Impact of starch and exopolysaccharide-producing lactic acid bacteria on the properties of set and stirred yoghurts

The impact of exopolysaccharide (EPS)-producing lactic acid bacteria with well-known structures and starch (0.75%) on the rheological properties (apparent viscosity and elastic modulus) and physical properties (syneresis) of set and stirred yoghurts was studied. Three EPS-producing strains with different structural characteristics were studied: Streptococcus thermophilus ST1 (anionic, stiff and linear EPS), Lactobacillus delbrueckii subsp. bulgaricus LB1 (neutral, stiff and ramified EPS) and Lb. delbrueckii subsp. bulgaricus LB2 (neutral, flexible and highly ramified EPS). The presence of linear, stiff, and anionic EPS from ST1 increased the elastic modulus in all yoghurt conditions, possibly owing to electrostatic interactions with caseins. Higher viscosity values were obtained with stiff and linear or slightly branched EPS from the ST1 and LB1 for all yoghurt conditions. Starch addition increased the values of the rheological and physical properties of all stirred yoghurts, probably due to the repulsion between proteins and polysaccharides favouring thermodynamic incompatibility

Characterization of syneresis phenomena in stirred acid milk gel using low frequency nuclear magnetic resonance on hydrogen and image analyses

Water retention is an important quality attribute for yogurt. Classically, stirred yogurt water retention is investigated using induced syneresis measurement (centrifugation), which does not characterize spontaneous syneresis. Low-frequency nuclear magnetic resonance (1H-LF-NMR) is a non-destructive technique to detect spontaneous syneresis. Experimental yogurt from pasteurized skim milk, and commercial stirred yogurts were analyzed with 1H-LF-NMR. After Laplace's transformation of the signal, hydrogen atoms pools were differentiated according to their mobility. Each hydrogen pool stood for a type of water mobility in the matrices characterized by a relaxation time (T2(i)), and a signal intensity (I2(i)). Yogurt water retention was assessed by induced syneresis and their structure was characterized using microscopy. Low frequency 1H-NMR detected four different water mobility groups in the matrices. Among these, there was a signal from bulk water, and another attributed to the separated serum (spontaneous syneresis). In experimental yogurts, spontaneous syneresis was visible, resulting in induced syneresis higher than 50%. Moreover, induced syneresis and spontaneous syneresis detected by 1H-LF-NMR were similar. In commercial yogurts, bulk water mobility reduced with increasing protein content and protein network density. Induced syneresis and bulk-water mobility correlated only in yogurts without gelatin. In the presence of gelatin, the network was more open, probably favoring bulk water mobility. This study shows that 1H-LF-NMR associated with microscopy image analysis efficiently assesses and describes yogurts water retention and spontaneous syneresis

Relationship between smoothing temperature, storage time, syneresis and rheological properties of stirred yogurt

Six different smoothing temperatures were compared for nonfat yogurt and the changes in syneresis and rheological properties observed for up to 22 days. Multiple linear regressions were used to describe the syneresis, firmness, flow time, viscosity, and flow resistance and the relationship between these properties, the smoothing temperature and the storage time. During storage, viscosity, firmness, and flow time increased; syneresis and flow resistance remained stable. Syneresis increased significantly (P ≤ 0.05) with smoothing temperature (10–35 °C). Other properties increased slightly (P > 0.05), and properties started to decrease above 30 °C. Syneresis, viscosity, and flow resistance were more sensitive to smoothing temperature; firmness and flow time were more sensitive to storage time. Lower smoothing temperature (10 °C) should be used to minimize syneresis while smoothing temperature ranging from 25 to 30 °C is better to improve rheological properties. Storage time must be considered to optimize these properties

The composition of Camembert cheese-ripening cultures modulates both mycelial growth and appearance

The fungal microbiota of bloomy-rind cheeses, such as Camembert, forms a complex ecosystem that has not been well studied, and its monitoring during the ripening period remains a challenge. One limitation of enumerating yeasts and molds on traditional agar media is that hyphae are multicellular structures, and colonies on a petri dish rarely develop from single cells. In addition, fungi tend to rapidly invade agar surfaces, covering small yeast colonies and resulting in an underestimation of their number. In this study, we developed a real-time quantitative PCR (qPCR) method using TaqMan probes to quantify a mixed fungal community containing the most common dairy yeasts and molds: Penicillium camemberti, Geotrichum candidum, Debaryomyces hansenii, and Kluyveromyces lactis on soft-cheese model curds (SCMC). The qPCR method was optimized and validated on pure cultures and used to evaluate the growth dynamics of a ripening culture containing P. camemberti, G. candidum, and K. lactis on the surface of the SCMC during a 31-day ripening period. The results showed that P. camemberti and G. candidum quickly dominated the ecosystem, while K. lactis remained less abundant. When added to this ecosystem, D. hansenii completely inhibited the growth of K. lactis in addition to reducing the growth of the other fungi. This result was confirmed by the decrease in the mycelium biomass on SCMC. This study compares culture-dependent and qPCR methods to successfully quantify complex fungal microbiota on a model curd simulating Camembert-type cheese

How do smoothing conditions and storage time change syneresis, rheological and microstructural properties of nonfat stirred acid milk gel?

Nonfat acid milk gel, acidified by GDL, was used to simulate microbial fermentation of milk to produce stirred yoghurt. Acid milk gel preparation at laboratory scale included stirring, pumping, smoothing and cooling operations. Two filters (pre-smoothed, 1 mm; smoothed, 500 μm), three smoothing temperatures (13, 22 and 35 °C) and two storage times (1 and 22 days) were studied. Syneresis, microgels size and smoothness of microgels were analysed for pre-smoothed and smoothed gels; viscosity, storage modulus, firmness and total pore area were only analysed for smoothed gel. After 1 and 22 days of storage, pre-smoothed gels developed lower syneresis and smaller microgels than smoothed gels at 22 °C. For smoothed gels, regardless of the smoothing temperature, syneresis, firmness, microgels size and smoothness increased during storage, while total pore area decreased and viscosity remained stable. Viscosity was lower when smoothing was performed at 35 °C and was correlated to rougher microgels

Quantitative PCR reveals the frequency and distribution of 3 indigenous yeast species across a range of specialty cheeses

Indigenous microorganisms are important components of the complex ecosystem of many dairy foods including cheeses, and they are potential contributors to the development of a specific cheese's sensory properties. Among these indigenous microorganisms are the yeasts Cyberlindnera jadinii, Pichia kudriavzevii, and Kazachstania servazzii, which were previously detected using traditional microbiological methods in both raw milk and some artisanal specialty cheeses produced in the province of Québec, Canada. However, their levels across different cheese varieties are unknown. A highly specific and sensitive real-time quantitative PCR assay was developed to quantitate these yeast species in a variety of specialty cheeses (bloomy-rind, washed-rind, and natural-rind cheeses from raw, thermized, and pasteurized milks). The specificity of the quantitative PCR assay was validated, and it showed no cross-amplification with 11 other fungal microorganisms usually found in bloomy-rind and washed-rind cheeses. Cyberlindnera jadinii and P. kudriavzevii were found in the majority of the cheeses analyzed (25 of 29 and 24 of 29 cheeses, respectively) in concentrations up to 104 to 108 gene copies/g in the cheese cores, which are considered oxygen-poor environments, and 101 to 104 gene copies/cm2 in the rind. However, their high abundance was not observed in the same samples. Whereas C. jadinii was present and dominant in all core and rind samples, P. kudriavzevii was mostly present in cheese cores. In contrast, K. servazzii was present in the rinds of only 2 cheeses, in concentrations ranging from 101 to 103 gene copies/cm2, and in 1 cheese core at 105 gene copies/g. Thus, in the ecosystems of specialty cheeses, indigenous yeasts are highly frequent but variable, with certain species selectively present in specific varieties. These results shed light on some indigenous yeasts that establish during the ripening of specialty cheeses

Viability of Bifidobacterium longum in cheddar cheese curd during manufacture and storage: effect of microencapsulation and point of inoculation

International audienceThe goal of this study was to assess the effect of methods of inoculation on the viability of probiotic bacteria during cheddar cheese manufacture as well as their stability during storage. Bifidobacterium longum ATCC 15708 was freeze-dried and microencapsulated by spray-coating. The effect of inoculation of free whole cell or microencapsulated cells at three points during manufacture (milk before renneting, at cheddaring or at salting) on the viable counts in cheese and whey was investigated. Microencapsulation had no effect on viable counts, chemical parameters (lactose, lactic acid, total nitrogen, nitrogen soluble in TCA, moisture) or sensory properties during manufacturing or storage of the fresh cheeses for 14 days. Inoculation of the bifidobacteria in milk before renneting resulted in higher viable counts in comparison to other points of inoculation. Bifidobacteria added at the salting step, which survived pressing, were subsequently more stable during storage than those inoculated in milk. The stability of B. longum 15708 during storage was greater in the pressed cheeses that in the free curds. The results of this study provides technological data for cheese makers on the optimum point of inoculation as well as the benefit of pressing the curds in order to ensure high levels of probiotics in fresh cheddar cheese

Transcriptome profiling of lactococcal mixed culture activity in milk by fluorescent RNA arbitrarily primed-PCR

Thermal treatment of milk is widely used to reduce milk contamination, while CO2 can be used to prevent bacterial growth and maintain milk quality during storage. These treatments applied before or during cheese manufacture could alter the metabolic activity of starter cultures. Changes in gene expression can be evaluated by differential display methods, so that effects on bacterial metabolic activity can be estimated by variation in transcription profiles. The aim of this study was to develop fluorescent RNA arbitrarily primed-polymerase chain reaction (RAP-PCR) as a method to evaluate the influence of milk CO2 acidification as well as rennet and salt concentrations on starter gene expression. Comparison with reference conditions showed that gene transcription was influenced according to the extent of thermal treatment as well as by CO2 acidification followed by different neutralization procedures. Thus, simple acid neutralization after CO2 acidification was not sufficient to regain the reference transcriptome profile. Starter gene transcription profiles showed important modifications following an increase in NaCl concentration or a decrease in rennet activity from standard conditions used in Cheddar cheese making. Increasing rennet activity results in small changes in the starter RNA profile. Fluorescent RAP-PCR is a promising method for obtaining a better understanding of gene expression profiles of mixed cultures during cheese making

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