In most cases, it was found that flame photometry was well suited for the determination of sodium, potassium and calcium in fresh fruits, using the wet ashing technique as given below.
There are significant differences in the composition between different varieties of the same fruit, although these differences are not usually considered to be nutritionally significant.
In general, calcium was shown to have the greatest variability between different varieties of the same fruit.
Based upon typical serving sizes, most fruits could not be considered as ‘good’ sources of calcium (defined as at least 10% of the Daily Value per serving). Most fruits contain very low levels of sodium and most could be considered as ‘sodium free foods’ (FDA, 1993), thus making fruit a good choice for individuals on sodium restricted diets.
Potassium is also an important element in human nutrition and several fruits such as strawberries, bananas, avocados, cantaloupes, honeydew melon and plantains are considered to be good sources (providing at least 10% of the Daily Value).
Preparation of Sample
A representative amount of fruit is prepared; in the case of individual fruits, a minimum of three is used or in the case of grapes or similar small fruits, 100g is prepared. Where appropriate the fruit is peeled and seeds removed.
The pulp is homogenised and the processed sample is stored in acid-cleaned polyethylene containers.
The prepared pulp samples are prepared in triplicate using a wet ash digestion. Calcium analyses are performed with the addition of 4% 8-hydroxyquinoline to serve as a protecting agent to prevent the formation of refractory species.
Typically, 5g samples are added to cleaned test tubes and 1ml of concentrated
sub-boiling distilled nitric acid is added.
The mixture is heated to 80ºC overnight. The digest is treated with 1ml of 50% hydrogen peroxide, added dropwise and heated at 100ºC for several hours, repeating the process until sample digests are clear.
Digest is heated overnight at 80ºC and 1ml HCl is added and the digests are heated for 3-4 hours at 80ºC.
The digest is filtered using ashless filter paper (Whatman No.41) and diluted to a final volume of 15ml and stored in polyethylene bottles until analysed.
Preparation of calibration standards
The calibration solutions should also contain a similar matrix of 5% v/v nitric acid and 5% v/v hydrochloric acid and prepared with the appropriate organometallic salts in an aqueous stock solution.
Preparation of Standard Graph
Set the flame photometer in accordance to MultiPoint/Single Ion Calibration found on page 24 of the BWB Technologies Installation and Operation Manual, to measure potassium emission. Nebulise the working standard solutions and adjust the controls until steady zero and maximum readings are obtained. Nebulise the intermediate working standard solutions and construct a graph relating raw emission data (known as RAW in BWB the flame photometer) to concentration of all the standard solutions.
1 Miller-Ihli, N.J., ‘Atomic Absorption and Atomic Emission Spectrometry for the Determination of the Trace Element Content
of Selected Fruits Consumed in the UnitedStates’, J. Food Composition and Anal.,
9, (1996), p.301-311.