Application
Note
Tip
Log #117
Applications Tip of the Week
Expressing Concentration on Per Serving Basis
In the analytical laboratory, we usually express the concentration results in units such as molarity (M),
normality (N), parts per million (ppm), mg/L, etc. In food and beverage industries, concentration is often
expressed not in these concentration units, but rather in mg per 100 g, mg per can, mg per serving, or in
percent Daily Value (%DV). The specific reason for the analysis often dictates in which units the
concentration of the ingredient should be expressed. For example, our choice for expressing salt content of
potato chips may be different if it is for quality control or for nutrition labeling. We will discuss here two
options that help express the analysis results in these “servings” units.
Option 1 – Perform analysis, obtain analysis results in concentration units, and then
make calculations to convert to the required serving units.
Example 1 for Solid Food reported on weight/weight basis - Sodium in crushed potato chips
•
3.5 g of potato chips was mixed with DI water in a 1 L volumetric flask.
•
50 mL of the prepared solution was analyzed.
•
Sodium concentration was 25.2 mg/L as displayed on the meter.
•
Now we want this concentration to be reported in “serving” units.
The first step is to identify the required units
•
If the required reporting units are mg per 100 g, calculate sodium (Na) concentration in the potato
chips using the following equation:
mg/100g = (Na conc. found in mg/L)*(volume of volumetric flask in L) *100 g *DF1 =
(Sample weight in g)
=
25.2 mg/L*1 L*100*1 = 720 mg/100g
3.5 g
•
If in the required reporting units are mg per serving, identify the serving size and use it in the above
equation (for example, serving size for potato chips =28 g):
mg/serving = (Na conc found in mg/L))*(volume of volumetric flask in L) *(serving size in g)*DF1
(Sample weight in g)
=
25.2 mg/L*1 L*28 g/serving*1 = 202 mg/serving
3.5 g
Example 2 for Liquids and Beverages reported on a volume bases - Sodium in Fitness Water
•
Sodium concentration in fitness water was 465 mg/L as displayed on the meter.
•
Now we want this concentration to be reported in “serving” units.
The first step is to identify the required units
•
According to Nutrition Facts label, the serving size for this water is 8 oz and the required reporting
units are mg per serving.
Environmental & Process Instruments Division (EPD)
Water Analysis Instruments (WAI)
2/3/2012
Part of Thermo Fisher Scientific
www.thermoscientific.com/waterlibrary
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Next, identify the conversion factor for converting liters to ounces.
•
We can take a look at the Internet and find that 1 US fluid ounce = 0.02957 L.
The final step in the calculations is to multiply the concentration units by that conversion factor and by the
number of ounces in a serving:
mg/serving = 465 mg/L*0.02957L/oz*8 oz = 110 mg/serving
1
Note: If sample concentration is greater than the method range, dilute the sample before the analysis - pipette
a small volume (aliquot) of a concentrated solution into the volumetric flask and dilute it with DI water. In this
case, calculate the dilution factor (DF) as shown below and multiply the analysis result on this dilution factor.
For example, if 200 mL of a sample is diluted in 1000 mL volumetric flask, the dilution factor will be 5.
DF
=
(volumetric flask volume, mL) = 1000 = 5
(aliquot volume, mL)
200
Option 2 – Set standards and sample preparation in such a way that the results are
reported in desired units.
Example 1 for Canned Food - Potassium in Tomato Sauce
Determine by direct method with potassium ISE and express the concentration in mg per serving:
•
The serving size is 1 can (or 100 g, or 1 cup).
•
The expected concentration is 811 mg/serving .
•
Prepare a sample by transferring one serving unit of a sample into a 1 liter volumetric flask and
diluting to the mark with DI water (some samples may require pre-blending in order to be mixed).
•
Prepare calibration standards bracketing the expected sample concentration as usual, in that weight unit
(mg) per liter. For our example, use 100 mg/L and 1000 mg/L calibration standards to bracket the
expected concentration of 811 mg/serving.
•
Perform a two point calibration of the electrode according to the electrode and meter user guides.
•
Transfer 50 mL of the diluted sample to the analysis beaker and add the appropriate ISA.
•
Measure the concentration.
•
The result is now in weight units per serving. No further calculation is necessary.
Note: If it is not practical to use a 1-L volume for a sample preparation, smaller volumes can be used by
mixing proportionally smaller quantities of a sample and DI water than those given above. See table below
for guidance.
Sample quantities for different volume of the volumetric flask
Volume of
volumetric flask, mL
100
250
500
Serving size=100 g
10 g
25 g
50 g
Environmental & Process Instruments Division (EPD)
Water Analysis Instruments (WAI)
Serving size=1 can
0.1 can
0.25 can
0.5 can
2/3/2012
Part of Thermo Fisher Scientific
Serving size=1 cup
NA
0.25 cup (2 oz)
0.5 cup (4 oz)
Serving size=8 oz
0.8 oz
2.0 oz
4.0 oz
www.thermoscientific.com/waterlibrary
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