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In recent years, the consumption of snack foods has been growing rapidly due to the development of new processing methods and the use of new raw materials. Some of them fall within the category of functional foods as they contain health promoting ingredients with proven nutritional benefits. One of the technologies used in the manufacture of new types of functional snacks is extrusion-cooking. The HTST (high temperature-short time) extrusion-cooking method allows for the manufacture of modern snacks with an improved nutritional value and specific quality characteristics that meet the needs of contemporary consumers, especially gluten-free products for celiac disease patients (Bouasla et al., 2016, 2017; Mitrus and Moscicki, 2014; Oniszczuk et al., 2015). Extruded snacks can be produced without salt or sugar and with a very low fat content, therefore they may be a preferred choice for the diets of children (Potter et al., 2013; Wójtowicz et al., 2017). In the extrusion-cooking process ready-to-eat snacks are manufactured. The basic ingredients for directly expanded products are usually corn or rice grits. However, sometimes corn grit is replaced by other raw materials (Ačkar et al., © 2019 Institute of Agrophysics, Polish Academy of Sciences A. WÓJTOWICZ et al. 408 2018). This is mainly due to the fact that starch – the main component of corn snacks – is energy-rich but has a very low nutritional value. Therefore, in order to improve the nutritional and functional characteristics of gluten-free snacks, it is advisable to add some ingredients with specific functional properties (Dehghan-Shoar et al., 2010). Such enriching additives can, however, have a varied influence on the appearance and texture of snack products (Anton et al., 2009). The beneficial effects of fruit and vegetables are attributed to their fibre content and antioxidant properties (Hidalgo and Almajano, 2017; Potter et al., 2013). One of the most promising additives are berries. Fruits require processing in order to extend their relatively short shelf life, they are dried, lyophilized, frozen or cooked with sugar to make jams or other preserves. Strawberries are primarily valued for their taste and intensive bright-red colour. Their consumption is widely promoted because they contain many antioxidants, including vitamin C. They also contain anthocyanins, fibre, and phenolic components (Bao et al., 2018). The most popular fruit in Poland is the strawberry (Gajdoš Kljusurić et al., 2016). Also, chokeberry and elderberry are becoming more and more popular due to their antioxidant properties (Michalczyk et al., 2009). The chokeberry in particular exhibits such potential (Lipińska et al., 2017). It contains many healthy compounds such as polyphenols or anthocyanins. However, the fruit is not particularly popular among consumers as it is bitter and tart in taste (Lachowicz et al., 2018a). Another valuable fruit is elderberry which grows under natural conditions both in Europe and in South Africa or Western Asia. The fruit contains such compounds as anthocyanins, flavanols, or phenolic acids. Cyanidin glycosides are among its predominant anthocyanins (Pliszka, 2017). Several reports have cited the possibility of the application of berry fruits in food products such as juices (Lachowicz et al., 2018b; Ricci et al., 2019; Tundis et al., 2018; Wang et al., 2019), smoothies (Guazi et al., 2019), snacks (Gramza-Michałowska and Człapka-Matyasik, 2011; Kowalska et al., 2018a,b), jams (Banaś et al., 2018), pie fillings (Threlfall et al., 2007), flour confectionery (Osipenko et al., 2019), cookies, natural colourants (Wathon et al., 2019) in the form of dried/lyophilized and powdered fruits (Gagneten et al., 2019; Oniszczuk et al., 2016), extracts (Cilek et al., 2019) or residues (Drożdż et al., 2019). The aim of this study was to process ready-to-eat cornbased gluten-free snacks supplemented with selected fruits (chokeberry, elderberry, strawberry) and to assess the physical properties and texture of the obtained crisps depending on the type and amount of additive and the screw speed applied during extrusion-cooking. MATERIALS AND METHODS Corn grits (purchased from Lubella Sp. z o.o. Sp. K., Lublin, Poland) were used as the basic raw material for the preparation of gluten-free snacks. Three types of fresh fruit were collected at the peak harvest time from local farmers. The black elderberries and black chokeberries were rinsed with tap water and dried using an air dryer at 40°C overnight. The fresh strawberry fruits were frozen in a laboratory freezer overnight and freeze-dried using an ALPHA 2-4 LD plus lyophilizer (Martin Christ GmbH, Germany) with a condenser temperature of -54°C and a heated shelf temperature of 40°C. Dry fruits with a moisture content below 5% were ground using a laboratory grinder TESTCHEM LMN10C (Radlin, Poland) to the size of particles with dimensions lower than 1 mm. Powdered fruits were used in the amounts of 5, 10, 15 and 20% (w/w) as a replacement for corn grits and mixed. The moisture content of the blends was set at 14% by the addition of distilled water. Extruded corn snacks with fruit addition were made using a single screw extruder-cooker TS-45 (Z.M.Ch. Metalchem, Gliwice, Poland). They were shaped with a 3-mm circular forming die and cut just after they left the die. The ratio of the screw length to its diameter was L/D = 12. The temperature of processing used during the extrusion-cooking process was set at: 132 and 142oC in section I and II, respectively and 135oC at the forming die. The snacks were extruded at a screw speed of 80 and 120 rpm. They were then collected, cooled down to ambient temperature and stored in sealed plastic bags at room temperature prior to testing. The obtained ready-to-eat snacks were tested to assess their expansion ratio, bulk density, and colour profile at the CIE-Lab scale and texture under different processing conditions and for the different types and quantity of additives used. The expansion ratio was calculated as the ratio of snack diameter to the diameter of the forming die. The calculation was replicated 10 times for each type of snack (Bisharat et al., 2013). The bulk density of the snacks was determined as the weight of a specific volume of the extrudates. The test was performed in triplicate (Wójtowicz et al., 2018). The colour profile of the supplemented snacks was measured using the Color and Appearance Measurements System Lovibond CAM-System 500 (The Tintometer Ltd., Amesbury, UK) at the CIE-Lab scale. The results were presented as the means of 10 replications of the coordinate L *, a* and b*. Parameter L* described the balance between white (100) and black (0). The coordinate a* indicates the balance between redness(+) and greenness(-), and b* is defined as the balance between yellowness(+) and blueness(-). The total colour change was calculated as ΔE according to Wójtowicz et al. (2017) and Pęksa et al. (2016). The texture profile was evaluated by using a Zwick/ Roell BDO-FB0.5TH texture apparatus (Zwick GmbH & Co., Ulm, Germany) equipped with an Ottawa cell. A double compression test of up to 50% of the sample height was used to evaluate firmness (F), crispness (CR) fracturability SNACKS ENRICHED WITH FRUITS 409 (F), springiness (S), cohesiveness (C), and chewiness (CH) with a working head test speed of 100 mm min. Firmness was represented by the top peak force at the first compression cycle. Crispness was described as the first force peak needed to destroy the cellular structure of the extrudates. Springiness is the physical property of the snack describing its tendency to restore its original shape upon the termination of the first penetration cycle. Cohesiveness may be understood as the tendency of a product to cohere or stick together, and was determined as the work needed at the second deformation relative to the resistance under the first deformation. Chewiness, typical of solid foods, was determined as the hardness and cohesiveness ratio and showed how the extrudates behave during biting (Shah et al., 2017). A statistical analysis ANOVA (at α = 0.05) was performed using Statistica 13.3 software (StatSoft, USA) with an F-test and p values to evaluate the effect of the level of fruit addition and the screw speed applied during the experiment. A post hoc Tukey test was used to compare the mean values of the test results and homogenous groups identified the significance of differences. RESULTS AND DISCUSSION The high quality of the extruded snacks is usually determined by the high expansion index (Wójtowicz et al., 2018). Moreover, the intensity of treatment in the extrusion-cooking process depends on such properties as the expansion ratio and porosity (Jakubczyk et al., 2017; Lucas et al., 2018). During extrusion-cooking, several components undergo transformations. One of the most important of them is gelatinization which occurs in the melted dough inside the barrel, the dough cooks at a high temperature in the presence of water. Water bonded inside the treated material is turned into steam and evaporates immediately after the pressure drops outside the forming die. The expansion of the extruded snacks is the effect of a porous structure forming in melted starch and the protein matrix becoming rigid after cooling down. Generally, additives may disturb the gelatinization and melting processes and cause less expansion (Bisharat et al., 2013; Makowska et al., 2015). The expansion ratio of the extruded snacks decreased along with higher supplementation levels of chokeberry, elderberry and strawberry in relation to the control corn sample with high regression coefficients ranging from 0.889 to 0.998 (Fig. 1). The maximum value of the expansion ratio was observed for corn snacks (5.88) processed at 120 rpm. For almost all the tested snacks, higher screw speeds during processing caused the snacks to expand to a greater extent, regardless of the type of fruit used. As reported by Y

Physical properties and texture of gluten-free snacks supplemented with selected fruit additions