can be transferred to a beaker. Around 50 mL deionized water is added and the sample is stirred over low heat, at approximately 55°C, for a time period of 10 to 15 minutes. The sample is then transferred to a 100 mL volumetric flask, using deionized water to help in the complete transfer. The flask is filled to the mark with deionized water and inverted for mixing.
  3. The 2900 Series instrument is calibrated with a 5.00 g/L lactose standard solution.
  4. At least once per day, the linearity of the membrane is checked via injection of a lactose linearity check solution (25.0 g/L). These recommendations can be found in the Operations Manual (Section 5) for specifications.
  5. The sample is prepared and assayed in B by aspiration into the 2900 series instrument. The linear range of the system is 0.05 to 25.0 g/L lactose. If the value attained is higher than this, further dilution is needed.
  6. Calibration should be done regularly, as described in the Operations Manual (Section 7).

IV. Calculations

To calculate % lactose, the reported result is multiplied by the appropriate dilution factor. For example, 9.96 g of cheese was prepared as described, and when assayed, the value reported was 2.16 g/L lactose.

Source: YSI

% Lactose:
2.16 g/L x 0.100
L/9.96 mg
= 0.0217 g lactose/cheese
= 2.17% (w/w)

 

Measuring Lactose Concentrations in Cheese

Image Credit: YSI

This information has been sourced, reviewed and adapted from materials provided by YSI.

For more information on this source, please visit YSI.

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