Rheology, microstructure and sensorial properties of probiotic yoghurts containing fiber from passion fruit by-product

Passion fruit rinds are a by-product of the fruit pulp industry and are rich in total dietary fiber, particularly pectins. The association of probiotic bacteria and total dietary fiber in a food product such as yoghurt can help to promote the intestinal normobiosis. Nevertheless, the addition of fruit dietetic fiber into yoghurt is a challenge due to the fragile structure of the product and to possible changes in its sensorial characteristics. So, the aim of this study was to evaluate the influence of the addition of passion fruit fiber obtained from the fruit by-product on the rheology of probiotic yoghurts on 1, 7 and 14 days of cold storage. Microstructure and sensorial characteristics were also evaluated on 1 day. To reach these goals, skim milk bases with 1% of passion fruit fiber and without fiber (control) were heat treated and inoculated with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (CY340), and divided into four groups according to the probiotic strain added: Lactobacillus acidophilus strains L10 and NCFM, Bifidobacterium animalis subsp. lactis strains Bl04 and B94. Fermentations were performed at 42ºC until pH 4.5. The rheological measurements were carried out in quintuplicate at 5°C, using a rotational rheometer with a cone-plate sensor. Two cycles of shear rate (0 – 15 s-1) in upward and downward curves were performed, and the corresponding shear stress (, Pa) data recorded. For the sensorial analysis three coded samples of passion fruit fiber yoghurts, namely without probiotic, or co-fermented by L. acidophilus L10 or B. animalis subsp. lactis Bl04, were evaluated in a 9-point structured scale by untrained panelists (n=170). The photomicrographs of three freeze-dried samples of each yoghurt type were made in a field-emission scanning electron microscope. Results were submitted to ANOVA using Tukey test at P<0.05. During 2 weeks of cold storage, the thixotropy of all passion fruit fiber yoghurts was higher (25-32%) than that of their respective controls (14-22%) in the first cycle. All yoghurts showed a dramatic decrease in the structure recovery in the second cycle but, again, the fiber yoghurts presented higher averages of thixotropy (3.5-8%) than their controls (1-4%). The same happened with apparent viscosity, which was significantly higher in fiber yoghurts co-fermented by the lactobacilli than in their controls at the end of cold storage. The rheological behavior of the yoghurts were better explained by the Power Law Model (R2 > 0.97). Differences in the microstructure of yoghurts with and without passion fruit fiber were evident in photomicrographs, which demonstrated that in passion fruit fiber yoghurts, the casein gel overlays the fiber and sometimes is nestled in it. On the other hand, filaments of exo-polysaccharides were more frequent in control yoghurts. The attributes of appearance, smell and color of the passion fruit fiber yoghurts were considered good or very good, and the intensity of the passion fruit flavor was considered weak by the panelist. Regarding the sensorial analyses, the results indicates that the passion fruit fiber is an almost neutral ingredient to the formulation of new high value-added yoghurt.