Rheological properties of currant purees and jams: Effect of composition and method of proceeding

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1 WFL Publisher Science and Technology Meri-Rastilantie B, FI-9 Helsinki, Finland Journal of Food, Agriculture & Environment Vol. (&) : -. Rheological properties of currant purees and jams: Effect of composition and method of proceeding Vilma Speicien, Daiva Leskauskaite, Pranas Viskelis * and Marina Rubinskiene KTU Food Institute, Taikos str. 9, LT-, Kaunas, Lithuania. lmai@lmai.lt. Kaunas University of Technology, Department of Food Technology, Radvil n str. 9, LT-, Kaunas, Lithuania. daiva.leskauskaite@ktu.lt. Lithuanian Institute of Horticulture, Kauno str., LT- Babtai, Kaunas distr., Lithuania. * biochem@lsdi.lt, viskelis@parabole.lt Received June, accepted September. Abstract The rheological properties of thawed purees made from blanched and cold-pressed red and black currants as well as jams made from thawed purees were investigated. The influence of thawing conditions on the rheological properties and syneresis of purees was evaluated. The purees were thawed in three different ways: at room temperature, in and with. The rheological properties of purees and jams were characterized by the flow curves obtained at s - shear rate range using a controlled stress rheometer. The results indicated that purees and jams behaved as non-newtonian fluids, well described by Power-Law model. Thawing with caused higher rheological parameters comparing to the longer thawing conditions that determined the decrease in rheological properties. The composition and method of pre-treatment of currants had influence on the rheological properties and syneresis of thawed purees: red currant purees showed lower viscosity and higher syneresis than black currant purees; purees from blanched currants had higher water retention capacity comparing to purees from cold-pressed currant. It was concluded that the structure of purees made from blanched black currants thawed with can be characterized as well-connected, viscous structure. The rheological properties of currant jams were influenced by the rheological properties of the purees they were made from. Jams made from blanched currant purees showed higher viscoelastic properties (storage modulus and loss modulus), rheological properties (consistency index K and thixotropy) and lower syneresis in comparison with jams made from cold-pressed currants. Key words: Red currants, black currants, puree, jam, thawing, rheological properties, syneresis. Introduction Frozen purees made from different berries are widely used in the food processing industry. Thawing is one of the most damaging stages in the processing of frozen foods because it dramatically affects the structure of the products. During freezing ice crystals, that form in the cellular medium of fruit cells, puncture the cell membrane and damage their structure. Thawing causes further damage to cell walls and membranes. These two processes lead to a loss of turgidity and, hence, fruit textural properties in a thawed product,. The size of ice crystals is critical and is determined by the freezing-thawing rates. Low thawing rates, as low freezing rates can determine grow of very large ice crystals and result in higher undesirable changes in texture of fruits. The quality of products made from currants depends on the properties of raw materials and the applied technologies for their manufacture. It can be predicted that the quality of jams produced from frozen purees will be affected by the properties of the purees after thawing and added ingredients (e.g., sugar, thickeners and others), 7. Unfortunately, we did not find data on the correlation between textural properties of currant jams and the purees they were made from. In addition to taste, texture is a quality attribute that is critical in determining the acceptability of jams by consumers. Texture is defined as the sensory and functional manifestation of the structural, mechanical and surface properties of foods detected through the senses of vision, hearing and kinesthetics. However, the properties affecting texture of foodstuffs are predominantly rheological properties such as elasticity, viscosity and fracture, which can be measured with instruments (rheometers, viscosimeters, etc.) 9. Thus, the rheological measures of jams are useful in the quality control of the product. The objectives of this study were: () to determine the influence of thawing conditions on the rheological properties and syneresis of purees made from blanched and cold-pressed red and black currants; () to determine the rheological properties and syneresis of jams made from purees with different compositions and different treatments and () to determine the relationship between rheological properties and syneresis of currant purees and the jams made from them. Materials and Methods Purees and jams made from the berries of Ben Lomond black currant cultivar and Hollandische red currant cultivar were investigated. The following purees were used for investigations: red currants, blanched; red currants, cold-pressed; black currants, blanched; Journal of Food, Agriculture & Environment, Vol. (&), July-October

2 red-black currants mixture (:), cold-pressed. Jams made from the following purees were used for the investigation: red currants, blanched; red currants, cold-pressed; red-black currants mixture (:), blanched; red-black currants mixture (:), cold-pressed; black currants, blanched. All experiments were performed in triplicate. Puree preparation: Purees were made in two ways: blanching and cold-pressing. For blanched purees, berries were processed by steam at a temperature of C for min in a conventional Unox oven (Italy), rubbed by a MP-M rubbing machine (Byelorussia) and strained through a sieve with holes of mm in diameter. For cold-pressed purees, berries were strained by a LZ-. industrial apparatus (China) through a sieve with holes of. mm in diameter. Prepared purees were poured into. L plastic containers and were frozen at a temperature of C. They were held for months before they were thawed and investigated. The thawing process for frozen currant purees was carried out in three different conditions: ) microwave thawing by two s duration pulses at W power; ) thawing at room temperature (± C) for hours; ) thawing in a (± C) for hours. Thawed purees were frozen second time and were held for month at a temperature of C. Then purees were thawed applying the same conditions as described before, and rheological properties of them were measured. Jam preparation: For the production of the jams, the frozen berry purees were thawed at room temperature and mixed with sugar to obtain dry soluble solids content of ±% and heated at 9 C for min. The hot mass was packed into. L jars and pasteurized at 9 C for minutes. The quality of the products was evaluated after months. Determination of total soluble solids and dry matter: Total soluble solids (TSS) were determined by the refractometer method using a digital refractometer (ATAGO PR-, Atago Co., Ltd., Tokyo, Japan). Dry matter (DM) content was determined after forced air convention drying at C to a constant weight (Air Oven Method) 9. Measurements of rheological properties and syneresis: Rheological measurements of the currant purees and jams were carried out using a CSL controlled stress rheometer (Carri- Med Ltd, Dorking, UK) with cone and plate geometry (diameter mm, cone, gap µm). Shear rate was increased from to s - for min and decreased from to s - for min at C. The Power-Law model was applied to fit the flow data: τ = K γ n, where τ is shear stress (Pa), γ is shear rate (s - ), K is consistency index (Pa sn), which corresponds to the apparent viscosity when the shear rate is equal to s - and n is the flow behavior index, which reflects the closeness to Newtonian flow. Thixotropic behaviour was evaluated by calculating the area of the hysteresis loop between the upward and downward shear stress/shear rate curves. For jam samples, the frequency dependence of storage (G ) and loss (G ) modulus as a function of the oscillatory frequency at Hz was measured by performing a frequency sweep test at a constant stress of. Pa. Syneresis of purees and jams was measured as the amount of separated free serum (juice) in a hour time period at room temperature (± C). Results and Discussion Table shows the average values of dry matter and dry soluble matter solid content in the red and black currant purees that were pre-treated before freezing and thawed under different conditions. The highest DM and TSS contents were obtained in the case of the blanched black currants thawed in a (.%) and by microwave (.-.%). The DM and TSS contents were lowest in the blanched red currant samples thawed in a and by microwave. Thawing at room temperature caused the lowest values of DM and TSS content in all cases. The rheological properties of purees were characterised by the flow curves obtained at - s - shear rate range using a controlled stress rheometer (Fig. ). The results indicated that all purees behaved as non-newtonian fluids, exhibiting shearing timedependent behaviour and shear rate thinning. Such a pseudoplastic behaviour exhibits in other fruit purees,. The sample of cold-pressed red currant puree was not suitable for this measurement as it was very inhomogeneous, with big particles that tended to separate. It was determined that blanched black currant samples had stronger structure compared to that of blanched red currant, or the mixture of cold-pressed red and black currant (:) samples. The rheological properties of different processed currant purees thawed in different conditions were described by Power-Law parameters (Table ). According to the literature data, a lot of other fruit purees showed flow behaviour well described by the Power-Law model -. By comparing consistency indices of purees, which correspond to the viscosity of the purees at small shear rates s -, it can be seen that the highest values of indices were shown by purees thawed by. Additionally, the longer thawing time caused a decrease in consistency index values. Also it is obvious that black currant purees are more viscose than those from red currants. When the flow properties of purees were measured after a second freezing and thawing, it can be seen that the consistency indices of purees decreased. By comparing flow indices, which indicate the similarity of the purees to Newtonian substances, it was determined that the least pseudoplastisity can be attributed to purees thawed in a. According to the data presented in Table, it also can be concluded that the difference in pseudoplastisity at room Table. Dry matter (DM) and total soluble solids (TSS) of currant purees thawed under different conditions. Thawing condition Currant puree Room temp. Refrigerator Microwave DM, % TSS, % DM, % TSS, % DM, % TSS, % Red currants, blanched Red currants, cold-pressed Black currants, blanched Red-black currant mixture (:), cold-pressed Journal of Food, Agriculture & Environment, Vol. (&), July-October

3 temperature and in -thawed purees was most expressed in cold-pressed purees. However, second freezing only slightly altered flow indices of all purees. Thixotropy of structured systems is caused by structural breakdown under shear. The return curves showed thixotropic behaviour for all purees (Fig. ). The influence of thawing conditions, as well as the pre-treatment of currants, on the thixotropic properties of purees was studied (Table ). Blanched black currant purees displayed a higher degree of hysteresis. Thixotropic behaviour was different for the purees thawed in different conditions. In all cases samples thawed in a a) b) c) Figure. Flow curves of purees of blanched red currants (a), blanched black currants (b), and cold-pressed red-black currant mixture (:) (c), thawed under different conditions. exhibited a lower degree of hysteresis, suggesting a lower connectivity of the formed structure of the system. The flow behaviour of purees frozen twice was characterized by a smaller area of the hysteresis loop between upward and downward shear stress/shear rate curves in comparison with the curves obtained for purees frozen only once. The syneresis of currant purees thawed at room temperature is shown in Fig.. The red currant purees showed greater syneresis than those made from black currants, and cold-pressed currant purees syneresis was greater than syneresis from blanched currant purees. Despite these findings the clear dependence of thawing conditions on the juice separation dynamic was not established. The differences determined in the rheological properties and syneresis of purees can be caused by differences in composition (e.g. soluble solids content, as well as pulp content and size and shape of particles), 7. According to the data presented in Table, there are smaller quantities of dry soluble solids in red currants than in black currants. This difference could determine the higher viscosity of black currant purees. A sieve with smaller diameter holes was used for the blanching process than for the cold-pressing process, and this caused the purees to contain particles of different sizes. Small particles conform to flow alignment more easily than larger particles. Thawing conditions also could alter both these two parameters. They could impact different cell damage and could change the size and shape of solid particles as well as the quantities of soluble solids released from damaged cells. The second part of our study was conducted on jams made from purees with different compositions and different processed. Flow curves for jams produced from different compositions of currant purees are shown in Fig.. All samples exhibited pseudoplastic behaviour with clear shear rate dependent flow. The influence of the method of currant treatment on the flow behaviour was similar in both jam recipes: the shear stress values of jams made from blanched currants were slightly higher than those produced from cold-pressed currants. The Power-Law model was used to model the shear stress data of the jams at increasing shear rate (Table ). This model was used by other authors as well to describe the flow behaviour for a number of fruit jams 7,, 9. The analysed jam samples showed differences in the Power-Law model parameters. Red currant jams, in comparison with those made from black currants, had a lower consistency index (K), slightly higher flow index values (n) and lower thixotropy values indicating lower connectivity of the jams structure. The effect of currants proceeding method on the Power- Law model parameters was also observed. The parameter that was most affected was K, which was lower for jams made from cold-pressed currants than for jams from blanched purees of all samples. According to the data presented in Table, it can be stated that thixotropy was influenced by the method of currant treatment. All samples made from blanched currant purees showed higher thixotropy, indicating higher resistance to flow and higher stability under the effect of shear. The mechanical spectra of jams were measured and storage modulus (G ) and loss modulus (G ) values at the frequency of Hz are presented in Table. For jams made from red currant purees, the storage modulus G and the loss modulus G values were significantly lower than those for jams made from black currant puree. Both modules were affected by the method of currant Journal of Food, Agriculture & Environment, Vol. (&), July-October

4 Table. s of different processed currant purees thawed in different conditions. Red currant purees, Black currant purees, Red-black currant purees mixture blanched blanched (:), cold-pressed Microwaves Micro- Micro- temp. rator waves temp. rator waves temp. rator time frozen purees K (Pa s n ) n Thixotropy (Pa s - ) R times frozen purees K (Pa s n ) n Thixotropy (Pa s - ) R time frozen times frozen Figure. Flow curves of the purees (red currants thawed by ) increasing and decreasing shear rate. a) red, blanched red, cold-pressed Shear rate, s - Amount of separated juice (ml). Time (h) red-black, blanched red-black, cold-pressed b) Shear rate, s - Figure. Syneresis of different processed currant purees (thawed at room temperature): - red, cold-pressed ; - red, blanched ; - red-black, cold-pressed; - black, blanched. Figure. Flow curves of jams made from different processed red currant (a) and red-black currant (b) mixtures. treatment. In all cases jams made from blanched currant purees showed higher G and G when compared with jams made from cold-pressed currants. The syneresis rate of the jams made from differently processed currants is presented in Fig. The process of syneresis, when physically not binding water separate in food system, is characteristic for many products that have a gel structure. It is evident that the biggest amount of water separated from jams made from cold-pressed red currant purees, and the best water retention was observed in the jam made from blanched black currants. Jams made from blanched currant purees showed lower syneresis than jams made from cold-pressed purees. This can be explained by higher viscosity of the former jams. By comparing rheological parameters presented in Tables and it can be seen that jams have higher values of consistency index and thixotropy than purees. Jams showed higher water retention capacity comparing to purees (Fig., Fig. ) that is determined by the well-connected, viscous structure. Since jams were prepared without thickening agents and sugar was the only ingredient added to the purees in the production of jams, the Journal of Food, Agriculture & Environment, Vol. (&), July-October

5 texture of jams could be affected by pectin presented in currants and sugar acting as dehydrating agent. The method of currant purees preparation had the significant effect on the rheological properties of jams as well. The highest consistency index, storage modulus and loss modulus were determined for the jam made from purees containing black currants processed by blanching before freezing. Table. s of jams of different processed currants. Jam of currants K (Pa s n ) n Thixotropy (Pa s - ) R Red, blanched..9.9 Red, cold-pressed Black, blanched..9.9 Red-black, blanched Red-black, cold-pressed Table. Storage modulus (G ) and loss modulus (G ) of jams of different processed currants at Hz. Jam of currants G' (Pa) G'' (Pa) Red, blanched.. Red, cold-pressed.7.9 Black, blanched.9.9 Red-black, blanched..9 Red-black, cold-pressed Amount of separated juice (ml). Time (h) Figure. Syneresis of jams made from different processed currants: - red, cold-pressed; - red, blanched; - red-black, blanched; - red-black, cold-pressed; - black, blanched. Conclusions ) Microwave thawing causes higher pseudoplastisity and viscosity of currant purees in comparison with thawing at room temperature and in a. Longer thawing times determined the decrease in rheological properties of currants purees. ) Red currant purees showed lower viscosity and higher syneresis than black currant purees. The method of pre-treatment of currants before freezing had influence on the properties of thawed purees: syneresis of cold-pressed currant purees was higher than syneresis of blanched currant purees. ) The rheological properties of currant jams were influenced by the rheological properties of the purees they were made from. Jams made from blanched currant purees showed higher G, G, consistency index K and thixotropy in comparison with jams made from cold-pressed currants. Acknowledgement This work was partly supported by Lithuanian State Science and Studies Foundation. References Alonso, J., Canet, W. and Rodriguez, T Thermal and calcium pretreatment affects texture, pectinesterase and pectic substances of frozen sweet cherries. J. Food Sci. :-. Sausa, M.B., Canet W., Alvarez, M.D. and Fernandez, C. 7. Effect of processing on the texture and sensory attributes of raspberry (cv. Heritage) and blackberry (cv. Thornfree). J. Food Eng. 7:9-. Grout, B.W.W., Morris, G.J. and McLellan, M.R. 99. The freezing of fruit and vegetables. In Bald, W. B. (ed.). Food Freezing: Today and Tomorrow. London, Springer, pp. -. Delgado, A.E. and Rubiolo, A.C.. Microstructural changes in strawberry after freezing and thawing processes. LWT :-. Flaumenbaum, B. 9. Basis of Conservation of Food Products. Moscow, 9 p. (in Russian). Afoakwa, E., Nartey, E., Ashong, J. and Annor, G.. Effect of sugar, pectin and acid balance on the quality characteristics of pineapple (Ananas comosus) jam. Proceeding of the th World Food Congress, Nantes, France, pp Maceiras, R., Alvarez, E. and Cancela, M.A. 7. Rheological properties of fruit purees: Effect of cooking. J. Food Eng. :7-79. Rosenthal, A.J Relation between instrumental and sensory evaluation of texture. In Food Texture: Measurements and Perception. Aspen Publishers, Gaithersburg, pp Tabilo-Munizaga, G. and Barbosa-Canovas, G.V.. Rheology for the food industry. J. Food Eng. 7:7-. AOAC 99. Solids (soluble) in fruits and fruit products. In Helrich, K. (ed.). Official Methods of Analysis. th edn. AOAC Inc., Arlington, VA, p. 9. Manuals of Food Quality Control 9. Vol. 7. Food Analysis: General Techniques, Additives, Contaminants, and Composition. FAO, Rome, p. Rao, A. (ed.)999. Rheology of Fluids and Semisolids Foods. Principles and Applications. Aspen Publishers, Inc., Gaithersburg, Maryland, p. Krokida, M.K., Maroulis, Z.B. and Saravacos, G.D.. Rheological properties of fluid fruit and vegetable puree products: Compilation on literature data. Int. J. Food Prop. :79-. Yeow, Y.L., Perona, P. and Leong, Y.K.. A reliable method of extracting the rheological properties of fruit purees from loop data. J. Food Sci. 7:7-. Guerrero, S.N. and Alzamora, S.M Effect of ph, temperature and glucose addition on flow behaviour of fruit purees: II. Banana puree. J. Food Eng. :9-. Guerrero, S.N. and Alzamora, S.M. 99. Effect of ph, temperature and glucose addition on flow behaviour of fruit purees: II. Peach, papaya and mango purees. J. Food Eng. 7: Nindo, C.I., Tang, J., Powers, J.R. and Takhar, P.S. 7. Rheological properties of blueberry puree for processing applications. LWT :9-99. Costell, E., Carbonell, E. and Duran, L. 97. Chemical composition and rheological behaviour of strawberry jams. Relation with fruit content. Acta Alimentaria :9-. 9 Grigemo-Miquel, N. and Martin-Belloso, O Influence of fruit fibre addition on physical and sensorial properties of strawberry jams. J. Food Eng. :-. Journal of Food, Agriculture & Environment, Vol. (&), July-October