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 85?C/20 min and 90?C/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 43?C. Samples were incubated until pH 4.6 was reached. Samples were immediately cooled to 4?C 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 90?C/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 90?C/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 termicki tretirani na 85?C/20 min i 90?C/10 min. Kao kontrolni uzorak koristeno je termicki netretirano mleko. Svi uzorci su inokulisani na 43?C sa 2.5% tecne 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 4?C i drzani na toj temperaturi do analize. Viskozitet je odredjivan nakon 1., 7. i 14. dana skladistenja pomocu Brookfield-ovog viskozimetra DV-E pri brzini rotacije spindla od 20 o/min. Nakon 1. dana skladistenja, uzorci cvrstog jogurta proizvedeni od netretiranog mleka imali su najvece a uzorci proizvedeni od mleka termicki tretiranog na 90?C/10 min najmanje inicijalne vrednosti viskoziteta, bez obzira na sastav i sadrzaj suve materije. Ujedno, srednja vrednost viskoziteta cvrstog jogurta se smanjuje sa povecanjem temperature primenjenog termickog tretmana. Tokom skladistenja uzorci cvrstog jogurta proizvedeni od mleka termicki tretiranog na 90?C/10 min imaju najmanje izrazeno smanjenje viskoziteta tokom vremena. Nakon 14 dana skladistenja uzorci jogurta proizvedeni od mleka standardizovanog DSUP imaju vece vrednosti viskoziteta od uzoraka proizvedenih od mleka A.