The effect of heteropolysaccharide‐producing strains of Streptococcus thermophilus on the texture and organoleptic properties of low‐fat yoghurt

Heteropolysaccharide‐producing Streptococcus thermophilus strains (LY03, ST 111, or 5077) were compared with nonropy S. thermophilus strains (NR or 5137), in respect of texture development of low‐fat yoghurt produced at 42°C. Whether milk solids were added or not, lower syneresis and improved texture and sensory properties were obtained in yoghurts produced with S. thermophilus LY03 or S. thermophilus 5077 compared with nonropy strains. Texture effects, lower syneresis and improved sensory properties because of the use S. thermophilus ST 111 were only noticeable in the presence of milk solids. Milk solids had no significant effect on the textural parameters of nonropy strains.     

Improving the structure and rheology of high protein,low fat yoghurt with undenatured whey proteins

The objective of this study was to investigate the effects of whey protein denaturation and whey protein:casein-ratio on the structural, rheological and sensory properties of high protein (8% true protein), low fat (<0.5% fat) yoghurt. Yoghurt milk bases were made by adding undenatured whey proteins from native whey protein concentrate (NWPC) to casein concentrate in different whey protein:casein-ratios. The degree of whey protein denaturation was then controlled by the temperature treatment of the yoghurt milk bases. Addition of NWPC in low (whey protein:casein-ratio 25:75) or medium levels (whey protein:casein-ratio 35:65) in combination with heat treatment at 75 °C for 5 min gave yoghurts with significantly lower firmness, lower storage modulus (G′), and better sensory properties (less coarse and granular and more smooth), compared with corresponding yoghurts produced from yoghurt milk bases heat-treated at 95 °C for 5 min or with control yoghurts (no addition of NWPC).     

The effect of heat treatment of ovine milk on the compositional and sensory properties of set yoghurt

The aim of this study was to investigate the effect of heat treatment of ovine milk at 60 °C/5 min and 90 °C/5 min (control group) on the compositional and sensory properties of set yoghurt (n = 40). The concentration of apparent casein and total whey protein were significantly higher while sensory properties (except consistency) were not significantly different from the yoghurts in control group (P < 0.05). The results showed that ovine set yoghurt produced by heat treatment at low temperature possessed higher amount of preserved inherent functional and nutritional properties of milk than yoghurt produced by heat treatment at high temperature.     

The physicochemical properties of yoghurt supplemented with microencapsulated peanut sprout extract,a possible functional ingredient

This study was designed to determine the physicochemical and sensory properties of yoghurt supplemented with powdered peanut sprout extract microcapsules (PPSEM) (3–10 μm) during storage at 4 °C for 16 days. The releasing rates of the polyphenol, resveratrol, from yoghurts were minimised at lower concentrations of PPSEM (0.25 and 0.5%, w/v). The viscosity decreased gradually with higher concentrations of PPSEM added. In the sensory test, there were significant increases in yellowness, peanut and cooked scores (P < 0.05). Based on the results, it is concluded that the low concentrations (0.25 and 0.50%, w/v) could be used to produce PPSEM‐supplemented yoghurt without significant adverse effects on the physicochemical and functional properties.     

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.     

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.     

Sensory properties of soy yoghurts prepared from yellow and black soymilk using Streptococcus infantarius 12 and Weisellia sp. 4

BACKGROUND: Soy yoghurts containing enhanced bioactive isoflavone aglycones were prepared from combinations of yellow (YS) and black soymilk (BS) (YS:BS = 100:0, 70:30, 50:50 and 0:100, v/v) by fermentation with Streptococcus infantarius 12 and Weisellia sp. 4 at 37 °C for 10 h. Sensory properties of the bioactive soy yoghurts were investigated. Furthermore, other characteristics including the contents of isoflavones, viable cell numbers, pH, colour and DPPH radical scavenging activity were investigated. RESULTS: One hundred percent YS yoghurt showed the highest scores of all sensory attributes while 100% BS yoghurt was the lowest for colour and smoothness. The overall acceptability scores were 5.9 and 5.2 for 100% YS and 100% BS yoghurts, respectively (9‐point scale). The L‐values of soy yoghurts decreased as the percentage BS increased, indicating darkening of soy yoghurts. The contents of bioactive isoflavone aglycones (daidzein and genistein) in soy yoghurts remarkably increased (>1300 mg kg^?1) by fermentation. Viable cell numbers and pH of soy yoghurts ranged from 8.4 × 10⁸ to 9.6 × 10⁸ CFU g^?1 and 4.43 to 4.54, respectively. DPPH radical scavenging activity significantly increased as the percentage BS increased (P < 0.05). CONCLUSION: One hundred percent YS yoghurt was better for overall sensory properties while 100% BS yogurt was more bioactive in terms of the isoflavone aglycone contents and DPPH radical scavenging activity compared to other yoghurts. Copyright © 2008 Society of Chemical Industry     

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.     

Influence of brodifacoum and bromadiolone on growth of yoghurt cultures in milk

The influence of brodifacoum and bromadiolone on the growth of yoghurt culture in milk was determined. Milk samples inoculated with a commercial yoghurt culture and serially diluted rodenticides were incubated at 42 °C for 6 h. The yoghurt cultures showed detectable sensitivity (significant change (P < 0.05) in pH in the medium) to brodifacoum at 0.12 mg/mL and to bromadiolone at 0.08 mg/mL. There were significant differences (P < 0.05) in the bacterial count and α‐galactosidase and β‐galactosidase activities in the presence of toxins in milk. The results indicate that brodifacoum and bromadiolone could inhibit the growth of yoghurt culture in milk.     

Effects of poly‐γ‐glutamic acid on the physicochemical characteristics of skim milk yoghurt

The physicochemical and sensory properties of skim milk yoghurts containing poly‐γ‐glutamic acid (PGA) at different levels (0.0025, 0.005 and 0.01%) were evaluated. Addition of PGA up to 0.01% to reconstituted skim milk (11%, w/v) did not affect the growth of lactic acid bacteria or the development of titratable acidity in yoghurt, whereas full‐fat control yoghurt had reduced acid production. No changes were found in viable cell counts of PGA yoghurts during storage (4 weeks at 4 °C). The addition of PGA (0.005%) significantly decreased syneresis in skim milk yoghurt and did not cause any undesirable effects in sensory acceptability.     

Comparison of the effects of high-pressure microfluidization and conventional homogenization of milk on particle size,water retention and texture of non-fat and low-fat yoghurts

The effect of high-pressure homogenization using a Microfluidizer^® on texture, water-holding capacity, and extent of syneresis on stirred yoghurts was compared with that of conventional homogenization. The effect of homogenization condition on particle size was also assessed in milk and in yoghurt. Stirred yoghurts were prepared from recombined milk samples (0 and 1.5% fat) heat-treated (95 °C, 2 min) and then treated by conventional valve homogenization at 25 MPa or microfluidization at 150 MPa. Homogenization conditions influenced the particle size in milk, gel particle size, and textural quality of stirred yoghurts in a manner dependent upon fat content. Milk microfluidized at 150 MPa had smaller particle size than homogenized milk, but resulted in larger particles in yoghurt. Microfluidization of low-fat milk modified the microstructure of yoghurt, giving more interconnectivity in the protein networks with embedded fat globules, but with similar texture profiles and water retention compared with yoghurt made from conventionally homogenized milk.     

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.     

Viability of Lactobacillus acidophilus in rice bran‐enriched stirred yoghurt and the physicochemical and sensory characteristics of product during refrigerated storage

This study was aimed at investigating the fortification of probiotic yoghurt with rice bran to increase nutritional properties of the product. The different levels of rice bran (0.3%, 0.6%, 0.9% and 1.2%) were incorporated into milk. The yoghurt samples were produced after pasteurisation, addition of starter culture and 1% Lactobacillus acidophilus suspension (6 × 10⁸ CFU mL^?1) and incubation. During sample storage in refrigerator, the viability of L. acidophilus, viscosity and physicochemical and sensory properties of product were investigated. Rice bran significantly increased the viability of L. acidophilus (P < 0.05). In addition, all probiotic yoghurts incorporating rice bran indicated higher viscosity and acidity and lower pH and syneresis compared to plain yoghurts. Furthermore, increments in rice bran incorporation levels resulted in a reduction in consumers' sample preferences. In general, the addition of rice bran at a suitable level could increase L. acidophilus viability and improve quality attributes of yoghurt.     

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.     

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.     

Potential application of Saccharomyces boulardii as a probiotic in goat's yoghurt: survival and organoleptic effects

Potential application of Saccharomyces boulardii in yoghurts could offer an alternative probiotic product to people suffering from antibiotic‐associated diarrhoea whilst on treatment. Three different yoghurts were made using goat's milk. Yoghurt 1 was the control with only starter culture added to the milk. For Yoghurt 2, S. boulardii was added in parallel with the starter culture, and for Yoghurt 3, the yeast was added after the yoghurt was coagulated by the starter. Duplicate counts for lactic acid bacteria (LAB) and S. boulardii alongside organoleptic assessment were carried out every 3 days following manufacture and storage at 6 °C. Our results demonstrate that S. boulardii can survive in high numbers for the period of 4 weeks tested without significant changes in taste. Very important is the fact that the presence of the yeast stimulated the survival of the LAB, which in both yeast yoghurts were significantly higher than in the control.     

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.     

Physico-Chemical,antioxidant activities,textural, and sensory properties of yoghurt fortified with different states and rates of pomegranate seeds (Punica granatum L.)

This study examines the effects of incorporating fresh, frozen, osmodehydrated, and dried pomegranate seeds, at different concentrations (5, 10, and 20%) on the physicochemical, antioxidant, textural, and sensory properties of yoghurt. Antioxidant activity, pH, acidity, syneresis, and color of yoghurt were also evaluated during the storage (28 days) at 4°C. The principal component analysis (PCA) was performed to assess the correlations between different yoghurt formulation and their sensory attributes. Data showed that the addition of pomegranate seeds reduced pH and modified the chromatics coordinate (L*, a*, b*, C*, h°) and firmness of the yoghurt samples. In addition, acidity, °Brix, and syneresis increased compared to the control. Thanks to their high anthocyanin content, pomegranate seeds considerably improved the antioxidant activity essentially for yoghurt enriched with frozen seeds. In addition, the supplementation of 20% of frozen seeds into yoghurt was the most appreciated by panelists and improved the sensory properties in comparison to other formulation. According to PCA, taste and appearance were the main criteria for the overall acceptability of yoghurt. After 28 days of storage, the reddish color was reinforced; however, the antioxidant activity of yoghurts was reduced. Overall, it can be concluded that pomegranate seeds can be used as a natural ingredient to develop a novel yoghurt with high nutritional properties.     

Comparison of volatile compounds in yoghurts made from cows’, buffaloes’, ewes’ and goats’ milks

The composition of the volatile fraction of yoghurts made from cows’, buffaloes’, ewes’ and goats’ milks was investigated during the 28 days of storage at 4 °C using solid‐phase microextraction technique (SPME) and gas chromatography coupled to mass spectrometry analysis (GC/MS). A total of 34 volatile compounds were identified in yoghurts during their storage at 4 °C, including aldehydes, ketones, alcohols, esters, acids, terpenes, hydrocarbons and sulphur compounds. In this study, acetaldehyde, diacetyl and acetoin, considered as the major compounds of yoghurt, were detected in all yoghurts.     

Influence of heat treatment of goat milk on casein micelle size,rheological and textural properties of acid gels and set type yoghurts

Acid gels and yoghurts were made from goat milk that was heated at 72°C/30 s, 85°C/5 min, and 95°C/5 min, followed by acidification with starter culture at 43C until pH 4.6. The rheological and textural properties of acid gels and yoghurts were analyzed using dynamic low amplitude oscillatory rheology and back extrusion texture analysis, respectively. The effect of goat milk heat treatment on the mean casein micelle diameter and protein profile was also determined by dynamic light scattering and SDS PAGE electrophoresis, respectively. The shortest gelation and fermentation time was recorded for yoghurt prepared from milk heated at 85°C/5 min. Also, the pH of gelation, the storage moduli (G′) and yield stress were higher for this yoghurt, compared with the other two. Textural properties of goat milk yoghurts such as firmness and consistency were strongly affected by milk heat treatment, and the highest values were recorded for yoghurt produced from milk preheated at 85°C/5 min, as well. The largest casein micelles were measured after 85°C/5 min treatment and their size decreased at higher temperature, despite higher denaturation of whey proteins at the most intense heat regime, indicating the structure changes that influence on the acid gelation.