Texture and Color of Crisps in Relation to Sugar – and Acrylamide Content during Storage

Mariane Lakehal-Ayat (1) Prof. Eva Tornberg *(1) and Jeanette Purhagen (1,2)

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

Abstract

During potato crisp production, the raw material changes in both texture and color. These changes depend on both potato cultivar and process parameters. In this study, the texture and color of crisps processed from potatoes with different sugar contents were analyzed. The acrylamide content, which is related to sugar content and the color of the crisps, was also measured. To see whether the properties change over time, the crisps were measured during a storage time up to eight weeks. The results showed that the sugar content was the main factor governing the quality aspects of the crisps. The brightness (L) decreased, while the redness (a) increased with increased sugar content. Also, the acrylamide formation increased with the sugar content. The texture seemed to be the only parameter affected by both the sugar content and the storage time, requiring higher break forces and longer breaking distances for crisps with low sugar content, and a decrease in firmness after eight weeks of storage.

Introduction

Crisps are a common snack food around the world and are most often produced from potatoes. The quality and consumer acceptance of crisps in terms of texture and color depend on both the raw material and the production process. Crispness and color have been shown to be the most important parameters for consumer perception (Krokida, et al., 2001, Pedreschi, et al., 2005, Santis, et al., 2007, Segnini, et al., 1999, van Vliet, et al., 2007).

The texture and color parameters (i.e. lightness (L), redness (a), and yellowness (b)) have been found to be influenced by both the production parameters, such as frying medium, temperature, and slice thickness, as well as by the potato cultivars (Abong, et al., 2011, Kita, 2014).

During the frying process, the water decreases from around 75% in the fresh potato slices to 2–3% in the crisps (Viklund, 2007). The water is then partly replaced by oil, which can constitute up to 40% of the finished product, which in turn affects the texture (Kita, et al., 2007, Lisinska & Leszczynski, 1989). How much oil is absorbed during the frying partly depends on the frying temperature, which influences the length of the process (Dobarganes, et al., 2000, Gamble, et al., 1987, Mellema, 2003, Moreira, et al., 1997, Saguy & Dana, 2003). In addition, crisps are generally fried at high temperatures (175–190°C), which is a favorable temperature range for acrylamide formation (Gertz & Klostermann, 2002, Grob, et al., 2003). Acrylamide is formed in foods containing reducing sugars (glucose and fructose) and amino acid (asparagine) at high temperatures and is considered to be a carcinogenic substance to humans (Codex Alimentarius, 2009, Napolitano, et al., 2008, Rodríguez-Ramiro, et al., 2011). There are several acrylamide formation mechanisms; however, the main pathway seems to be based on a chemical reaction called the Maillard reaction, which contributes to the color of the crisps. It has been shown that the formation of acrylamide can be reduced (about 73%) by washing the potatoes in water, however, this method causes a negative impact on the final product, resulting in less crispness (Viklund, 2007).

In this paper, the texture and color of crisps during storage was investigated in relation to sensory analysis and the amount of acrylamide formation at different sugar contents.

Material and Methods

Material
Potatoes stored at 8°C (10°C during 8 hours of transfer) and sunflower oil, supplied by a commercial producer, were used.

Crisp production
Washed non-peeled potatoes were cut into slices of 2 mm thickness with a slicing machine (Berkel 800, The Netherlands). About 50 g of potatoes were used for each batch (about 8 to 16 slices).

For frying, a Julabo HC was used with an oil temperature of 156°C for 6 min. For each of the sugar contents (SC), the oil was changed. After frying, crisps were cooled down and the excess grease was eliminated with air and absorbent paper. Samples were put into aluminum bags and sealed with heat for storage measurements. The samples were stored in ambient temperature except for the samples where the amount of acrylamide should be determined. These ones were stored at -18°C (Viklund, 2007). Analyses were performed after 0, 2, 4, 6, and 8 weeks of storage.

Dry matter determination
The dry matter of the potato slices was measured before and after frying for each SC for week 0 and week 8. 5 g of potatoes slices were put into a metal cup and then placed in an oven (Termaks) for 18 h at 105°C, while 2 g of crushed crisps were put into a metal cup and placed in the oven for 5 h at 105°C.

Glucose measurement
A diabetic machine (Accu-Check sensor blood glucose meter) was used to measure the concentration of glucose in the potatoes. Five randomly chosen potatoes were tested for each range of concentration. A slice was cut in the middle of the potato (the glucose is unevenly distributed in the potatoes, so the same part of the potatoes was always tested). The slice was crushed with a mortar and the juice was tested five times for each potato.

Color determination of crisps
A spectrophotometer ‘L.a.b’ (CM-700d) was used to determine the color of the crisps. It is a non-destructive machine and so the crushed crisps were then used for other analysis. Crisps were crushed and put into metal cups; a plastic film was used to protect the spectrophotometer. Five replicates were done for each sample.

Acrylamide measurements
The acrylamide determination was performed in an external lab according to the LC MS/MS method (Chromedia, 2015). The sample endures a solid phase extraction (SPE) before the separation unit with a liquid of chromatography (LC). The final measure of the acrylamide was done with a mass spectrometry (MS).

Texture measurements
A TVT-texture analyzer (Perten Instruments, Sweden) equipped with a three-point bend rig and break probe was used to determinate the fracture behavior of the crisps by using a single cycle compression method. The maximum peak force and the fracture distance was determined, Figure 1. Profile settings are displayed in Table 1.

Sensory analysis
Two crisps were tested for each of the two sugar contents (Low and High) after 0, 5, and 8 weeks of storage. An untrained panel was used and the sensory parameters were color, crispness, off-flavor, and total impression. The grading was 1–9 where 1 corresponded to ‘none’ and 9 to ‘very high’.


Figure 1: Texture parameters

Table 1: Profile settings for texture measurements


Results and Discussion

Dry matter content and glucose determination
The dry matter content of the potato slices and crisps can be seen in Table 2 together with the glucose determination of the potato slices. No clear correlation is seen between the dry matter content and the sugar content. In addition, the storage time does not seem to influence the dry matter.

According to a study by Williams (2005), glucose concentration in raw potatoes is correlated with the acrylamide level in fried crisps with R² = 0.97. In that study, no correlation was found between asparagine and acrylamide concentration. That means that as long as asparagine is present, the concentration of reducing sugars is the limiting factor in formation of acrylamide. The importance of using potatoes with low sugar content is thus the main factor. Only potatoes with SClow and SCmedium-low can be used to cope with the recommendations of less than 1 g/kg fresh weight (Biedermann-Brem, et al., 2003).

Color measurements
The brightness of the crisps decreased with increased sugar content, which was expected since sugar is the main reacting compound in the Maillard reaction, resulting in brown pigments and a more colorful product, hence an increase in redness, Figures 2 and 3. Furthermore, in most cases both the brightness and the redness decreased in the samples during storage. Figure 4 shows the difference (Δ) between week 8 and week 0 for the different sugar contents.


Figure 2: A - sugar content Low, and B - sugar content High


Figure 3: Brightness and redness for different sugar contents (L low, M-L medium-low, M-H medium-high and H high).

Table 2: Dry matter and glucose level in potato slices and crisps at four different sugar contents

* mean and std dev


Figure 4: Δ values between week 8 and week 0 for brightness and redness at different sugar contents (L low, M-L medium-low, M-H medium-high and H high).

Acrylamide formation
Figure 5 clearly shows that more acrylamide is formed with higher sugar content. This high concentration of acrylamide could be explained by a long transport time, bad weather while growing the potatoes, or lengthy storage before the frying procedure.


Figure 5: Acrylamide formation at different sugar contents (L low, M-L medium-low, M-H medium-high and H high).

Texture of the crisps
The crisps were not equal in shape and some were more homogenous than others, which resulted in different types of fracture behavior. Two different example graphs can be seen in Figure 6. The blue curve crisp had a sharp break point while the red curve crisp displayed several smaller fractures before the break point.

The sugar contents gave different behaviors for the fracture during storage, with their maximum firmness between week 2 and week 6. However, all sugar contents had a decreasing firmness in week 8 compared to their maximum firmness from the storage time. Since no differences were found in dry matter between week 0 and week 8, water absorption from the air within the sealed bags could not solely explain the decrease in firmness. It is therefore likely that a water distribution of the unbound water within the crisps occurred. Furthermore, crisps from the sugar content Low required significantly higher forces and a longer distance to reach the break point compared to crisps with sugar content High.


Figure 6: Example graphs of the fracture behavior of crisps.

Sensory analysis

The sensory panel gave higher color scores to the crisps with the sugar concentration High than for crisps with sugar concentration Low. This is in agreement with the instrumental results which displayed a decrease in brightness and increase in redness for the higher sugar content.

The crispness tended to decrease with increased sugar content while the off-flavor seemed to increase. However, the variations in scores are largely due to the fact that the panel is untrained. The total impression corresponds to overall appreciation and includes the color, the crispness, the off-flavor, and the crisp appreciation, which seemed to decrease with higher sugar concentrations.

Conclusions

Quality aspects of crisps have been analyzed concerning sugar content of the potato and the storage time. The results showed that the sugar content was the main factor governing the quality aspects of the crisps. The brightness (L) decreased, while the redness (a) increased with increased sugar content. Also the acrylamide formation increased with the sugar content. The texture seemed to be the only parameter affected by both the sugar content and the storage time, requiring higher break forces and longer breaking distances for crisps with low sugar content, and a decrease in firmness after eight weeks of storage.

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