Changes in the rheological properties of stirred acid milk gels as induced by the acidification procedure

Abstract

-The aim of this study was to compare the rheological properties of stirred acid milk gels, acidified using two different acidification procedures. Pre-heated milks were acidified either by means of glucono-δ-lactone (GDL gel) at 20• C or by incubation with lactic acid bacteria (lactic gel) at 38• C. The latter procedure gave a harder acid set gel with a more heterogeneous structure. Once formed, the gels were filtered and stirred, then immediately characterised using the low-amplitude dynamic oscillation method for 20 h at 4• C or creep measurement at 4• C. The stirred gels were stored at 4• C for 28 d, during which the pH and the viscosity at 64 s −1 were measured. After stirring, the lactic gel made at 38• C had a higher instantaneous elastic modulus, a higher viscosity and showed a greater increase in the elastic modulus over 20 h compared with the stirred GDL gel. The viscosity of both stirred gels progressively increased during storage at 4• C, but that of GDL acidified gels increased 3.5 times more than that of lactic gels. The procedure used to prepare the set acid gels had notable effects on the interactions formed within 20 h and 28 d of storage of stirred gels. Résumé -Effet de la procédure d'acidification sur les propriétés rhéologiques des gels acides brassés. Le but de ce travail est de comparer les propriétés rhéologiques des gels acides brassés acidifiés par 2 procédures différentes. Le lait traité thermiquement était acidifié soit par addition de glucono delta lactone à 20• C (gel GDL), soit par incubation avec des bactéries lactiques à 38• C (gel lactique). Cette dernière procédure donnait des gels plus fermes avec une structure davantage hétérogène. Une fois formés, les gels étaient filtrés et brassés, puis immédiatement caractérisés en utilisant la méthode dynamique d'oscillations en faible amplitude pendant 20 h à 4• C et par fluage à 4• C. Les gels brassés étaient stockés à 4• C pendant 28 j pendant lesquels le pH et la viscosité à 64 s −1 étaient mesurés. Après brassage, le gel lactique préparé à 38• C avait un plus grand module élastique instantané, une plus grande viscosité et montrait une plus grande augmentation du module élastique pendant 20 h, comparé au gel GDL brassé. La viscosité des 2 types de gel brassé augmentait progressivement pendant le stockage à 4• C, mais elle augmentait 3,5 fois plus dans le gel GDL que dans le gel lactique. La procédure appliquée pour préparer le gel acide avait des effets notables sur les interactions formées à 20 h et à 28 j de stockage des gels brassés. yaourt brassé / yaourt ferme / gel acide / rhéologie

Rheological properties of stirred yoghurt as affected by gel pH on stirring, storage temperature and pH changes after stirring

“…The increase in G 0 after stirring was found as a two-stage process. G 0 increased first rapidly for 2 h then slowly for at least the following 22 h (result not shown), in accordance with other authors (Arshad et al, 1993;Cayot et al, 2003;Renan et al, 2008aRenan et al, , 2008b.…”

“…Set-style yoghurts were produced according to Renan et al (2008a). Milk was reconstituted at 140 g dry matter per kg, heattreated (90 C for 10 min) and inoculated with strains of LB340 and TA060 at 0.002 and 0.02 units L À1 (1 unit ¼ 2 Â 10 9 ), respectively, after 0, 1 or 2 days after milk heat-treatment.…”

“…were forced under compressed air through a mesh of 350-400 mmholes and stirred in a home food processor for 10 s at 300 rpm according to Renan et al (2008a). Half of each stirred gel was withdrawn, then 2.05 g kg À1 NaN 3 was added and stirring of the remainder was continued for another 2 s. The experimental error was estimated by performing 3 repetitions of the centre point of the experimental design.…”

“…The gel is first cooled from the incubation temperature to approximately 20 C in a heat exchanger, then stored at 20 C prior to filling of the retail container and further cooling at 4 C. Filling at this rather high temperature could prevent excessive structural breakdown. Viscosity of the product decreases during mixing and pumping (stirring) but increases again during cold storage (Dejmek and Cuvelier, 1998;Renan et al, 2008a;Sodini et al, 2004;Steventon et al, 1989). This phenomenon is called ''rebodying'' or structure recovery.…”

The rebodying of stirred yoghurt: interactions between proteins

“…Milk was reconstituted from ultra low heat skim milk powder as described in Renan et al (2008a) at 140 g dry matter per kg for samples with no further addition or at 147 g per kg for samples where salt or reagent addition induced a slight dilution of yoghurt. Milk was heat-treated as described in Laligant et al (2003).…”