At the end of 2011, the world population had reached 7 billion and it is supposed the increase of one billion people to 2025. Around one billion people in the 21st century are starving and another billion people are suffering from micronutrient deficiency. On the other hand, over a billion people worldwide are deemed overweight or obese, with increased risk of associated diseases. Even with sufficient intake or excessive energy, consumed food can remain a poor source of essential nutrients. Opportunities to improve the nutritional profile and sustainability of the diet are found throughout the food chain, farm production, retail, and households. These include crop diversification, food enrichment, improvements in the efficiency of transport, waste minimisation, and last but not least food reformulation (Buttriss, 2013).

The food industry has two main approaches to product reformulation: gradual reduction of nutrient content without introducing further changes in product formulation or partial/total replacement with other nutrients such as hydrocolloids, fat substitutes, and sweeteners (Ares et al., 2018). However, some studies have found that products without added salt or sugars leave a negative sensory impression (Phelps et al., 2006; DuBois and Prakash, 2012). Added sugars are defined as sugars that are added to foods during processing, preparation, or “on the table”. They contribute to the energy value of the diet but have a little nutritional benefit and their high intake is associated with increased dietary energy, dental caries, and other adverse health effects such as excessive weight gain and reduced bone density. The current WHO recommendation is that the intake of added sugars should be less than 10% of the total energy intake (WHO, 2015; Yeung et al., 2017). Replacing sugars in reformulated foods could represent a strategy to reduce sugar intake in the population without a significant change in the normal diet. However, reducing the sugar content of food is not easy because it results in changes to taste, texture, functionality and shelf life (van Raaij, Hendriksen and Verhagen, 2009; Cruz et al., 2010). The hygroscopic nature of sugar plays an important role in reducing water activity in foods and helps maintain and prolong the shelf life of foods. Sugar prevents for example microbial spoilage of jams after opening. It is therefore always necessary to understand the function of sucrose in a particular food product before it is replaced (Goldfein and Slavin, 2015).

Fruit, vegetables, and related products are often perceived by the consumer as “healthy” and consumable in any quantity. However, fruit and in particular for fruit products should be consumed with caution. Fruit products (compotes, fruit spreads, candied fruit, and other) often contain high sugar content. The effort to reformulate is then significant for the aforementioned reasons (Rýdlová et al., 2019). The term “fruit spreads” is generally used in the food processing industry for marmalades, jams, jellies, and similar products. Fruit spreads have traditionally high sugar content (60 – 65%), and there is the possibility for their reformulation. The formation of the desired jelly consistency of a fruit spread with reduced sugar content can be achieved by the use of low-esterified pectins or alternatively gelling agents based on gums or extracellular products of microorganisms (Kadlec, Melzoch and Voldřich, 2012).

Carrageenan C (CEAMGEL 9205; CEAMSA, Spain), citric acid (Hubka-Petrášek a vnuci s.r.o., Czech Republic), sucrose (DIAMANT; 1. Cukerní společnost Praha, s.r.o., Czech Republic), strawberry puree (100%) (Fruta Podivín, a.s., Hamé s.r.o., Czech Republic), distilled water, potassium hydrogen phthalate (SIGMA-ALDRICH, USA).

According to the literature review (Tang et al., 2019; Ellis et al., 2019; Ortiz-Tafoya et al., 2018), carrageenan was chosen as a gelling agent for reformulated strawberry jam. The model samples of carrageenan gel were prepared (Table 1), and gel stiffness was measured. The results of the carrageenan gel stiffness are shown in Figure 2, Figure 3, and Figure 4. The results show that carrageenan, sugar, and citric acid content influence the gelling ability and gel stiffness of carrageenan gels. The gel stiffness increased significantly with increasing amounts of carrageenan. Likewise, the addition of sugar slightly increased the gel stiffness, as confirmed by the results of studies by Lopez- Sanchez et al. (2018), Maurer, Junghans, and Vilgis (2012), Deszczynski, Kasapis and Mitchell (2003) and Watase et al. (1990). However, the addition of citric acid significantly reduces the gel stiffness of carrageenan gels (p = 0.05).

The results show that it is not necessary to add citric acid to strawberry spreads because of the natural occurrence of organic acids in a strawberry puree to improve the texture parameters (Kallio et al., 2000; Sturm, Koron and Stampar, 2003). However, the addition of citric acid is necessary for the sensory parameter of the final product (balance of sweet and sour taste). Therefore, the same final concentration of 0.5% of citric acid was chosen for the preparation of all strawberry spreads.

One of the parameters of the overall sensory perception of this type of product for the consumer is gel stiffness (Pérez-Herrera et al., 2020). The results of the strawberry spreads gel stiffness (Figure 5) show, that both, sugar and strawberry puree content contribute to this ability to form a gel (p = 0.05). The maximum force required to compress the gel was higher than that of carrageenan gels with the same sugar and citric acid addition (p = 0.05). Samples containing 40 – 50% of strawberry puree and 30 – 40% of sugar achieved the highest gel stiffness. Only samples without strawberry puree and low sugar content (10 – 20%) showed low gel stiffness.

Colour is one of the most important parameters of strawberries and strawberry products for their final perception and attractiveness to consumers (Bursać Kovačević et al., 2015). Parameter (L*) of prepared strawberry spreads J1 – J16 is shown in Figure 6. The results show that the addition of sugar significantly reduces product lightness (p = 0.001). This fact is probably due to the amount of sugar, which contributes to the Maillard reaction (Li et al., 2020; Liao et al., 2020; Shen, Chen and Li, 2018; Basu, Shivhare and Singh, 2013). The addition of strawberry puree also had a significant effect on parameter L* of strawberry spreads (p = 0.05), but less than that of sugar.

The parameter a* (redness, Figure 7) was not affected by the amount of sugar added (p = 0.05). Significantly lower was the redness (a*) of the strawberry spreads with a strawberry puree content of 20%. On the other hand, the addition of strawberry puree affected the colour parameter a* (p = 0.05).

Colour parameter b* (yellowness, Figure 8) was lower for samples with a strawberry puree content of 20% (p = 0.05). Like the parameter L*, parameter b* decreased with the addition of sugar. This difference is statistically significant (p = 0.05).

The results of the sensory analysis of strawberry spreads (J1 – J16) are shown in Table 3. It is visible, that the evaluation of all samples was very subjective. The results of sensory evaluation of texture and sweet taste show that panellists identify differences between samples. In contrast to this fact, the differences in acid taste and overall acceptability (p = 0.05) were not perceived between samples. The overall acceptability values ranged across the whole hedonic scale for all samples. No statistically significant differences between the overall acceptability value of tested samples were observed (p = 0.05).

The reduction of sugar content in strawberry spreads was carried out with a focus on people with special nutritional requirements, specifically suffering from obesity and type 2 diabetes. Carrageenan is a suitable gelling agent for the preparation of reformulated strawberry spreads. The applicable concentration of carrageenan C in strawberry spreads for the ideal stiffness of strawberry spreads was 2%. The results of the maximum compression force show a statistically significant increase of gel stiffness (p = 0.05) with the increasing addition of strawberry puree. As the sugar content in the model increases, the maximum compression force values increase. This effect is also statistically significant (p = 0.05) but not as much as the effect of strawberry puree. It was possible to reduce the sugar content in strawberry spreads up to 10% with acceptable sensory perception. The production of this type of fruit spreads with standard characteristics is a complex task influenced by many factors, including the type of used gelling agent and characteristics of input raw material. These reformulated strawberry spreads may be an alternative to jams, which are not recommended because of their high sugar content, for people suffering from these diseases.