2013 Potato analytical program

Background

Food Standards Australia New Zealand (FSANZ) has a collection of analytical data from the 1980s to the present that was used to develop nutrient profiles for potatoes consumed during the 2011-12 National Nutrition and Physical Activity Survey (NNPAS) component of the 2011-13 Australian Health Survey (AHS).

In previous national nutrition surveys surveys, potatoes have been reported as a major contributor to vitamin C intakes. However, recent analytical data commissioned by FSANZ in 2008 indicated that levels of vitamin C in cooked potatoes were actually quite low.

In June 2013, FSANZ undertook a small analytical program to collect nutrient data on a range of raw and boiled, peeled and unpeeled potatoes, to check estimates of the likely vitamin C content of potatoes and to determine how vitamin C levels vary in potatoes as a result of washing and boiling.

Sampling

FSANZ selected three types of potatoes for analysis of ascorbic acid (vitamin C): white skinned purchased already washed, white skinned unwashed and pink skinned washed. For each type of potato, twenty potatoes were purchased from a range of outlets including supermarkets and independent grocers. FSANZ purchased the samples in Brisbane on 24 May and in Canberra on 29 May 2013. Potatoes were stored in a cool dark place after purchase.

Preparation and analysis

FSANZ grouped the potatoes into 12 composite samples, with each sample comprising five potatoes purchased from five different locations including two in Brisbane and three in Canberra, as shown in Table 1. FSANZ weighed and photographed the 12 composite samples before sending them to the National Measurement Institute (NMI) where they were received on 5 June 2013.

Table 1: Potato samples selected for analysis

The NMI prepared and analysed the samples on 6 June 2013 in accordance with specific instructions set out by FSANZ. These instructions included how the samples should be washed, cut, cooked and weighed. 

The NMI conducted the analyses at their Melbourne laboratories using methods of analysis that have been accredited by the National Association of Testing Authorities.

Results

FSANZ considered the results of the analyses conduced on 6 June (Table 2 and Table 3 below) with reference to older analytical values. There were minor differences in the analysed vitamin C values between white skinned, pink skinned, peeled and unpeeled potato samples. An unexpected difference was identified with notably lower vitamin C levels in raw potatoes compared to boiled potatoes. While the results for boiled potatoes were in line with older Australian and overseas data, the result of much lower vitamin C in raw potatoes was not.

After consultation with the analysts, we agreed that the most likely reason for this unexpected result was excessive time taken at the laboratory to prepare the five individual samples that formed one composite sample for analysis. Potatoes contain enzymes that degrade vitamin C and this process accelerates when the potato flesh is exposed to air. Exposure to air is hard to avoid when preparing raw potatoes for analysis, especially given the time it can take to prepare a composite sample. As a result, a substantial amount of vitamin C can be lost from raw potatoes during the procedure. Cooking processes, on the other hand, deactivate the enzymes that degrade vitamin C. If the cooking is started quickly, as soon as a potato has been peeled, the potato flesh gets little exposure to air and destruction of vitamin C by enzymes is minimised.

FSANZ considered that the preliminary results from analyses conducted on 6 June indicated too much time had been taken in the preparatory steps of the analysis of raw potatoes. Therefore, a modified sample preparation procedure was developed by FSANZ and NMI where each sample would be from one potato only, homogenised and then analysed immediately.

Additional samples of the same varieties of potatoes were purchased in Victoria by NMI. The NMI undertook a new analysis of raw potatoes on 25 June using the modified preparation procedure, to minimise degradation of vitamin C in raw potatoes.

The results from the second analysis of raw potatoes were in line with older Australian and overseas data and consistent with the levels of vitamin C found in boiled potatoes.

Table 2: Moisture and vitamin C content of raw analysed foods

Table 3: Moisture and vitamin C content of boiled analysed foods

Updates to the food nutrient database

FSANZ used the analytical results from the 2013 program to review its existing data and validate its use of overseas data for a range of potatoes. As a result of this small analytical program:

• An average of the results from the second analysis of raw potatoes was incorporated into the raw potato records developed for the NNPAS.
• Where raw potato records have been used as an ingredient in a cooked food, FSANZ updated the cooked food also.
• An average of the boiled potato results was also incorporated into the boiled potato records previously developed using analytical data.

Conclusion

The results of the potato analytical program enabled FSANZ to refine records in its database with regard to levels of vitamin C in potatoes. The results will also feed into future releases of the FSANZ reference database NUTTAB.