RELATION BETWEEN SENSORY TEXTURE PROPERTIES AND EXOPOLYSACCHARIDE DISTRIBUTION IN SET AND IN STIRRED YOGHURTS PRODUCED WITH DIFFERENT STARTER CULTURES

Set yoghurts produced with seven different exopolysaccharide‐producing starter cultures were subjected to sensory texture profiling including texture evaluation before and after a stirring procedure carried out by trained assessors. The microstructure of four selected yoghurts was investigated by confocal laser scanning microscopy with special focus on the distribution of exopolysaccharides (EPS) in the protein network. Two different types of microstructures were observed, one in which EPS were associated with the protein network and another where EPS appeared to be incompatible with the protein; as a consequence, the EPS were situated in the void spaces in the protein network. The latter yoghurts were less ropy and had higher serum separation than the yoghurts where EPS were associated with the protein, and they exhibited a significant increase in mouth thickness as a result of stirring. Yoghurts in which the EPS were associated with protein had high ropiness, low serum separation and appeared more resistant to stirring. The nature of EPS‐protein interactions is extremely important for the textural effects of EPS in fermented milk products, and such interactions depend on the specific starter culture used.     

Modifying the microstructure of low-fat yoghurt by microfluidisation of milk at different pressures to enhance rheological and sensory properties

The effects of microfluidisation of milk at different pressures, prior to heat treatment, on structural and sensory properties of low-fat stirred yoghurt, were investigated. Low-fat yoghurts prepared from microfluidised milk were compared with low-fat (1.5%) and full-fat (3.5%) control yoghurts made with homogenised (20/5 MPa) milk. The microstructure of low-fat yoghurts prepared with microfluidised milk consisted of smaller and more uniform fat globules, well incorporated into more interconnected fat-protein gel networks, compared with those of control yoghurts. This modification in microstructure caused significant changes in gel particle size, sensory profile and rheological behaviour. Microfluidisation increased the gel particle size, gel strength and viscosity; marked beneficial effects were found at higher pressures (50–150 MPa). Microfluidising milk at 50–150 MPa increased the gel strength by 171–195% and viscosity by 98–103%, creating low-fat yoghurts with creaminess and desirable texture properties similar to, or better than, full-fat conventional yoghurt.     

Incorporation of selected nutraceuticals and probiotic bacteria into a fermented milk

Yoghurts were produced from a base milk containing three important nutraceuticals, namely ω-3-fatty acids, isoflavones and phytosterols. The cultures employed to make the yoghurts were single probiotic strains of Lactobacillus gasseri or Bifidobacterium infantis and, to achieve a short production time, a two-stage fermentation procedure was used with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus providing the rapid acidification. Yoghurts containing counts of >1.0×10⁸ cfu mL⁻¹ of the individual probiotics and high counts of the traditional species from yoghurt were awarded overall scores for sensory acceptability >4.0 out of 5.0; the nutraceuticals appeared to have no adverse effect on flavour. Storage trials at 5 °C showed that the viability of the probiotic cultures was retained over 15 days.     

Physical characteristics of set yoghurt made with altered casein to whey protein ratios and EPS-producing starter cultures at 9 and 14% total solids

This study investigated firmness and syneresis of set yoghurts made at the CN to WP ratios of 4:1, 3:1, 2:1 and 1:1 using non-EPS-, capsular EPS- and ropy EPS-producing starter cultures. The yoghurts were made at 9 and 14% (w/w) total solids. The total solids, total protein, the concentration of lactose and the ratios of CN to WP as well as the protein's profiles (native- polyacrylamide gel electrophoresis) in heated and unheated milk blends were investigated. The level of soluble denatured whey protein aggregates in heated milk was also determined. The concentration of EPS, firmness and level of spontaneous syneresis in set yoghurt were monitored weekly throughout 28 days of storage. The microstructure of the set yoghurt made with milk blends at the CN to WP ratio of 4:1 and using three types of starter cultures was carried out after 1 day of storage. There was no difference in the total solids, total protein and lactose concentration of liquid milk blends, except the CN to WP ratios. There was no difference in whey protein denaturation between milk blends. The level of soluble denatured whey protein aggregates in heated milk blends decreased with reducing CN to WP ratio. The firmness and the level of spontaneous syneresis in set yoghurts decreased as the CN to WP ratios were reduced. The use of EPS-producing starter cultures reduced firmness and syneresis and changed the protein matrix in the microstructure of set yoghurts made at 9% (w/w) total solids compared to the control products. These were not observed in set yoghurts made at 14% (w/w) total solids.     

Physical characteristics of yoghurts made using exopolysaccharide-producing starter cultures and varying casein to whey protein ratios

This study investigated the physical characteristics of set and stirred yoghurts made at 9% (w/w) total solids with various casein (CN)-to-whey protein (WP) ratios and with exopolysaccharide (EPS)-producing starter cultures (capsular or ropy) during storage. The yoghurt was evaluated for composition, firmness and syneresis of set yoghurt, and for the flow curve and the area of hysteresis loop between the upward and downward curve of stirred yoghurt. Viable counts of starter bacteria and concentrations of lactic acid and EPS in the yoghurt were also determined. EPS concentration did not decrease during storage for 28 d. Firmness and syneresis of set yoghurt decreased when the CN-to-WP ratio was reduced from ratio 4:1 to 1:1 and when EPS starter cultures (especially ropy EPS) were used. Stirred yoghurt with a CN-to-WP ratio of 3:1 and made using ropy EPS-producing starter cultures had a higher shear stress and hysteresis loop area than yoghurt made using capsular EPS- or non-EPS-producing starter cultures. The results suggested that the physical characteristics of set and stirred yoghurts can be improved by varying CN-to-WP ratio and by the use of EPS-producing starter cultures.     

Interactions between EPS-producing Streptococcus thermophilus strains in mixed yoghurt cultures

Mixed cultures of different EPS-producing Streptococcus thermophilus strains in combination with a Lactobacillus delbrueckii subsp. bulgaricus strain with negligible EPS-production were used for yoghurt production. The yoghurt texture was characterised with respect to sensory, rheological and microstructural properties and the EPS-concentrations were determined. The cultures resulted in yoghurts with highly different texture properties, and positive interactions between certain Streptococcus thermophilus strains were observed. The underlying properties of yoghurt texture are multidimensional, but a number of microstructural characteristics were apparent in the yoghurts with the highest mouth thickness, creaminess and viscosity. A strong protein network, not too dense and with medium size pores containing EPS, seems associated with these properties. The presence of capsular polysaccharides (CPS) also appeared to be beneficial as did a combination of EPS types, which were distributed differently in the protein network (in serum pores, respectively in association with protein). Obviously, a certain concentration of EPS must be present to provide for these effects on yoghurt texture, but other factors than concentration per se seem more important.     

Rheological,microstructural and sensory characterization of low-fat and whole milk set yoghurt as influenced by inulin addition

The effect of inulin addition (0–4%) upon texture and microstructure of set yoghurt with different levels of fat (0.2%–3.5%) was investigated. A two-factor experimental design with four treatments was used for data analysis. Skimmed milk with various inulin and cream concentrations was standardized to 4% protein content, homogenized, heated to 92 °C and fermented at 42 °C until a pH of 4.6 was reached. The chemical composition, pH, consistency and microstructure properties of the yoghurts were analysed after 6 days of storage at 5 °C. The statistical analysis showed that inulin and fat significantly affected the rheological and sensory results. Higher yield stress, “firmness” and “creaminess” values were observed in yoghurt produced with higher inulin additions, whereas the pH value was not affected. A significant correlation was found between yield stress and sensory determined firmness (r = 0.91). The microstructure examined by confocal laser scanning microscopy (CLSM) was only slightly affected by the concentrations of inulin in the range studied, possibly due to weak protein interactions between the inulin and the milk protein network.     

Flavour profiles and survival of starter cultures of yoghurt produced from high-pressure homogenized milk

Volatile carbonyl compounds, organic acids and yoghurt bacteria counts were investigated in yoghurts made from ultra-high pressure homogenized milk. Yoghurts were manufactured from milk treated using ultra-high pressure homogenization at 200 or 300 MPa and at 30 °C or 40 °C, and compared with those produced from heat-treated milk with 3% added skim milk powder. To study the evolution of these parameters, samples were analysed after days 1, 14 and 28 of storage. Yoghurts from milk heat-treated or treated at 300 MPa had very similar profiles of organic acids and volatile compounds, as well as similar bacterial counts of both starter cultures. In comparison, yoghurts from milk treated at 200 MPa at either 30 °C or 40 °C gave different profiles, together with a sharp decrease in counts of lactobacilli. During storage, only slight differences in flavour compounds and yoghurt bacteria counts were detected, except in those samples from milk treated at 200 MPa.     

Impact of oxidoreduction potential and of gas bubbling on rheological properties of non-fat yoghurt

The aim of this work was to study the effect of different gaseous conditions on the physico-chemical properties of yoghurt. Four conditions of oxidoreduction potential (Eh₇), +433 mV (milk gassed with air), +405 mV (ungassed milk), +283 mV (milk gassed with N2) and ?349 mV (milk gassed with N₂–H₂) were applied to milk. The rheological properties and microstructure of these yoghurts were determined by measuring apparent viscosity, whey separation (WS) and using confocal laser scanning microscopy. Exopolysaccharide (EPS) production by lactic acid bacteria (LAB) was also studied, and production was increased for extreme Eh₇ values: the highest values were obtained for air (oxidizing condition) and for N₂–H₂ (reducing condition). Yoghurts made under bubbled N₂–H₂ clearly led to low WS with higher gel aggregation and a lower proportion of pores.     

Probiotic stability of yoghurts containing Jerusalem artichoke inulins during refrigerated storage

Experimentally prepared Jerusalem artichoke inulins (JAI) were compared with two commercial chicory root inulins for their prebiotic potentials in media broth model and growth-sustaining ability in non-fat yoghurts. Experimental yoghurts were made with 12% reconstituted skim milk (RSM) supplemented with 4% inulin powders, inoculated with mixed cultures of Lactobacillus casei LC-01, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (1:0.5:0.5 based on supplier’s recommendation) and incubated overnight at 37 °C. Non-supplemented yoghurt was prepared from 16% RSM and used as control. The survival and acidifying activity of lactic and probiotic cultures in all yoghurts were investigated on weekly intervals during the shelf life of 28 days at 4 °C. Incorporation of JAI resulted in improved viability of LC-01, maintaining >7.0 log CFU/g during cold storage but did not affect the viability of yoghurt bacteria in comparison with the control.     

Characteristics of stirred low-fat yoghurt as affected by high pressure

The effects of high pressure on the physicochemical, chemical, microbiological and sensory characteristics of stirred low-fat yoghurt were studied. Laboratory-made yoghurts were treated at high pressure (100–400 MPa) for 15 min at 20°C. No significant changes in pH and total organic acids were observed after pressuring the yoghurt. Pressures over 200 MPa prevented post-acidification of the yoghurt during chilled storage. Pressurized yoghurts exhibited higher viscosity and amino acid contents than did the untreated controls, and the differences were maintained after chilled storage. High-pressure treatments at 300 and 400 MPa reduced the number of viable cells of lactobacilli to below the legal minimum permitted in many countries. Significant differences in sensory characteristics between untreated and pressurized yoghurts (200–300 MPa) were detected after chilled storage.     

Survival and activity of selected probiotic organisms in set-type yoghurt during cold storage

The growth and metabolism of two probiotic organisms (L. acidophilus LAFTI^® L10 and Lactobacillus casei LAFTI^® L26) and a regular yoghurt culture (L. delbrueckii ssp. bulgaricus Lb1466 and Streptococcus thermophilus St1342) were studied in yoghurt containing 0.5%, 1.0%, and 1.5% (w/v) of high amylose corn starch powder (Hi-maize^®) or inulin. Viable cell counts of probiotic organisms, their metabolites and proteolytic activities, and viscosity of the yoghurts were determined during refrigerated storage for 28 d at 4 ^oC. In the presence of inulin, cultures showed better retention of viability (8.0 log cfu g⁻¹) in comparison with that of Hi-maize, which had a reduction by one log cycle. Lower concentrations of 0.5–1.0% Hi-maize improved (P<0.05) the production of propionic acid and also increased proteolytic activity of probiotic organisms substantially. A greater release of free amino acids may have sustained better growth of the organisms in yoghurts. Supplementation with either Hi-maize or inulin increased the viscosity of probiotic yoghurts significantly (P<0.05).     

Rheology,spontaneous whey separation,microstructure and sensorial characteristics of probiotic yoghurts enriched with passion fruit fiber

Beyond demonstrated beneficial health attributes, passion fruit rinds are a by-product of the fruit pulp industry, rich in total dietary fiber, particularly pectin. The aim of this study was to evaluate the influence of the addition of passion fruit fiber on the whey formation, rheological parameters, microstructure and sensorial characteristics of probiotic yoghurts. Skim milk bases enriched with 1% of passion fruit fiber or not were heat treated and inoculated with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, and divided into four groups according to the probiotic strain added — Lactobacillus acidophilus strains L10 and NCFM and Bifidobacterium animalis subsp. lactis strains Bl04 and B94. Fermentations were performed until the pH reached 4.5. Rheological characteristics of yoghurts were determined by a rotational rheometer in two cycles of shear rate ranging from 0 to 15 s^? 1 in both upward and downward curves. Sensorial analysis of passion fruit fiber yoghurts, either without any probiotic or co-fermented by L. acidophilus L10 or B. animalis subsp. lactis Bl04, was evaluated against a control yoghurt without fiber. Photomicrographs of freeze-dried yoghurts were made by field-emission scanning electron microscope (SEM). Thixotropy of enriched yoghurts was higher than that of their respective controls in the two cycles of shear rate. Apparent viscosity was significantly higher in fiber yoghurts co-fermented by the lactobacilli than in their controls at the end of cold storage. Photomicrographs demonstrated that in passion fruit fiber yoghurts the casein gel was more compact and overlaid the fiber, while filaments of exopolysaccharides were more frequent in control yoghurts. Appearance, odor and color of the passion fruit fiber yoghurts received scores as ‘good’, and the intensity of the passion fruit flavor was considered weak by the sensory assessors. Results indicate that the passion fruit fiber is an almost neutral ingredient for the design of new high value-added yoghurt.     

Production of traditional Greek yoghurt using Lactobacillus strains with probiotic potential as starter adjuncts

Lactobacillus plantarum ACA-DC 146 and L. paracasei subsp. tolerans ACA-DC 4037 were examined for their potential application as adjuncts in the production of traditional Greek set-type yoghurt. Both strains displayed low milk acidification activity, while no inhibition was observed towards or from the yoghurt starters used. Yoghurt produced with L. paracasei subsp. tolerans ACA-DC 4037 exhibited the best sensory properties, with a rich traditional smooth taste, and the strain was selected for further trials. Yoghurt produced with this strain as an adjunct had good physicochemical properties. After 2 weeks of refrigerated storage, microbial loads (>7.0 log cfu g⁻¹) were in accordance with international recommendations and guidelines for probiotic and starter cultures in milk products. Increasing the microbial load further, using concentrated and encapsulated inocula (10–11 log cfu g⁻¹), gave yoghurt with long fermentation times and poor organoleptic properties.     

Properties of low-fat stirred yoghurts made from high-pressure-processed skim milk

Physical properties of stirred yoghurt made from reconstituted skim milk that was high-pressure (HP)-treated at 100, 250 or 400 MPa, at 25, 70 or 90 °C, for 10 min, prior to inoculation with yoghurt cultures, were studied; portions of milk HP-treated at 25 °C were also heat-treated at 90 °C for 10 min before or after pressure treatment. Control yoghurts were made from skim milk given a heat treatment at 90 °C for 10 min. Fermentation time was not affected by treatment applied to the milk. HP treatment of skim milk at 25 °C, before or after heat treatment, gave stirred yoghurts of similar viscosities to that made from conventionally heat-treated milk. Lower viscosities were obtained when stirred yoghurts were made with milk HP-treated at elevated temperatures. A model is proposed to correlate properties of yoghurt with HP/heat-induced changes in interactions and structures of protein in the milk samples.Industrial relevanceTo meet end user expectations, the dairy industry needs to diversify its product range by tailoring specific functionalities. To meet these expectations, new processing methods such as high-pressure processing are of interest for their potential to achieve specific and/or novel functionalities and/or improve efficiencies, including reduced chemical and water use. In this paper, an investigation of the use simultaneous pressurization and heating of milk before the manufacture of stirred yoghurt is presented.     

Influence of different exopolysaccharide-producing strains on the physicochemical, sensory and syneresis characteristics of reduced-fat stirred yoghurt

Reduced-fat stirred yoghurts were prepared by using ropy and nonropy exopolysaccharide (EPS)-producing strains or nonexopolysaccharide-producing strains. Yoghurts were evaluated for pH, titrable acidity, acetaldehyde, whey separation, viscosity, EPS concentration, sensory and microbiological analyses. Determinations of these parameters were made at 1, 7, 14 and 21 days of storage. Reduced-fat yoghurts made with EPS⁺ strains exhibited lower syneresis and acetaldehyde contents, but higher viscosity than those made with the EPS ⁻ strain. Physico-chemical results correlated with the sensory results in that panellists scored the EPS⁺ yoghurts as having an overall better mouthfeel, but a worse flavour than EPS ⁻ yoghurt.     

How do different cooling temperatures affect the characteristics of set-type yoghurt gel?

The impact of cooling temperatures on gel characteristics of set-type yoghurts was investigated through 28 days of storage. Yoghurt samples were cooled at four different temperatures (−10 °C, −5 °C, 0 °C, 4 °C) after incubation until the central temperature of yoghurt cups reached 4 °C. Cooling temperature and storage period were determined as significant factors affecting the yoghurt gel characteristics (p < 0.05). Yoghurts cooled at 0 °C clearly differed from the other yoghurts in terms of gel firmness and consistency. Confocal images of yoghurts cooled at 0 °C showed more homogeneous protein matrix than the other samples. Microbial counts and the acidity of the yoghurt samples increased with storage period. Textural parameters, water holding capacity and susceptibility to syneresis increased until 14th day of storage and then decreased. Sensory analysis of yoghurt samples confirmed the results obtained by microbiological and textural measurements.     

ACE-inhibitory activity of probiotic yoghurt

In this study, the in vitro angiotensin-converting enzyme (ACE)-inhibitory (ACE-I) activity of peptide fractions from different yoghurt batches was assessed. Inhibition of ACE activity resulted in an overall antihypertensive effect. Yoghurts were prepared either using a sole yoghurt culture including Lactobacillus delbrueckii ssp. bulgaricus Lb1466 and Streptococcus thermophilus St1342, or L. acidophilus L10, L. casei L26 and Bifidobacterium lactis B94 in addition to yoghurt culture. ACE-I activity was determined at weekly intervals during 28 days of cold storage. Peptide fractions showing high ACE-I activity were further purified using multiple-steps of RP-HPLC. All probiotic yoghurts showed appreciable ACE-I activity during initial stages of storage compared with the control yoghurt, with a significant (p<0.05) decrease afterwards. The ACE-I activity ranged from IC₅₀ of 103.30–27.79 μg mL⁻¹ with the greatest ACE inhibition achieved during first and third week of storage. The in vitro ACE-I activity could be related to the peptide liberation via degradation of caseins. In total, 8 ACE-I peptides were characterized originating from α_s2-casein (1), κ-casein (2) and β-casein, of which two well-known ACE-inhibiting peptides, namely Val–Pro–Pro (VPP) and Ile–Pro–Pro (IPP), were identified. These peptides are already used in commercial products.     

Physical properties and microstructure of yoghurts supplemented with milk protein hydrolysates

The physical properties and the microstructure of yoghurts containing probiotic bacteria, and supplemented with milk protein hydrolysates, were studied. Three casein hydrolysates and three whey protein hydrolysates were added to milk at a concentration ranging from 0.25 to 4 g L⁻¹. The milks were then fermented with either of two different cultures. The resulting yoghurts with added hydrolysates were compared to the control yoghurt without supplementation. For both cultures, addition of hydrolysates decreased the complex viscosity and graininess in yoghurts. The addition of hydrolysates also reduced fermentation time. Microstructural observations showed a more open and less branched structure in yoghurts when milk protein hydrolysates were incorporated. The difference in fermentation time between milks with different levels of added hydrolysates could partially explain the differences in microstructure and physical properties of the final yoghurts.     

Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt

This study investigated the effects of two Streptococcus thermophilus strains, ST 285 and ST 1275, on selected technological and rheological characteristics of set-type yoghurt. The strains were selected for their capability to produce distinctly different exopolysaccharides (EPS) and were thus coded as capsular (ST 285) or ropy-capsular (ST 1275). The culture performance and physico-chemical properties of yoghurt were assessed in relation to different fermentation temperatures (30, 37 or 42 °C) and prolonged storage (up to 30 days) at low temperature (4 °C). ST 1275 showed faster growth and acidification rates, resulting in yoghurt with lower syneresis and higher-flow behaviour index, than ST 285. EPS production appeared to be growth associated with the maximum given at growth temperatures of 37 and 42 °C for ST 285 and ST 1275, respectively; however, EPS concentration declined considerably during storage. Prolonged cold storage increased several rheological characteristics of yoghurt including G′, consistency index and hysteresis loop area. A weak correlation between EPS concentration and textural properties of yoghurt was observed.