Evaluation of Tough Surface of Potatoes Served at Swedish Schools

Klara Sjölin* (1) and Dr. Jeanette Purhagen (1,2)

(1) Lund University, Department of Food Technology, Engineering and Nutrition, Lund, Sweden
(2) Perten Instruments AB, Garnisonsgatan 7a, 254 66 Helsingborg, Sweden
*Corresponding author: klara.sjolin@food.lth.se


Potatoes served at Swedish schools sometimes develop a tough surface, which is considered impaired quality. In this study, the tough surface of industrially pre-treated potatoes of the Fakse variety were prepared by steam cooking or conventional boiling and evaluated by tex-ture analysis. Depending on whether the results are analyzed based on peak force, gradient, or peak distance, the results differ. In total the results show that pre-treatment has more im-pact on quality than cooking method. However, further studies need to be done to identify the exact parameters that consumers perceive as reducing quality.


In Sweden, all children are served a free hot lunch every day. Several times per week potatoes, which are a staple food in Sweden, are one of the main components of the meal. In re-cent years the overall consumption of potatoes in Sweden has decreased, and it also has at Swedish schools (Jordbruksverket & Statistiska Centralbyrån, 2012). One reason for this is most likely the increasing awareness of quality. Unfortunately, the quality of potatoes served at Swedish schools is often considered poor, which increases the risk of unnecessary food waste and reduced nutritional intake among Swedish children (Svensk Potatis AB, 2015).

The potatoes are usually prepared in large-scale kitchens. The type and condition of the equipment varies. Usually, the potatoes are prepared by steaming cooking (SC), since the handling is easier compared to conventional boiling (CB) in a boiling vessel. It has been shown that SC gives rise to a firmer texture than CB in general (Thybo, et al., 1998), and the general opinion in Sweden is that SC also contributes to a tougher surface (Svensk Potatis AB, 2015).

To increase the efficiency of the cooking, the potatoes are usually industrially pre-peeled a few days before being delivered to the kitchens. Pre-peeling and storage temperature affects susceptibility to enzymatic browning caused by polyphenol oxidase for several kinds of fruits and vegetables, including potatoes (Kahn, 1977, Vitti, et al., 2011). One method to avoid en-zymatic browning is treatment with preservatives. Different kinds of acids and/or sodium metabisulphite (SMS) are commonly used. The acids reduce the pH and have an antioxidant effect. SMS also causes a slightly reduction in pH and acts as an antioxidant in combination with a bleaching effect. SMS has been used widely due to its efficiency and low price. In the past years though, health risks and allergies connected to sulfites have been studied, which resulted in restrictions in use of sulfites as a preservative (FDA, 1994).

Another drawback with pre-peeling is that the quite rough peeling methods, eventually in combination with preservatives, might cause an unpleasant, tough layer at the surface of the tuber. According to Kaack, Kaaber, et al. (2002), subsurface hardening depends on formation of suberin at the surface, which seems to increase if the tubers are treated with acids. Svensson (1971), on the other hand, suggests that SMS prevents the formation of suberin, but the tough layer is still formed. One theory is that the activity of the enzyme pectin meth-ylesterase increases, which causes cross-linkage of pectin causing the tough surface (Kaaber, et al., 2007).

The tough surface of potatoes can be evaluated by sensorial analysis by a panel or mechani-cally by a texture analyzer. To evaluate the cooked potatoes mechanically, the peak force is usually used to determine the hardness of the surface (Calder, et al., 2011, Kaack, Larsen, et al., 2002, Ross, et al., 2010), but other parameters such as peak distance and gradient could be used to evaluate the quality.
The aim of this study was to evaluate subsurface hardening as a measurement of quality of boiled and steam-cooked potatoes by using a texture analyzer. Both industrial and hand-peeled tubers were investigated.

List of abbreviations
SC  steam cooking
CB  conventional boiling
SMS sodium metabisulfite

Materials and Methods

Potato samples
Potatoes of the variety Solanum tuberosum cv. Fakse were used for analysis. All tubers were grown in the south of Sweden in the same field.

Preparation of samples
The tubers were pre-treated with similar equipment as for industrial pre-treatment, but on a smaller scale. The tubers were washed and then peeled with an abrasion peeler for 2 min, followed by peeling with a knife peeler for 40 s. The tubers were then treated with a solution consisting of 0.1% citric acid and malic acid at 8°C for 20 min, followed by treatment with 1.5% SMS for 30 s at 14°C. The tubers were stored in plastic bags at 8°C for 24 h until analy-sis. As reference, tubers were hand-peeled right before analysis.

Cooking techniques
Steam cooking
A combi-steamer (SCC WE 61, RATIONAL AG) was preheated to 100°C with 100% steam. The industrially pre-treated tubers and the reference tubers were loaded into the oven, and cooked until the center had reached 96°C. The temperature was monitored both with the built-in temperature sensor and by external thermocouples (type K).

Conventional boiling
Industrially pre-treated and reference tubers were soaked in boiling water in lab scale, and cooked until the center had reached 96°C. The temperature was monitored by thermocouples (type K).

Texture analysis
Texture analysis was performed with TVT-300XP (Perten Instruments AB) equipped with a 7 kg load cell. The tubers were cut on one side, and the flat side was placed downwards on the instrument board. A 2 mm cylindrical probe was used to penetrate the tuber 9 mm with a rate of 1 mm/s. The required force was monitored. For each variety and treatment, 5 tubers were analyzed with 5 or 6 tests per tuber.

Results and Discussion

 In Figure 1, two industrially pre-treated tubers cooked with different cooking techniques can be seen.

Figure 1: Industrially pre-treated tubers cooked with different cooking techniques: CB to the left and SC to the right.

A layer separating from the rest of the tuber can be seen on the surface of the conventionally boiled tuber. This was not detected for the steam-cooked tubers on a visual inspection.

An example of the results from the texture measurements of a steam-cooked reference sample can be seen in Figure 2. The peak force was observed within 3 mm from the surface of the tuber. For some of the samples there were difficulties in determining the correct peak force due to non-specific peaks. In total 106 measurements were performed. For 7 meas-urements the peak force could not be determined properly, and therefore the data was ex-cluded from the study. The number of data points for each set-up ranges from 21 to 26.

Figure 2: Example graph from steamcooked reference sample

The results were evaluated in terms of peak force, gradient for the first third of the slope until the peak force, gradient to peak force and peak distance, see Figure 2. The results are pre-sented in Table 1. Depending on the chosen evaluation criteria, the results differ. According to peak force, the industrially pre-treated tubers cooked by CB have a significantly harder sur-face, while evaluation of the gradient shows that it is the industrially pre-treated tubers cooked by SC that differ. According to peak distance, there is a difference depending on pre-treatment, showing that industrially pre-peeled tubers get a more elastic surface, while cook-ing method does not affect the result.

As can be seen in Table 1, the gradient differs for some set-ups depend on which part of the curve is studied. In the graph in Figure 1, it is clear that the slope decreases closer to the peak force. At this position, the probe is probably affected by the texture underneath the tough sur-face. This part of the potato is softer, and therefore requires less force to penetrate, which results in a lower gradient closer to the peak.

Since the gradient is the ratio of peak force and peak distance, and both of these are sup-posed to be small to obtain a high quality, there is most likely an optimum for the value of gra-dient. However, tubers of poor quality, where both peak distance and peak force have larger values, can still have the optimum gradient. The gradient is therefore not suitable for deter-mining the quality of boiled potatoes.

Table 1: Evaluation of subsurface hardening based on peak force, gradient for the first third and the total gradient of the slope until peak force, and peak distance. Means of 21-26 repli-cates. 

Values within a column not having the same letter (a-b) and a row (x-y) are significantly different (p<0.05).

Even if the evaluation of the tuber based on peak force and peak distance does not agree completely, it is clear that the tough surface depends more on pre-treatment than cooking method, and that it is increased by industrial pre-treatment.


Pre-treatment and cooking methods affect different properties of the tuber. Therefore it is critical to identify the purpose of the analysis in detail, and also which parameters represent quality, and thereafter select one or several of them. Probably both elasticity and hardness are related to quality loss, but this has to be confirmed by sensorial analysis where the con-sumer determines the quality of the tubers in combination with mechanical analysis.


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