Effect of freezing on viscoelastic behaviour of heat-treated whey protein suspensions

B. E. MEZA; R. A. VERDINI; A. C. RUBIOLO

Viña del Mar - Chile

Congreso; International Congress of Engineering of Food; 2008

Ministerio de Agricultura y Universidades de Chile

Whey proteins (WP) are well known for combining high nutritional value with functional properties. Heat treatments can improve WP functionality and increase their application as food ingredients. When WP suspensions are heated, proteins partially unfold and aggregate. If protein concentration is sufficiently low, aggregates remain soluble and modify rheological properties of WP suspensions. Freezing is widely used for food preservation and has recently been introduced for food texturization. Crioconcentration that occurs during freezing can produce modifications in the rheological behavior of heat-treated WP suspensions.  In this study, the effect of freezing on viscoelastic behavior of heat-treated WP  suspensions was analyzed. Suspensions containing 5 and 9% w/v of total protein (TP) were prepared from a commercial WP concentrate. Samples were heat-treated at 72.5 and 77.5ºC (HS) to obtain 60% of denatured protein (DP). A group of HS was frozen  at -25ºC for 24 h, and thawed at 20ºC (HS-F). Frequency sweeps were performed in the range 0.01-10 Hz at 20ºC. Frequency (w) dependence of elastic (G’) and viscous (G”) moduli was modeled with power law equations (G’=awx, G”=bwy), and characteristic relaxation times (tc) were calculated as the inverse of the crossover frequency.  Power law parameters related to G’ were influenced by freezing only in suspensions with 9% of TP when heating temperature was 77.5ºC, being coefficient a lower and slope x higher for HS-F. However, parameters related to G” were much more influenced by freezing. Coefficient b was lower for HS-F in both suspensions with 9% of TP (72.5 and 77.5ºC) and slope y was higher for HS-F in all HS-F suspensions. Characteristic relaxation times were influenced by freezing only in suspensions with 5% of TP, being higher for  HS-F. Consequently, the effects of freezing on viscoelastic properties of heat-treated WP suspensions prepared with the same content of DP were strongly dependent on both heating temperature and TP content. These results could increase the knowledge of the rheological properties of WP suspensions that is necessary to improve texture of food products.