Perspectives on the yogurt rheology

The oral processing of yogurt is a dynamic process involving a series of deformation processes. Rheological knowledge is essential to understand the structure and flow properties of yogurt in the mouth and to explore its relationship with sensory perception. Yogurt is rheologically characterized as a non-Newtonian viscoelastic material. The rheological properties of yogurt are affected by many factors, from production to consumption. Therefore, rheological measurements are widely used to predict and control the final quality and structure of yogurts. Recent studies focus on the elucidation of the effects of cultures and processes used in production, as well as the design of different formulations to improve the rheological properties of yogurts. Moreover, the science of tribology, which dominates the surface properties of interacting substances in relative motion to evaluate the structural sensation in the later stages of eating in addition to the rheological properties that give the feeling of structure in the early stages of eating, has also become the focus of recent studies. For a detailed comprehension of the rheological properties of yogurt, this review deals with the factors affecting the rheology of yogurt, analytical methods used to determine rheological properties, microstructural and rheological characterization of yogurt, and tribological evaluations.

Assessment of the Physicochemical, Antioxidant, Microbial, and Sensory Attributes of Yogurt-Style Products Enriched with Probiotic-Fermented Aronia melanocarpa Berry Juice

The aim() of this study was to create() various formulations of yogurt enriched with freeze()-dried adjuncts, namely() (i) probiotic Lactobacillus plantarum ATCC 14917 culture(), and (ii) L. plantarum ATCC 14917 fermented black chokeberry juice, along with a commercial() starter culture(). The goal was to enhance() functionality and optimize the nutritional() value() of the products. These new yogurt-style() formulations were subsequently() compared with commercially produced yogurt. All products demonstrated() favorable() physicochemical properties, and the probiotic strain() consistently() maintained viable() levels exceeding 7 log() cfu/g throughout() the entire() storage() period(). The fermented milk produced with the adjunct-free L. plantarum cells, as well as the yogurt produced with the proposed() lactobacilli-fermented chokeberry juice, exhibited the highest lactic acid() production() (1.44 g/100 g yogurt by the end of storage()). Levels of syneresis were observed at lower() values() in yogurt produced with freeze()-dried fermented chokeberry juice. Yogurts prepared() with the lactobacilli-fermented freeze()-dried chokeberry juice displayed elevated total() phenolic content() and antioxidant capacity() (25.74 µg GAE/g and 69.05 µmol TE/100 g, respectively()). Furthermore, sensory tests revealed a distinctive() fruity flavor() in samples incorporating fermented juice. The results demonstrate() that probiotic L. plantarum-fermented chokeberry juice enhances() both the antioxidant capacity() and the viability of beneficial() bacteria() in yogurt while it can be readily() applied and commercialized, especially in the form of a freeze()-dried formulation.

Whey proteins as a functional food: Health effects, functional properties, and applications in food

Functional foods are defined as foods and ingredients that exhibit health benefits beyond their nutritional value. Research on functional foods is increasing rapidly as they may help prevent and manage some non-communicable diseases. Whey proteins are recognized as a high-quality nutrient source and known to contain some bioactive components. They are rich in essential amino acids such as cysteine, branched-chain amino acids such as leucine, valine, and isoleucine, and bioactive peptides. Whey proteins look promising as a potential functional food, given its antioxidant, anti-inflammatory, blood pressure lowering, anti-obesity, and appetite suppressing effects that is discussed in the literature. Whey proteins also show functional properties that play an essential role in food processing as an emulsifier, fat-replacer, gelling and encapsulating agent and are known to improve sensory and textural characteristics of food. This review focuses on the functional food aspects of whey proteins, associated health effects, and current food applications.

Influence of Post-Heating Treatment on the Sensory and Textural Properties of Stirred Fermented Milk

The purpose of this study was to investigate the post-heating induced changes in the textural and sensory characteristics of stirred fermented milk. The textural and rheological properties of post-heating fermented milk (55-85 °C, 25 s) with respect to viscoelastic behaviors, viscosity, textural parameters, etc., were monitored, and the friction behaviors and sensory attributes were assessed. Treatments below 65 °C/25 s increased the textural properties of fermented milk such as gel strength, firmness, and viscosity, due to the moderate aggregation and increased linkages of microgels. In this case, despite the size and amount their aggregates increased (~15-~21 μm), they exhibited similar frictional behaviors and sensory attributes. However, treatments above 65 °C/25 s degraded textural properties due to excessive aggregation (~46-~63 μm), accompanied by unacceptable grainy attributes, which could be characterized by their good correlations with tribological coefficients and particle size parameters. These findings could provide an understanding of the quality formation of post-heating fermented milk and a perspective to improve the textural defects of ambient fermented milk products.

Effect of the Heat Exchanger Type on Stirred Yogurt Properties Formulated at Different Total Solids and Fat Contents

In this work stirred yogurts were produced using a technical scale pilot in which the cooling step was processed using either a tubular (THX; low shear) or a plate (PHX, high shear) heat exchanger. The aim was to determine how total solids (TS, adjusted using lactose) and fat contents (FC) impact stirred yogurt properties during storage, depending on the heat exchanger used. Using raw milk, cream, skim milk powder, and lactose, four yogurts were formulated at 16.5% TS and 4.2% proteins, with different FC (0.0, 1.3, 2.6, and 3.9%); one more control yogurt was formulated at 14% TS, 4.2% proteins, and 0.0% FC. Analyses of yogurts (firmness, viscosity, induced syneresis) were realized at days 1, 3, 7, 21, and 34 after production. The addition of lactose between the non-fat yogurt at 14 or 16.5% TS had little to no effect on stirred yogurt properties. Increasing FC reduced syneresis while increasing firmness and viscosity. The use of PHX reduced the syneresis compared to THX; however, it also tended to reduce the firmness of the yogurts with 3.9% FC.

Effect of Yogurt Enrichment with Wood Tannin during Shelf Life: Focus on Physicochemical, Microbiological, and Sensory Characteristics

Six food-grade tannins obtained from different woods were used as a source of polyphenolic compounds at two concentrations (0.5% and 1% w/w) in yogurt formulations and monitored during 3 weeks of storage. Yogurt containing tannins showed significantly higher total phenolic content (+200%), antioxidant activity (+400%), and syneresis (+100%) than control. These changes were higher with fortification at 1%. Tannin origin also significantly influenced the yogurt composition and yogurt obtained from a Turkish gall showed higher values of total phenolic content (4 mg GAE/g) and antioxidant activity (17 μM Trolox/g). Yogurt color was evaluated by CIELab parameters, and their values were influenced by tannin origin and concentration. The addition of tannins did not significantly affect the number of lactic acid bacteria. Yogurt with a lower amount of tannins (0.5% w/w) received higher consumer acceptability but significant differences in preferences were due to tannin origin. In particular, yogurt added with tannin obtained from Quebracho wood at 1% w/w showed higher consumer preference. The obtained results would provide an opportunity for dairy producers to develop a novel dairy food with high nutritional quality.

Optimization of pea protein and citrus fiber contents for plant based stirred soymilk yogurt using response surface methodology

This study investigated the optimization of pea protein (PP) and citrus fiber (CF) contents with the goal of producing a clean-label plant-based stirred soymilk yogurt that is free of additives. If CF is absent, a greater PP concentration tends to produce soymilk yogurt with improved physical properties (viscosity, flowability and water holding capacity). A CF concentration of 0.1% helped to improve the physical properties necessary in the production of stirred yogurt; however, an increase in CF concentration to 0.2% or higher would instead cause the physical properties to become unfavorable. The lactic acid bacteria (LAB) count was unaffected by CF content and increased proportionally with PP content. Response surface methodology was employed to investigate how the physical properties were affected by the mixing ratio, and an optimization technique was used to obtain the optimal yogurt mixing ratio. According to the optimization process, the optimal contents of 4% PP and 0.1% CF was obtained with a desirability of 87.1%. This result could provide the basic and fundamental information for developing clean-label plant-based stirred soymilk yogurt as a reference in the future.

Colloidal and Acid Gelling Properties of Mixed Milk and Pea Protein Suspensions

The present study aims to describe colloidal and acid gelling properties of mixed suspensions of pea and milk proteins. Mixed protein suspensions were prepared by adding pea protein isolate to rehydrated skimmed milk (3% w/w protein) to generate four mixed samples at 5, 7, 9, and 11% w/w total protein. Skimmed milk powder was also used to prepare four pure milk samples at the same protein concentrations. The samples were analyzed in regard to their pH, viscosity, color, percentage of sedimentable material, heat and ethanol stabilities, and acid gelling properties. Mixed suspensions were darker and presented higher pH, viscosity, and percentage of sedimentable material than milk samples. Heat and ethanol stabilities were similar for both systems and were reduced as a function of total protein concentration. Small oscillation rheology and induced syneresis data showed that the presence of pea proteins accelerated acid gel formation but weakened the final structure of the gels. In this context, the results found in the present work contributed to a better understanding of mixed dairy/plant protein functionalities and the development of new food products.

Encapsulation of Different Types of Probiotic Bacteria within Conventional/Multilayer Emulsion and Its Effect on the Properties of Probiotic Yogurt

Microencapsulation of probiotic cells within emulsion is an efficient method to enhance the viability of probiotic bacteria. In the present study, free and encapsulated probiotic cells (Lactobacillus rhamnosus and Lactobacillus plantarum) in simple and multilayer emulsions were used to produce a set of probiotic yogurts. In all samples, an increasing trend in syneresis and acidity values and a decreasing trend in pH and viability of probiotic cells were observed during the storage time. However, the changes in these parameters were more significant for free-loaded probiotic samples. Moreover, the free cells showed poor survival in the yogurt samples by decreasing the viable cell count of probiotics from 7.71–7.59 logs CFU/mL to 6.93–6.82 log CFU/mL during storage, while encapsulation in the multilayer emulsion showed an insignificant reduction from 7.65–7.59 logs CFU/mL to 7.55–7.45 log CFU/mL at the end of storage. The obtained results showed that the type of probiotic bacteria had no significant effects on the physicochemical and structural properties of samples. However, encapsulating probiotics in multilayer emulsion led to a more homogenous structure in yogurt. The sensorial properties were also not affected by the probiotic type and the encapsulation method. Consequently, the multilayer emulsion can provide an ideal delivery carrier for encapsulating probiotic bacteria in dairy products.

Fat-free fermented concentrated milk products as milk protein-based microgel dispersions: Particle characteristics as key drivers of textural properties

The popularity of fat-free fermented concentrated milk products, such as fresh cheeses and high-protein yogurt, has increased over the recent years, attributed to greater availability and improvements in taste and texture. These improvements have been achieved through modifications and new developments in processing technologies, for example, higher heat treatment intensities and incorporating different membrane filtration technologies. Though numerous processing parameters are discussed in the literature, as well as reasons behind the developments, detailed examinations of how process modifications affect the final textural attributes of these products are lacking. To draw links between processing parameters and texture, we review the literature on fat-free fermented concentrated milk products from the perspective of fermented milk protein-based microgel particles as the basic structural unit. At each main processing step, relationships between process parameters, micro- and macrostructural and sensory (textural) properties are discussed.An overview of particle characteristics that drive structural changes at each processing step is developed in relation to textural characteristics. Using this approach of assessing relationships between structural characteristics of concentrated dispersions of fat-free fermented milk protein-based microgel particles and processing parameters provides a basic context for the selection of optimal parameters to achieve a desired texture.

Smoothing temperature and ratio of casein to whey protein: Two tools to improve nonfat stirred yogurt properties

Consumers are not always ready to compromise on the loss of texture and increased syneresis that nonfat stirred yogurts display compared with yogurts that contain fat. In this study, we investigated milk protein composition and smoothing temperature as a means to control nonfat yogurt microstructure, textural properties, and syneresis. Yogurts were prepared with different ratios of casein to whey protein (R1.5, R2.8, and R3.9). Yogurts were pumped through a smoothing pilot system comprising a plate heat exchanger set at 15, 20, or 25°C and then stored at 4°C until analysis (d 1, 9, and 23). Yogurt particle size and firmness were measured. Yogurt syneresis and water mobility were determined, respectively, by centrifugation and time domain low-frequency proton nuclear magnetic resonance (¹H-LF-NMR). Increasing the smoothing temperature increased gel firmness and microgel (dense protein aggregates) sizes independently of the whey protein content. Also, yogurt microgel sizes changed with storage time, but the evolution pattern depended on protein ratio. Yogurt R1.5 showed the largest particles, and their sizes increased with storage, whereas R2.8 and R3.9 had smaller microgels, and R3.9 did not show any increase in microgel size during storage. Micrographs showed a heterogeneous gel with the empty area occupied by serum for R1.5, whereas R2.8 and R3.9 showed fewer serum zones and a more disrupted gel embedding microgels. Induced syneresis reduced with greater whey protein content and time of storage. This is in agreement with ¹H-LF-NMR showing less bulk water mobility with increasing whey protein content during storage. However, ¹H-LF-RMN revealed higher values of spontaneous serum separation during storage for R1.5 and R3.9 yogurts, whereas these were lower and stable for R2.8 yogurt. Microgels play an important structural role in yogurt textural attributes, and their characteristics are modulated by whey protein content and smoothing temperature. Optimization of these parameters may help improve nonfat stirred dairy gel.

Synergistic Effect of Heating pH and Transglutaminase on the Gelation Kinetics and Texture of Yak Skim Milk Gels

Textural defects (including syneresis and poor consistency) often occur in yogurt gels produced from yak milk. In this research, the synergistic effects of transglutaminase (TGase) and heating pH on the textural properties of acidified yak skim milk gels, as well as the related mechanism of action, were investigated. The pH values of yak skim milk were adjusted to 6.3, 6.7, and 7.1, respectively. The samples were heated at 80°C for 30 min and then cooled to 42°C. After treatment with different contents of TGase (0, 3, and 10 U TGase per gram proteins), the samples were acidified with glucono-delta-lactone. For a given TGase content, the final storage modulus (G′) of gels was positively related to the heating pH, whereas the opposite trend was observed for the gelation time. This effect was obvious between pH 6.3 and 6.7. At a definite heating pH value, the final G′ of the gels was highest at 3 U TGase per gram proteins. The highest water holding capacity and firmness value were observed in gels prepared using pH 7.1 and 3 U TGase per gram proteins. In the samples treated with 3 U TGase per gram proteins (preheating pH 7.1), more rigid network structures were seen in the gel than 0 or 10 U TGase per gram proteins. Therefore, adjusting the heating pH values and TGase contents is an effective way of improving the textural properties of yak milk gels.