Viscosity of set-style yogurt as influenced by heat treatment of milk and added demineralized whey powder

Abstract

Skim milk powder was reconstituted to obtain milk A (with 8.44% TS). Milk sample A was standardized with different amounts of demineralized whey powder (DWP) to obtain milk B (with 9.71% TS) and milk C (with 10.75% TS). Milk samples were heat treated at 85ºC/20 min and 90ºC/10 min, respectively. Untreated milk was used as control. Milk samples were inoculated with 2.5% of commercial yogurt culture (containing Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus in the ratio 1:1) at 43ºC. Samples were incubated until pH 4.6 was reached. Samples were immediately cooled to 4ºC and held at that temperature until analyses. Measurements of viscosity were done with Brookfield DV-E Viscometer. Spindle No 3 at 20 rpm was used for all samples. After 1 day of storage, set-style yogurt samples produced from untreated milk had the highest, while samples produced from milk heat treated at 90ºC/10 min the smallest initial viscosity, regadless of the dry matter content and composition. Average viscosity of set-style yogurts decreased with intensifying temperature of applied heat-treatment. During storage, set-style yogurt samples produced from milk heat treated at 90ºC/10 min had the least pronounced decrease of viscosity during shearing. After 14 days of storage, set-style yogurt samples produced from milk standardized with demineralized whey powder had higher viscosity than samples produced from basis milk.

Obrano mleko u prahu je rekonstituisano i dobijeno je mleko A (sa 8.44% SM). Mleku A je dodavana demineralizovana surutka u prahu i dobijeni su uzorci mleka B (sa 9.71% SM) i mleka C (sa 10.75% SM). Svi uzorci su termički tretirani na 85ºC/20 min i 90ºC/10 min. Kao kontrolni uzorak korišteno je termički netretirano mleko. Svi uzorci su inokulisani na 43ºC sa 2.5% tečne jogurtne kulture (Lactobacillus delbrueckii subsp. bulgaricus i Streptococcus thermophilus u odnosu 1:1) i inkubirani do postizanja pH 4.6. Nakon fermentacije uzorci su ohladjeni na 4ºC i držani na toj temperaturi do analize. Viskozitet je određivan nakon 1., 7. i 14. dana skladištenja pomoću Brookfield-ovog viskozimetra DV-E pri brzini rotacije spindla od 20 o/min. Nakon 1. dana skladištenja, uzorci čvrstog jogurta proizvedeni od netretiranog mleka imali su najveće a uzorci proizvedeni od mleka termički tretiranog na 90ºC/10 min najmanje inicijalne vrednosti viskoziteta, bez obzira na sastav i sadržaj suve materije. Ujedno, srednja vrednost viskoziteta čvrstog jogurta se smanjuje sa povećanjem temperature primenjenog termičkog tretmana. Tokom skladištenja uzorci čvrstog jogurta proizvedeni od mleka termički tretiranog na 90ºC/10 min imaju najmanje izraženo smanjenje viskoziteta tokom vremena. Nakon 14 dana skladištenja uzorci jogurta proizvedeni od mleka standardizovanog DSUP imaju veće vrednosti viskoziteta od uzoraka proizvedenih od mleka A.