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.     

Effect of heat treatment and casein to whey protein ratio of skim milk on graininess and roughness of stirred yoghurt

The aim of this work was to study how heat treatment and casein (CN) to whey protein (WP) ratio of skim milk affect physical characteristics of stirred yoghurt. Different heat treatments (95 °C/256 s, 110 °C/180 s and 130 °C/80 s) were applied to the yoghurt milk with the CN to WP ratios of 1.5:1, 2:1, 3:1 and 4:1. Physical properties, including graininess and roughness, of stirred yoghurt were determined during storage at 4 °C for 15 days. Visual roughness, number of grains, perimeter of grains, storage modulus, and yield stress decreased, when heating temperature or CN to WP ratio increased.     

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.     

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.     

Effects of high-pressure and heat treatments on physical and biochemical characteristics of oysters (Crassostrea gigas)

Physical and biochemical changes in oysters following high-pressure (HP) treatment at 260 MPa for 3 min or heat treatment (cool pasteurisation (CP) at 50 °C for 10 min or traditional pasteurisation (TP) at 75 °C for 8 min) were investigated and compared to changes in untreated oysters. HP or TP oysters had higher (P < 0.05) pH values (6.49–6.58) than untreated or CP oysters (6.45–6.46). HP and heat treatment both modified the gross composition of oyster tissue. The protein content of HP-treated oysters (6.9%) was significantly (P < 0.05) lower compared to control or heat-treated oysters (7.9–9.1%). The moisture content of HP-treated whole oyster tissue (86.5%) was higher than that of heat-treated or untreated oysters (83.5–84.7%), but HP or CP treatments did not affect the salt content or water activity of oysters. However, all treatments increased Hunter L- (66.3–68.9) while decreasing a- (− 1.6 to − 2.4) and b- (15.8–14.5) values of oyster tissue; overall, HP treatment had less negative effects on tissue colour of oysters than thermal treatments. HP-treated, CP and TP oysters had higher shucking yields (15.5%, 12.5% and 2.6%, respectively) than untreated oysters. One significant advantage of HP treatment over heat treatment of oysters was that the former process opened the oyster and separated the muscle of the oyster from the shell.     

Acid coagulation properties and suitability for yogurt production of cows’ milk treated by high-pressure homogenisation

The effects of high-pressure homogenisation (HPH) of cows’ milk were investigated for suitability for yogurt manufacture, compared with the processes currently applied in industry. Milk at different inlet temperatures (30 °C or 40 °C) was subjected to HPH treatment at 100, 200 or 300 MPa (one stage) and 130, 230 or 330 MPa (two-stage). HPH-treated milk was compared with milk heat-treated (90 °C for 90 s) and homogenised at 15 MPa, and with milk treated under the same thermal conditions and also fortified with 3% skim milk powder. Milk treated at 300 or 200 MPa showed higher gel strengths on coagulation, higher gel firmness in texture analysis, less syneresis and lower titratable acidity compared with conventionally treated milk fortified with 3% skim milk powder.     

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.     

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.     

Graininess and roughness of stirred yoghurt as influenced by processing

The aim of this work was to study how heat treatment, the type of starter culture, incubation temperature, and storage time can affect the physicochemical characteristics of stirred yoghurt. A four-factor experimental design was used for data analysis. Yoghurt milk was heated at 95 °C for 5 min or 130 °C for 80 s. Yoghurts were produced with three different starter cultures that had been incubated at 37, 42 or 45 °C and stored at 4 °C for 15 days. Visual roughness, number of grains, perimeter of grains, storage modulus, and yield stress all decreased when heating temperature was increased, when an exopolysaccharide-producing starter culture was used, or when incubation temperature was decreased. Storage time did not affect any of the physicochemical properties of yoghurt, except for the pH.     

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.     

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).     

Rheological properties of stirred yoghurt as affected by gel pH on stirring,storage temperature and pH changes after stirring

The rheological properties of stirred yoghurt were studied as a function of the delay between milk heat-treatment and inoculation (0, 1 and 2 days), of pH in the acid gel on stirring (4.4, 4.7, and 5.0), of the storage temperature (4, 12, and 20 °C) for 24 h following stirring and of over-acidification (allowed or inhibited). At low pH values, the gels exhibited higher elastic modulus (G′) and fracture strength. They yielded stirred yoghurts with higher G′ and viscosity, and higher increase in G′ and viscosity during storage (“rebodying”). Rebodying was only partially explained by over-acidification and cooling. Changing the storage temperature had no impact on the evolution of G′ after stirring; hydrophobic interactions were therefore probably not involved in rebodying. Electrostatic interactions seemed to play a major role in rebodying, as pH on stirring was the significant factor.     

Plasmin activity in direct-steam-injection UHT-processed reconstituted milk: Effects of preheat treatment

Ultra-high-temperature (UHT)-processed reconstituted milk that is subjected to a minimal preheat treatment during the direct-steam-injection heating process may have a shortened shelf-life as a result of plasmin-mediated proteolysis. Some manufacturers apply a preheat treatment before UHT treatment (140 °C for 4 s) with the aim of prolonging the shelf-life. Preheat treatments are, however, often arbitrary in terms of temperature and holding time. The aim of the current work was to determine guidelines for the minimum preheat treatment that will effectively inhibit or prevent plasmin-type enzyme activity in UHT milk. A selected range of preheat treatments was applied to milk preparations reconstituted from several batches of low-heat skim milk powder. Increased plasmin-type proteolysis was observed after intermediate preheat treatments at ⩾80 and <90 °C. Effective inhibition of plasmin-type proteolysis was obtained by preheating at 90 °C for 30 or 60 s.     

Cheese made from instant infusion pasteurized milk: Rennet coagulation,cheese composition,texture and ripening

Milk subjected to instant infusion pasteurization (IIP) at 72 °C, 100 °C and 120 °C (holding time 0.2 s) exhibited increased rennet coagulation time and decreased curd firming rate for increasing heat treatment temperature, when compared with raw or high temperature short time pasteurized (HTST) milk. However, addition of 4.5 mm or 9.0 mm of calcium restored the impaired rennet coagulation ability. Open texture cheeses produced from IIP milk (100 °C and 120 °C) contained significantly more moisture, had lower pH and shorter texture than similar cheese from IIP at 72 °C and HTST pasteurized milk. Cheese ripening was also affected by heat treatment, and different patterns of casein breakdown and peptide formation resulted from cheeses made from milk treated to IIP at 100 °C and 120 °C compared with cheeses made using IIP at 72 °C or HTST.     

Formation of soluble protein complexes and yoghurt properties influenced by the addition of whey protein concentrate

The effects of whey protein concentrate (WPC) on the formation of soluble protein complexes and yoghurt texture were evaluated. Skim milk (SM) and skim milk enriched with 1% WPC (SM + 1%WPC) or 2% WPC (SM + 2%WPC) were left unheated or heated and then made into yoghurt gels. Yoghurt prepared from heated SM + 2%WPC had significantly higher storage modulus, water holding capacity and firmness values and a denser microstructure than those prepared only from skim milk. Electrophoretic analysis of the milk showed that the level of β-lactoglobulin and κ-casein in the serum phase increased with increasing WPC concentration, indicating that the content of disulfide-linked β-lactoglobulin and κ-casein was higher in SM + 2%WPC than in SM, suggesting that more soluble protein complexes had been formed. Consequently, yoghurt prepared from heated SM enriched with WPC may have more bonds and more protein complexes in the protein network than yoghurt prepared only from SM, thus resulting in firmer gels.Practical applicationsYoghurt, one of the most popular fermented milk products, is of high economic importance to the dairy industry worldwide. In particular, high-protein yoghurt, such as Greek-style or set-type yoghurt, has been driving its ongoing popularity over recent years. In current industrial production of high-protein yoghurt, protein fortification and heat treatment of milk are two of the most important processing parameters affecting yoghurt texture. Whey protein concentrate has been added to milk to reduce whey separation and to increase the firmness of the yoghurt. From a technological point of view, the interaction of the denatured whey proteins with casein micelles or with κ-casein in the serum phases is regarded as responsible for obtaining a good yoghurt structure. The present research has shown that it is possible to produce yoghurt with a range of textural properties by precisely controlling the rate of whey protein fortification during its manufacture. Therefore, this study provides a better understanding of the effect of WPC fortification and aims to extend this insight for the production of good-quality yoghurt.     

Effect of high pressure processing and storage on the free amino acids in seedlings of Brussels sprouts

The potential of high pressure (HP) to affect the content of free amino acids (FAA) using seedlings of Brussels sprouts as a simple non-chopped vegetable system was examined. Firstly, the effect on FAA composition during growth was assessed and it was found that the composition of total free amino acids (TFAA) and individual FAA changed dramatically during growth of the seeds to the seedling at 7 days with the highest content of TFAA. Secondly, 7-day-old seedlings were HP-treated at various pressure levels (200–800 MPa for 3 min at 5 °C). As expected the HP-treatment did not affect the amino acids as no changes in TFFA were found immediately after pressurisation. In this line, HP-treatment up to 800 MPa had minor, but significant, effect on the FAA concentrations of 10 FAA (Ala, Asp, Glu, Gly, Leu, Phe, Pro, Ser, Trp and Tyr) and no significant changes were found for 7 of the FAA (Asn, Gln, His, Ile, Lys, Thr, and Val) concluding that the short pressure time (3 min) was insufficient to activate indigenous proteolytic enzymes. Furthermore, changes in the FAA content and composition of HP-treated seedlings during storage (0, 1, 2, and 4 days at 4 ± 2 °C) were evaluated in order to assess changes in the proteolytic enzyme activity. It was found that the changes in FAA differed according to the specific amino acids as well as the HP processing conditions and the subsequent storage time. These results suggest that HP treatment affects proteolysis and/or certain amino acids metabolism pathways in Brussels sprouts seedlings after HP treatment and during subsequent storage.     

The effect of high hydrostatic pressure on the flow behaviour of skim milk–gelatin mixtures

The flow behaviour of aqueous solutions of gelatin, and skim milk–gelatin mixtures treated by high-pressure processing (HPP) were investigated. HPP was carried out at 5 °C for 15 min, at 150 MPa, 300 MPa, 450 MPa and 600 MPa, and the gelatin concentrations were varied from 0 to 1 wt.%. Viscosity measurements showed that the HPP treatment did not affect the flow behaviour of gelatin alone, nor that of the skim milk–gelatin mixtures made with < 0.4 wt.% gelatin. However, at gelatin concentration > 0.4 wt.%, the mixtures treated with 300 and 450 MPa exhibited a peculiar flow behaviour, where at intermediate shear rates the viscosity was higher than that of the non-treated mixture or the mixtures treated at 150 MPa and 600 MPa. Particle size measurements showed that for gelled mixtures (> 0.4 wt.% gelatin) 300 MPa HPP treatment resulted in an increase in the particle size, while at all other pressure treatments (> 150 MPa), a shift in particle size distribution to lower sizes was observed. Confocal microscopy showed that these skim milk–gelatin mixtures were phase-separated with a gelatin continuous phase, this was confirmed by dynamic rheological measurements which showed that qualitatively the viscoelastic properties of the mixtures were the same. A mechanism of the effect of high-pressure treatment on the casein micelle in skim milk–gelatin mixtures is proposed.Industrial relevanceThis fundamental work, dealing with the effect of high pressure on the physicochemical properties skim milk–gelatin mixtures could be relevant to the industry in several ways. Firstly, skim milk–gelatin mixtures are widely used in the dairy industry, particularly in yoghurt manufacture, where gelatine is used as a stabiliser. In addition the application of High Hydrostatic Pressure to such a system is also relevant, as this technology could be used as a substitute to the conventional heat treatment processes. Secondly, an important finding of this study is that under certain conditions of high pressure and gelatine concentration, an increase in viscosity is observed at intermediate shear-rate (between 10 and 100 s^?1). This is highly relevant to Industry if the system requires subsequent pumping. Thirdly, from a sensory view point, this range of shear rates (10 and 100 s^?1) is comparable to that experienced by a food bolus during swallowing. Thus, this effect of high pressure on the viscosity can influence sensory attribute of the skim milk–gelatin food system.     

Kinetic parameters of Escherichia coli O157:H7 survival during fermentation of milk and refrigeration of home-made yoghurt

The survival parameters of Escherichia coli O157:H7 during milk fermentation (carried out by the LIM or “longer incubation method” at 30 °C, or by the SIM or “short incubation method” at 43 °C) and storage of home-made yoghurt at refrigeration temperatures (2, 4, or 8 °C) were studied. The E. coli O157:H7 counts increased slightly during fermentation by the LIM, from 5.1 to 5.4 log cfu mL⁻¹, and it was not found after 21 d of storage at 2 or 4 °C, and after 10 d at 8 °C. The microorganism counts increased from 4.8 to 5.4 log cfu mL⁻¹ during the SIM, and it was not detected after 7 d stored at 8 °C. The microorganism grew faster at 43 °C (generation time=0.93 h) than at 30 °C (4.12 h) during the fermentation period. The death time decreased with the increase of the storage temperature (from 38.1 h at 2 °C to 30.1 h at 8 °C) in the yoghurt produced by fermentation at 30 °C; however, a clear relationship between death time and storage temperature was not evident at 43 °C. The pH values of the yoghurt ranged from 4.0 to 4.7.     

Manufacture of Cheddar cheese from high-pressure-treated whole milk

The cheese-making characteristics of high-pressure (HP)-treated milk were examined. The rennet coagulation time of pasteurised milk decreased after HP treatment at 400 MPa but increased after treatment at 600 MPa. The L-value (whiteness) of milk decreased directly after HP treatment but, over the course of coagulation, whiteness of HP-treated milk increased to the same level as in the control. Cheddar cheese was then manufactured from raw whole milk or whole milk treated by high-pressure (HP) at 400 MPa (HP400) or 600 MPa (HP600) for 10 min at 20 °C. HP treatment of raw milk at 600 MPa resulted in a 3.66 log reduction in the initial counts of non-starter lactic acid bacteria (NSLAB), decreased protein and fat content, as well as a lower pH compared to the control. Furthermore, higher treatment pressures resulted in increased incorporation of β-lactoglobulin into the cheese curd, with parallel increases in yield by 1.23% and 7.78% for HP400 and HP600 cheeses, respectively. Overall, this study showed that the effects of HP treatment on milk proteins increased rennet coagulation times and changes in cheese composition at day 1.Industrial relevanceHigh-pressure treatment is a novel technology which has been applied to a number of commercial food products. In this study, HP-induced changes in milk proteins resulted in increased cheese yields and increased cheese whiteness. In addition, HP treatment significantly reduced the microflora of raw milk cheese. Those attributes could be of interest for both industry and consumer.     

High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h⁻¹ homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100–300 MPa) and milk inlet temperature T_in (4°C, 14°C or 24°C) on milk temperature T₂ at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The T_in values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High T_in and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d_4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at T_in=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (T_in=4°C, 14°C or 24°C) further decreased d_4.3 diameters to about 0.2 μm and narrowed the size distribution. At all T_in tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than 1 log cycle, provided homogenisation pressure was ⩾200 MPa at T_in=24°C (T₂∼60°C), 250 MPa at T_in=14°C (T₂∼62°C), or 300 MPa at T_in=4°C (T₂∼65°C). At all T_in tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria.