IJFS#1858_bozza Ital. J. Food Sci., vol. 32, 2020 - 945 PAPER PROPERTIES OF STRAWBERRIES PUREE STORED IN THE FREEZER V. OBRADOVIĆ, M. ERGOVIĆ RAVANČIĆ*, H. MARČETIĆ and S. ŠKRABAL Agricultural department, study of Food technology, Polytechnic in Požega, Vukovarska 17, 34 000 Požega, Croatia *Corresponding author: mergovic@vup.hr ABSTRACT Four different strawberry varieties were used for puree production and stored at -18 ºC for 12 months. Every three months samples were tested for rheological parameters, polyphenols content and antioxidant activity. Mathematical models for rheological behaviour of the samples were determined together with consistency coefficient (k) and flow behaviour index (n). Fluidity of the samples increased over time, but pseudoplastic behaviour remained through tested period. The biggest decrease of polyphenol content was observed between 9 and 12 months of storage, while antioxidant activity decreased the most during first three months by DPPH and ABTS method. Keywords: antioxidant activity, polyphenols, rheology, strawberries Ital. J. Food Sci., vol. 32, 2020 - 946 1. INTRODUCTION Current guidelines for fruit and vegetable consumption recommend five portions per day (HARTMANN et al., 2008). Due to seasonal character of many fruits, as well as hectic lifestyle, these recommendations are not easily achievable especially when targeting fresh fruit and vegetables. Thus, replacement of one or several of portions by fruit juices, concentrates or purees is suggested (HARTMANN et al., 2008). Consumers have increased interest to high-value food products, especially fruit, vegetables and other functional foods (BISHARAT et al., 2013). Therefore, food manufacturers are facing the challenge to create new products which, beyond basic nutrients, also provide certain health-promoting properties (OBRADOVIĆ et al., 2015). Strawberries are one of the most consumed berries worldwide. In European market average strawberries intake is 2.16 kg/year per person including raw and processed fruit (GASPEROTTI et al. 2015). These popular fruits are favoured for their attractive taste, and are considered as rich source of micronutrients and phytochemical compounds such as water soluble vitamin C and polyphenols (phenolic acids, anthocyanins, flavonols, tannins and other) (KLOPOTEK et al., 2005; OSZMIAŃSKI et al., 2009; BODELÓN et al., 2013; ŻEBROWSKA et al., 2019). Diet rich in fruit and vegetables is beneficial for human body. It lowers the risk of many diseases: diabetes, atherosclerosis, cardiovascular disease, inflammatory-related illnesses, and cancer. This is attributed to vitamins, dietary fiber and polyphenols (NOWICKA et al., 2019). Strawberries have very strong antioxidant activity. They have 1.3 times activity of oranges, 2 times that of red grapes, 5 times that of apples and bananas and 13 times that of honeydew melon (OSZMIAŃSKI et al., 2009). For all mentioned reasons, strawberries are considered as a functional food although exact mechanism involved is still generally unclear (GASPEROTTI et al., 2015). Unfortunately, strawberries are very perishable due to high water content and soft structure, and consequently have extremely short postharvest shelf-life (HOLZWARTH et al., 2012; PEINADO et al., 2012). Therefore, there is a huge demand for strawberries puree for use as a base product for preparation of juices and soft drinks, for addition to ice-creams and yoghurts (BODELÓN et al., 2013), or it can be sold directly to consumers in canned or frozen forms (DIAMANTE et al., 2016). Purees are usually preserved by freezing or by heat. Freezing results in cell destruction allowing reactions between genuine enzyme activities and their corresponding substrates. Thawing is especially critical since polyphenoloxidases (PPO) are responsible for polyphenols destruction (HOLZWARTH et al., 2012). In order to create puree of satisfying quality and nutritional value it is necessary to determine optimal storage time. Therefore, the objective of this study was to explore influence of freezing on strawberry purees. Rheological properties, polyphenol content and antioxidant activity were evaluated. Changes in the rheological properties of fruit purees that have undergone freezing or freeze–thaw treatments are of practical significance for their acceptance and consumption (DIAMANTE et al., 2016). Reduction in antioxidant activity during processing and storage may reduce the health beneficial effects of food products (OSZMIAŃSKI et al., 2009) and that was the reason for test puree samples for above mentioned parameters. Some works have reported the rheological characterization of different fruits like mango and papaya (EL-MANSY et al., 2005), blueberry (ANTONIO et al., 2007, NINDO et al., 2007), raspberry, strawberry, prune, peach (MACEIRAS et al., 2007, ERGOVIĆ RAVANČIĆ et al., 2012), nectarine and blackberry (ERGOVIĆ et al., 2009; ERGOVIĆ et al., 2010), but to the best of our knowledge they haven’t followed rheological parameters together with nutritional characteristics over a period of time in order to determine how storage time in the freezer affects mentioned characteristics. Ital. J. Food Sci., vol. 32, 2020 - 947 2. MATERIALS AND METHODS 2.1. Sample preparation Sample S-1 was prepared from wild strawberries harvested in woods near Požega town, Slavonia region, Croatia. Strawberries for sample S-2 (Albion variety) and S-3 (Clery variety) were purchased from the local farmers and for sample S-4 (Joly variety) in the local supermarket. Fruits were cleaned and blended in kitchen blender at room temperature for 3 minutes, divided in small portions (100 mL), sealed in polyvinyl chloride freezer bags (at atmospheric pressure and temperature, vacuum or modified atmosphere haven’t been used) and kept in chamber freezer at -18 ºC, until the analysis. Every three months three bags were thawed at room temperature as parallels for the analysis. 2.2. Sugar and acidity determination Total and reducing sugars were determined according to the Luff-Schoorl method (GAFTA, 2018). Total acidity was determined by potentiometric titration. 2.3. Extract preparation 1 g of strawberry puree was extracted with 20 mL of acidified methanol (methanol/2% HCl, 95:5) at room temperature for 60 min with constant shaking in temperature- controlled shaker (Kottermann labortechnik) at 200 rpm and centrifuged (Tehtnica, Centric 322A). Glasses were covered with aluminium folium to prevent evaporation of solvent. 2.4. Total phenol content Polyphenols were determined according the Folin-Ciocalteu method (OBRADOVIĆ et al. 2015, with modifications). An aliquot of the extract (200 µL) was mixed with 2 mL water and 100 µL Folin-Ciocalteu reagent (Kemika, Croatia). The mixture was allowed to equilibrate for 5 min, and then 300 µL of sodium carbonate solution (20%) was added. After incubation at room temperature in dark for 30 min, the absorbance of the mixture was read at 725 nm (Camspec M501, UK). Acidified methanol was used as a blank. Total polyphenols were determined with 3 replications. Gallic acid (Carlo Erba reagents, Italy) was used as a standard (calibration curve y = 1.1979x - 0.0188, R2 = 0.9984), and results were expressed in mg of gallic acid equivalents per 100 g of sample. 2.5. Antioxidant activity determination (ABTS) ABTS·+ radical was obtained by mixing 7.4 mM ABTS (Fluka, Switzerland) solution and 2.6 mM solution of ammonium persulfate in 1:1 ratio. Solution was left in dark through the night in order to develop stable radical, and then radical solution was diluted with ethanol in 2:70 ratio to obtain absorbance approximately 1.100 (AABTS). An aliquot of extract (0.2 mL), was mixed with 3.2 mL of diluted ABTS·+ radical. After incubation at room temperature in dark for 95 min, the absorbance of the mixture was read at 734 nm (AEXTR), and ∆A was calculated as AABTS - AEXTR. Trolox (Sigma Aldrich, USA) was used as a standard. Decrease in absorbance caused by trolox was done in the same way as for the samples, and standard curve ∆A/trolox concentration was created (y = 489.13x - 17.903, R2 = 0.9952). Ital. J. Food Sci., vol. 32, 2020 - 948 Determination of antioxidant activity was done in 3 replications. Results were expressed in µmol of the trolox equivalents per gram (OBRADOVIĆ et al., 2015). 2.6. Antioxidant activity determination (DPPH) An aliquot of extract (50 µL) was mixed with 2 mL DPPH radical solution (0,1mM in ethanol). The absorbance of the mixture was read at 517 nm during period of 30 min, results were expressed as the mean of 3 replications. Pure ethanol was used as a blank. % inhibition = [(A0 - At)/A0] x 100 (1) A0 - absorbance of DPPH radical solution, At – absorbance after 30 minutes. 2.7. Rheological properties determination The rheological properties were measured before storage in the freezer and after 3, 6, 9 and 12 months of storage by rotation rheometer, model VT 550 362-0001 HAAKE with concentric cylinders (RheoWin Pro 2.91 software). Diameter of inner rotating cylinder was 36 mm, inner diameter of outer stationary cap was 40 mm, gap between cylinders was 4 mm. The measurements were carried out in triplicate at 40°C, at shear rates 0 – 60 1/s. Temperature of 40°C has been selected as the closest to the temperature of “ready to eat” food containing puree. Puree is not expected to be eaten alone, it is usually used as a filling for some cakes like strudel, or as a dressing for pancakes and this is temperature of “ready to eat” product, close to the body temperature. For each shear rate computer recorded shear stress which was provided by the strawberry puree during rotation of the measuring cylinder of the rheometer. Flow curves are presented as the mean value of recorded results. Rheological parameters determined with experimental flow curves were fitted to Ostwald-de Waele (power law) model using a software (Excel 2016, USA). τ = k · Dn (2) τ - shear stress (Pa), k - consistency coefficient (Pasn), D - shear rate (1/s), n - flow behavior indeks. Samples were taken from the freezer and after thawing and reaching room temperature, rheological parameters were determined. Relation between shear rate and shear stress were presented graphically and determination coefficient (R2) was calculated for each curve. 2.8. Data analysis Chemical composition data were analysed by Statistica 12 software, using post hoc LSD at 95% level. Ital. J. Food Sci., vol. 32, 2020 - 949 3. RESULTS AND DISCUSSION Initial composition of strawberries puree samples is presented in Table 1. Parameters were tested as an indicator of ripeness to initially assess the starting material. S-1 sample, wild strawberries, had the highest sugar content and the lowest acidity. Sample S-2 is following with slightly lower content of sugars, but with much higher acidity, and the samples S-3 and S-4 were similar in sugar content, but sample S-3 had the highest acidity among all samples. These parameters were not expected to be significantly changed during storage, so they were not measured every three months. Table 1. Composition of purees before storage. S-1 S-2 S-3 S-4 Total sugars (%) 15.40±0.22 13.42±0.16 8.22±0.26 8.64±0.04 Reducing sugars (%) 10.56±0,06 8.96±0.12 7.64±0.08 8.16±0.06 Total acidity (mmol/100g) 7.96±0.04 17.52±0.02 24.40±0.06 13.60±0.10 There are diverse phenolic compounds in strawberries, not only coloured anthocyanins, but also colourless phenols like ellagic acid, ellagitannins, p-coumaric acid and quercetins (HARTMANN et al., 2008). GASPEROTTI et al. (2015) identified and quantified 56 individual compounds in strawberries, with concentrations ranging from 1 µg/100 g to 40 mg/100 g. They also highlighted that this is not a complete list of polyphenols present in strawberries. Total phenol content in puree samples before and after 3, 6, 9 and 12 months of storage is presented in Table 2. Table 2. Total phenol content in samples during 12 months of storage A, B. Sample Total phenols (mgGAE/100 g) Before storage After 3 months After 6 months After 9 months After 12 months S-1 422.59e±4.72 415.10d±2.82 404.60c±5.06 400.89b±1.55 368.98a±0.44 S-2 196.92d±6.96 192.66c±0.16 189.96b±0.98 188.12b±0.46 170.49a±1.47 S-3 164.68d±0.99 152.15c±0.83 151.62c±3.39 147.49b±1.41 135.21a±0.47 S-4 185.70d±3.12 165.69c±0.89 163.35b±3.94 163.91b±4.53 151.47a±1.19 AResults are expressed as mean of three repetitions ± standard deviation. BMeans followed by the same letter in the lines are not statistically different at 5% probability. As presented by YILDIZ et al (2014) and DIAMANTI et al (2014), initial polyphenols content in wild strawberries puree (sample S-1) was more than double compared to cultivated strawberries purees (samples S-2 till S-4). Polyphenols content is in direct relation to the ripeness stage (sugar content and acidity presented in Table 1). Obtained results are similar to the values presented by GALOBURDA et al. (2014) and KLOPOTEK et al. (2005). It is already documented that strawberry phenolics such as pelargonidin, ellagic acid, p-coumaric acid, quercetin and kampferol derivatives are very unstable and undergo destruction during fruits transformation in frozen products especially in the thawing process by native and microbiological enzymes and by nonenzymatic oxidation Ital. J. Food Sci., vol. 32, 2020 - 950 (AABY et al., 2007; OSZMIAŃSKI et al., 2009), but we couldn’t find recommendations for storage time in freezer in order to preserve reasonable high level of polyphenols, and if initial value influences degree of degradation. As it can be seen in Table 3, samples S-1 and S-2 had relatively low level of polyphenols destruction during first three months in the freezer (1.77 and 2.17%, respectively). Level of degradation continued over next months of storage, so both samples had percentage of degradation approximately 5% after 9 months of storage, which can be considered as good result. Between 9 and 12 months of storage a large decrease in polyphenol content can be seen and percentage of degradation during that period was higher than in previous 9 months. On the other hand, samples S-3 and S-4 had relatively high degradation during period of first three months (7.61 and 10.78%, respectively), which haven’t changed significantly until the 9 months, so at the end of that period degradation was 10.44 and 11.74%. Still, the highest degradation was between 9 and 12 months. Therefore, it can be concluded that storage time of strawberry puree shouldn’t be longer than 9 months in order to preserve phenolic compounds. In the end, percentage of degradation after 12 months of storage was higher in samples with lower initial values of polyphenols. It can be explained by the protective role of polyphenols (higher level of polyphenols-higher level of protection). Similar effect can be found in wines where red wines are less susceptible to degradation and require less chemical protection than white wines. HARTMANN et al. (2008) concluded that every processing step during production of juices and purees reduces the content of polyphenols, but they are better retained in purees than in juices. They also recommended a short enzymatic treatment of the mash with maceration enzymes in order to achieve maximal yield of polyphenols and antioxidant capacity. It is very important to obtain short enzymatic treatment because longer mash standing actually increases the loss. At the same time enzymatically treated puree was less viscous and smoother. While the nonenzymatically treated puree registered a water phase separation in less than 3 weeks of storage, which wasn’t the case in this research. HOLZWARTH et al. (2012) reported that freezing technique did not have significant influence on polyphenols as well as on colour and ascorbic acid. Thawing method was, on the other hand, very important factor affecting mentioned parameters. Thawing at 20ºC and microwave thawing were favourable methods (compared to 4 ºC and 37ºC thawing). As previously explained, in this research samples were thawed at room temperature. Table 3. Percentage of polyphenols degradation during storage compared to the initial value. Sample Degradation (%) After 3 months After 6 months After 9 months After 12 months S-1 1.77 4.26 5.13 12.68 S-2 2.17 3.54 4.47 13.42 S-3 7.61 7.93 10.44 17.90 S-4 10.78 12.04 11.74 18.43 ABTS and DPPH methods have gained popularity for the study of antioxidant activity due to their speed and simplicity, and both of them are based on free-radical scavenging activity. Antioxidant activity by ABTS method is presented in Table 4 and by DPPH method in Table 5. Values are in accordance with the results presented by KLOPOTEK et Ital. J. Food Sci., vol. 32, 2020 - 951 al. (2005) and NOWICKA et al. (2019). As expected, wild strawberries puree (S-1) had much higher antioxidant activity than other samples. Samples with higher content of polyphenols also had higher level of antioxidant activity by both methods. Contrary to polyphenols degradation, antioxidant activity by ABTS method in sample S-1 had the biggest decrease in first six months of storage. Sample S-3 (which had the lowest antioxidant activity at the beginning) lost almost 50% of the initial value by the end of a storage. Results obtained by DPPH method also show the biggest decrease in antioxidant activity during first three months. During the rest of the storage period further decrease was slower compared to first three months. Although polyphenols are the most important and the most popular antioxidants, there are also other molecules with antioxidant properties like products of Maillard reactions (OBRADOVIĆ et al., 2015), so direct correlation between polyphenols content and antioxidant activity is not always the case and it depends on method used. BAIANO et al. (2009) showed low correlation between amount of polyphenols in wines and antioxidant activity. They also concluded that beside previously mentioned antioxidants, antioxidant activity depends not only on the phenolic concentration, but also on the specific chemical structure of each phenolic compound. Table 4. Antioxidant activity of samples during 12 months of storage (ABTS method) A, B. Sample Antioxidant activity (ABTS) (µmol TE/g) Before storage After 3 months After 6 months After 9 months After 12 months S-1 30.20d±0.59 27.65c±0.53 24.31a±0.21 25.70b±0.45 25.71b±0.32 S-2 10.79e±0.37 9.96d±0.56 9.16c±0.33 7.98b±0.20 7.16a±0.18 S-3 7.62e±0.08 6.22d±0.04 5.46c±0.15 4.52b±0.39 3.94a±0.29 S-4 8.63d±0.04 5.52c±0.14 5.59c±0.13 5.10b±0.48 4.70a±0.41 AResults are expressed as mean of three repetitions ± standard deviation BMeans followed by the same letter in the lines are not statistically different at 5% probability. Table 5. Inhibition of DPPH radical after 30 minutes A, B. Sample Inhibition (%) Before storage After 3 months After 6 months After 9 months After 12 months S-1 80.02d±1.82 71.80c±2.04 70.54b±1.94 68.38a±2.20 68.13a±1.56 S-2 49.39c±0.96 39.63b±1.14 39.93b±1.55 39.49b±1.24 38.95a±0.93 S-3 43.94d±1.06 36.14c±0.58 36.60c±1.86 32.75b±0,64 31.44a±0.88 S-4 49.74d±1.58 40.35c±0.98 38.20b±0.60 38.27b±1,46 37.21a±1.38 AResults are expressed as mean of three repetitions ± standard deviation. BMeans followed by the same letter in the lines are not statistically different at 5% probability. Beside chemical and nutritional properties, knowledge of the rheological properties of food products is important for process design, control of the process, and consumer acceptability of a product. Rheological properties provide information on how to control flow properties of the product so that the desired product can be prepared. Rheological Ital. J. Food Sci., vol. 32, 2020 - 952 properties are explained by rheological parameters: flow behavior index (n) and consistency coefficient (k) (LOVRIĆ, 2003; OSORIO et al., 2008). Fruit purees are suspensions of solid matter in fluid media and have been categorized as time-independent non-Newtonian fluids showing a pseudoplastic behavior (RUDRA et al., 2007; SOROUR et al., 2016). Rheological properties of strawberry puree samples are presented in Fig. 1. Shape of the curve in Fig. 1. shows that strawberry puree is pseudoplastic system. Pseudoplastic non- Newtonian flow behavior occurs when shear stress is increasing at a diminishing rate, while increasing shear stress decrease when fluid is subjected to higher shear rates. It leads to a convex profile curve in which tangential slope is decreasing with increasing shear rate (KREITH, 1999; FIGURA and TEIXEIRA 2007). This behavior is caused by decreasing molecular interactions within the molecular structure of the fluid during flow. Pseudoplastic behavior of all samples remains regardless of storage time. It can be seen that freezing caused decrease of shear stress values in all samples compared to the starting sample. Deviations presented in stress-strain graphs are result of a samples’ inhomogeneity, but still, measured values fit well to the Ostwald de Waele model for pseudoplastic systems (Table 6, R2 ranging from 0,911 till 0,994). The same trends were obtained by several authors. ALVAREZ et al. (2006) studied the rheological behavior of strawberry jam, MACEIRAS et al. (2007), BUKUROV et al. (2012) and YALÇINÖZ and ERÇELEBI (2016) researched the rheological properties of strawberry puree. All mentioned authors fitted results of rheological measurements to the same model. Figure 1. Rheological properties of strawberry purees during storage in the freezer. Ital. J. Food Sci., vol. 32, 2020 - 953 Shear stress values were the highest in sample S-4 before and during storage with maximum shear stress 921 Pa and the lowest in sample S-3 with maximum shear stress 379 Pa at the shear rate 60 s-1. Table 6. Rheological parameters of strawberries puree samples during storage. Sample S-1 S-2 S-3 S-4 Before storage n 0.573 0.572 0.753 0.758 k, Pa·sn 91.285 79.818 19.289 41.910 R2 0.953 0.911 0.994 0.991 After 3 months of storage n 0.575 0.545 0.837 0.769 k, Pa·sn 56.559 58.425 8.322 32.848 R2 0.985 0.930 0.994 0.979 After 6 months of storage n 0.588 0.646 0.878 0.778 k, Pa·sn 45.301 36.274 5.785 32.674 R2 0.941 0.953 0.988 0.983 After 9 months of storage n 0.701 0.675 0.873 0.873 k, Pa·sn 26.044 24.786 5.957 17.495 R2 0.987 0.962 0.961 0.971 After 12 months of storage n 0.895 0.831 0.879 0.867 k, Pa·sn 11.622 12.956 6.067 14.375 R2 0.990 0.9863 0.998 0.961 As shown in Table 6, flow behavior index values are within 0 and 1 (0,545-0,878), characteristic for pseudoplastic systems. Pseudoplastic fluid behavior is explained by cracking of the molecule structure when exposed to hydrodynamic forces and increasing the alignment of the constituent molecules. It can be seen that from the start samples S-1 and S-2 had lower values for flow behavior index than samples S-3 and S-4. This difference is obvious till 9 months of a storage, while after that flow behavior index is practically the same for all samples. Strawberry purees before storage in the freezer had the lowest values of flow behavior index and the highest values of consistency coefficient. Consistency coefficient is constantly decreasing during time in all samples. Before storage, samples S-1 and S-2 had higher k values than samples S-3 and S-4 (lower fluidity). Decrease in k value overtime indicates that fluidity of the sample increased during storage time. Moreover, weak physical bonds like electrostatic and hydrophobic forces might have been destroyed easily during shearing (ISANGA AND ZHANG, 2009). Destruction of cellular structure during freezing and thawing caused increase of flow behavior index value and decrease of consistency coefficient value. Although it is obvious that rheological behavior is changed during storage with increased fluidity, pseudoplastic behavior remained. Depending on the final purpose of puree, improvement of fluidity can be achieved by addition of hydrocolloids (ERGOVIĆ et al., 2010). Ital. J. Food Sci., vol. 32, 2020 - 954 4. CONCLUSIONS Based on the results of this paper it can be concluded that strawberry puree shouldn’t be stored in the freezer longer than 9 months to avoid excessive polyphenol degradation because after 9 months of storage degradation of polyphenols accelerated in all samples. Regardless of the method used, antioxidant activity of all samples decreased significantly within the first three months of storage and continued to decrease further during the rest of the time. Fluidity of all samples increased during time, consistency coefficient decreased, flow behaviour index increased, but rheological parameters stayed within the limits for pseudoplastic systems. After all, depending on the final purpose of the puree optimal values for flow behaviour index and coefficient of consistency should be determined. Shelf life cannot be evaluated only based on polyphenols and additional tests are required. REFERENCES Aaby K., Wrolstad R.E., Ekeberg D. and Skrede G. 2007. Polyphenol composition and antioxidant activity in Strawberry Purees: Impact of Achene level and storage. J. Agric. Food Chem. 55:5156-5166. Álvarez E., Cancela M.A. and Maceiras R. 2006. Effect of temperature on rheological properties of different jams. Int. J. Food Prop. 9: 135-146. Antonio G.C., Faria F.R., Takeiti C.Y. and Park K.J. 2007. Rheological behavior of blueberry. Cienc. Tecno. Aliment. 29:723-737. Baiano A., Terracone C., Gambacorta G. and La Notte E. 2009. Phenolic content and Antioxidant Activity of Primitivo wine: Comparison among Winemaking Technologies. J. Food Sci. 74:258-267. Bisharat G.I., Oikonomopoulou V.P., Panagiotou N.M., Krokida M.K. and Maroulis Z.B. 2013. Effect of extrusion conditions on the structural properties of corn extrudates enriched with dehydrated vegetables. Food Res. Int. 53:1-14. Bodelón O.G., Avizcuri J.M., Fernández-Zurbano P., Dizy M. and Préstamo G. 2013. Pressurization and cold storage of strawberry purée: Colour, anthocyanins, ascorbic acid and pectin methylesterase. LWT-Food Sci. Technol. 52:123-130. Bukurov M., Bikić S., Babić M., Pavkov I. and Radojčin M. 2012. Rheological behavior of senga sengana strawberry mash. Journal on Processing and Energy in Agriculture 16:142-146. Diamante L.M. and Liu H. 2016. Rheological properties of green and gold kiwifruit purees at different temperatures. J. Food Chem. Nanotechnol. 2: 50-56. Diamanti J., Mazzoni L., Balducci F., Cappelletti R., Capocasa F., Battino M., Dobson G., Stewart D. and Mezzetti B. 2014. Use of Wild Genotypes in Breeding Program Increases Strawberry Fruit Sensorial and Nutritional Quality. J. Agric. Food Chem. 66:13397-13404. El-Mansy H.A., Sharoba A.M., Bahlol H.E.L.M. and El-Desouky A.I. 2005. Rheological properties of mango and papaya nectar blends. Ann. Agric. Sci. 43:665-686. Ergović M., Obradović V., Škrabal S. and Jakobović S. 2010. Influence of starches on Rheological properties of blackberry puree. Works of the Faculty of Agricultural and Food Sciences University of Sarajevo 60:135-140. Ergović Ravančić M., Obradović V. and Škrabal S. 2012. Change of plum puree rheological parameters during storage in the freezer. In “47th Croatian and 7th international symposium of agronomists - Proceedings Book 2”. M. Pospišil (Ed.), p. 839. University of Zagreb, Faculty of Agriculture, Zagreb, Croatia. Ergović M., Obradović V. and Škrabal S. 2010. Rheological properties of blackberry puree with additives during refrigerated storage. In “2nd international conference "Vallis Aurea", focus on regional development/Proceedings”. B. Katalinić (Ed.), p. 331. Polytecnic in Pozega and DAAAM International Vienna, Požega, Croatia. Ergović M., Obradović V., Jakobović S., Škrabal S., Troha F. and Šnajder I. 2009. Influence of storage on the rheological properties of nectarine (prunus persica var. nucipersica l) puree. Journal on Processing and Energy in Agriculture 13:64-66. Ital. J. Food Sci., vol. 32, 2020 - 955 Figura L.O. and Teixeira A.A. 2007. “Fluid Physics”. Springer-Verlag, Berlin, Heidelberg. Gafta 2018. Register of Analysis Methods. www.gafta.com/write/MediaUploads/Contracts/2018/METHOD_10.1_ SUGAR_-_LUFF_SCHOORL_METHOD.pdf Galoburda R., Boca S., Skrupskis I. and Seglina D. 2014. Physical and chemical parameters of strawberry puree. In “Proceedings of the 9th Baltic Conference on Food Science and Technology Food for Consumer Well-Being”. E. Straumite (Ed.), p. 172. Latvia University of Agriculture, Faculty of Food Technology, Riga, Latvia. Gasperotti M., Maseuro D., Mattivi F. and Vrhovsek U. 2015. Overall dietary polyphenol intake in a bowl of strawberries: The influence of Fragaria spp. in nutritional studies. J. Funct. Foods 18:1057-1069. Hartmann A., Patz C.D., Andlauer W., Dietrich H. and Ludwig M. 2008. Influence of Processing on Quality Parameters of Strawberries. J. Agric. Food Chem. 56:9484-9489. Holzwarth M., Korhummel S., Carle R. and Kammerer D.R. 2012. Evaluation of the effects of different freezing and thawing methods on color, polyphenol and ascorbic acid retention in strawberries (Fragaria × ananassa Duch.). Food Res. Int. 48:241-248. Isanga J. and Zhang G. 2009. Production and evaluation of some physicochemical parameters of peanut milk yoghurt. LWT Food Sci. Technol. 42:1132-1138. Klopotek Y., Otto K. and Böhm V. 2005. Processing Strawberries to Different Products Alters Contents of Vitamin C, Total Phenolics, Total Anthocyanins, and Antioxidant Capacity. J. Agric. Food Chem. 53:5640-5646. Kreith F. (Ed.). 1999. “Fluid Mechanics” 1st ed. CRC Press, USA. Lovrić T. 2003. “Procesi u prehrambenoj industriji s osnovama prehrambenog inženjerstva”. Hinus, Zagreb, Croatia. Maceiras R., A´lvarez E. and Cancela M.A. 2007. Rheological properties of fruit purees: Effect of cooking. J. Food Eng. 80:763-769. Nindo C.L., Tang J., Powers J.R. and Tlakhar P.S. 2007. Rheological properties of blueberry puree for prossesing application. LWT 40:292-299. Nowicka A., Kucharska A.Z., Sokół-Łętowska A. and Fecka I. 2019. Comparison of polyphenol content and antioxidant capacity of strawberry fruit from 90 cultivars of Fragaria × ananassa Duch. Food Chem. 270:32-46. Obradović V., Babić J., Šubarić D., Jozinović A., Ačkar Đ. and Klarić I. 2015. Influence of dried Hokkaido pumpkin and ascorbic acid addition on chemical properties and colour of corn extrudates. Food Chem. 183:136-143. Osorio O., Martínez-Navarrete N., Moraga G. and Carbonell. J.V. 2008. Effect of thermal treatment on enzymatic activity and rheological and sensory properties of strawberry purees. Food Sci. Tech. Int. 14(5):103-108. Oszmiański J., Wojdyło A. and Kolniak J. 2009. Effect of l-ascorbic acid, sugar, pectin and freeze–thaw treatment on polyphenol content of frozen strawberries. LWT-Food Sci. Technol. 42:581-586. Peinado I., Rosa E., Heredia A. and Andrés A. 2012. Rheological characteristics oh healthy sugar substituted spreadable strawberry product. J. Food Eng. 113:365-373. Rudra S.G., Sarkar B.C., Shivhare U.S. and Basu S. 2007. Rheological properties of coriander and mint leaf puree. J. Food Process. Eng. 31:91-104. Sorour M.A., Rabie S.M.H. and Mohamed A.Y.I. 2016. Rheological properties of some fruit spreads. Int. J. Nutr. Food Sci. 5:14-22. Yalçınöz S.K. and Erçelebi E. 2016. Rheological and sensory properties of red colored fruit sauces prepared with different hydrocolloids. Agriculture & Food 4:496-509. Yildiz1 H., Ercisli S., Hegedus A., Akbulut M., Topdas E.F. and Aliman J. 2014. Bioactive content and antioxidant characteristics of wild (Fragaria vesca L.) and cultivated strawberry (Fragaria × ananassa Duch.) fruits from Turkey. J. Appl. Bot. Food Qual. 87:274-278. Żebrowska J., Dyduch-Siemińska M., Gawroński J., Jackowska I. and Pabich M. 2019. Genetic estimates of antioxidant properties in the conventionally and in vitro propagated strawberry (Fragaria × ananassa Duch.). Food Chem. 299:125110. Paper Received April 6, 2020 Accepted October 2, 2020