J.A. Lucey

Formation and physical properties of acid milk gels: a review

Acidified milk products like yoghurt are an important food product but there are relatively few reports on the mechanisms involved in gel formation and the effects of processing variables such as heat treatment and gelation temperature on the important physical properties (such as whey separation) of acid-induced gels. Most previous reviews have described the microbiology of the starter cultures and technologies used in yoghurt manufacture. Recent developments are reviewed including the use of techniques such as dynamic low amplitude oscillatory rheology to monitor the gel formation process; confocal scanning laser microscopy to examine gel microstructure; and various models for the aggregation of particle gels are discussed in terms of possible mechanisms involved in the formation of acid-induced milk gels.

Contribution of the indigenous microflora to the maturation of cheddar cheese

Abstract Cheddar cheeses were made from raw milk, pasteurised milk (72°C, 15 s) or milk produced from skim milk which had been microfiltered using an Alfa-Laval MFS-1 MF unit and mixed with pasteurised cream (72°C, 30 s). Microfiltration (MF) reduced the total bacterial count (TBC) by > 99% and MF cheesemilk had a lower TBC than pasteurised milk; counts of non-starter lactic acid bacteria (NSLAB) were

Rheological properties at small (dynamic) and large (yield) deformations of acid gels made from heated milk

The effect of a range of milk heat treatments on the rheological properties, at small and large deformations, of acid skim milk gels was investigated. Gels were made from reconstituted skim milk heated at 75, 80, 85 and 90°C for 15 or 30 min by acidification with glucono-δ-lactone at 30°C. Gels were also made from skim milk powder (SMP) samples that had been given a range of preheat treatments during powder manufacture. Heating milks at temperatures [ges ]80°C for 15 min increased the storage moduli (G′) compared with unheated milk and produced gels with G′ in the range 300–450 Pa. Acid gels made from high-heat or medium-heat SMP had higher G′ than gels made from low-heat or ultra-low-heat SMP. Cooling gels to low temperatures resulted in an increase in G′. The yield stress of gels slightly decreased with mild heat treatments of milk, and then increased again to a maximum, finally decreasing slightly with very high heat treatments of milk. The strain at yielding decreased markedly with increasing heat treatment of milk, making these gels brittle and easier to fracture. We propose that denatured whey proteins aggregated with casein particles during the acidification of heated milk and were responsible for most of the effects observed in this study.

Effect of interactions between denatured whey proteins and casein micelles on the formation and rheological properties of acid skim milk gels

The effect of interactions of denatured whey proteins with casein micelles on the rheological properties of acid milk gels was investigated. Gels were made by acidification of skim milk with glucono-δ-lactone at 30°C using reconstituted skim milk powders (SMP; both low- and ultra-low-heat) and fresh skim milk (FSM). The final pH of the gels was ∼4·6. Milks containing associated or ‘bound’ denatured whey proteins (BDWP) with casein micelles were made by resuspending the ultracentrifugal pellet of heated milk in ultrafiltration permeate. Milks containing ‘soluble’ denatured whey protein (SDWP) aggregates were formed by heat treatment of an ultracentrifugal supernatant which was then resuspended with the pellet. Acid gels made from unheated milks had low storage moduli, G ′, of <20 Pa. Heating milks at 80°C for 30 min resulted in acid gels with G ′ in the range 390–430 Pa. The loss tangent (tan δ) of gels made from heated milk increased after gelation to attain a maximum at pH ∼5·1, but no maximum was observed in gels made from unheated milk. Acid gels made from milks containing BDWP that were made from low-heat SMP, ultra-low-heat SMP and FSM had G ′ of about 250, 270 and 310 Pa respectively. Acid gels made from milks containing SDWP that were made from ultra-low-heat SMP or FSM had G ′ values in the range 17–30 Pa, but gels made from low-heat SMP had G ′ of ∼140 Pa. It was concluded that BDWP were important for the increased G ′ of acid gels made from heated milk. Addition of N -ethylmaleimide (NEM) to low-heat reconstituted milk, to block the —SH groups, resulted in a reduction of the G ′ of gels formed from heated milk but did not reduce G ′ to the value of unheated milk. Addition of 20 m m -NEM to FSM, prior to heat treatment, resulted in gels with a lower G ′ value than gels made from reconstituted low-heat SMP. It was suggested that small amounts of denatured whey proteins associated with casein micelles during low-heat SMP manufacture were probably responsible for the higher G ′ of gels made from milk containing SDWP and from milk heated in the presence of 20 m m -NEM, compared with gels made from FSM.

A comparison of the formation, rheological properties and microstructure of acid skim milk gels made with a bacterial culture or glucono-δ-lactone

Abstract Gels were made by acidification of milk with glucono-δ-lactone (GDL) or a starter culture, at two gelation temperatures. Rheological properties and microstructure of these gels were determined using oscillatory rheometry, permeability and confocal laser scanning microscopy. On addition of GDL to milk the pH decreased rapidly and it stabilized at pH ∼4.6. After the addition of starter culture to milk initially the pH decreased slowly and then decreased steadily but stabilized at pH ∼4.0. GDL-induced gels had much shorter gelation times but higher storage moduli (G′), yield stresses and strains, permeability and whey separation than gels formed by a bacterial culture. Gels formed at 42°C had shorter gelation times but higher pH at gelation, G′, permeability and whey separation. Loss tangent of all gels increased to a maximum shortly after gelation. Microstructure of gels formed with a bacterial culture was not greatly affected by gelation temperature in contrast to GDL-induced gels.

Properties of acid casein gels made by acidification with glucono-δ-lactone. 1. Rheological properties

The effects of gelation temperature (20, 30 or 40 °C), assay temperature, concentration of glucono-δ-lactone (GDL) added, and NaCl concentration on the rheological properties of acid casein gels were studied at small and large deformations. Gels prepared at a high incubation temperature had very low storage moduli (G′), whereas those made at a low incubation temperature had extremely high G′ values. A higher concentration of GDL resulted in faster gelation but slightly lower G′ values of aged gels. Addition of NaCl resulted in longer gelation times and a slower rate of increase of G′. Cooling of gels prepared at 30 or 40 °C resulted in an increase in G′. However, for gels formed at 20 °C, G′ decreased initially on cooling but returned to its original value on holding at 5 °C. The loss tangent (tan δ) of gels formed at 20 or 30 °C was independent of frequency; however, for gels formed at 40 °C, tan δ was lower at low frequencies. As gels were cooled to 5 °C, tan δ increased. Fracture stress (σfr) of gels formed at 20 °C was much greater than that of the gels formed at higher temperatures. Heating gels to temperatures higher than the gelation temperature resulted in a decrease in σfr. At low gelation temperatures, young gels had very high (σfr values.

The relationship between rheological parameters and whey separation in milk gels

Abstract The relation between whey separation of rennet-induced gels and rheological properties of those gels is reasonably well understood. A low fracture stress and a high value for the loss tangent at low frequencies have been correlated with a tendency to exhibit syneresis in rennet gels. In contrast, little is known about the relationship between mechanical properties of gels and whey separation in acid-induced milk gels, such as yoghurt, although this continues to be a major defect. In recent work, it has been found that conditions such as high milk heat treatment, fast rates of acidification and high incubation temperatures all gave high levels of whey separation compared with gels made from unheated milk that were incubated at low temperatures and where the rate of acidification was slow (i.e. when bacterial cultures were used instead of the acidogen, glucono-δ-lactone). The tendency to exhibit whey separation in acid gels made from heated milk was related to a low fracture strain and an increase in the loss tangent (observed even at high frequencies) during the gelation process (a high value indicates conditions favouring relaxation of bonds). Excessive rearrangements of particles in the gel network before and during gelation were implicated as being responsible for whey separation and rheological conditions that appeared to indicate this defect are described. It was also concluded that techniques that measure the spontaneous formation of surface whey should be distinguished from those that measure the expression of whey from networks under pressure as the latter tests only measure gel rigidity.

Formation of Whey Protein Isolate (WPI)−Dextran Conjugates in Aqueous Solutions

The conjugation reaction between whey protein isolate (WPI) and dextran in aqueous solutions via the initial stage of the Maillard reaction was studied. The covalent attachment of dextran to WPI was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with both protein and carbohydrate staining. The formation of WPI-dextran conjugates was monitored by a maximum absorbance peak at approximately 304 nm using difference UV spectroscopy. The impact of various processing conditions on the formation of WPI-dextran conjugates was investigated. The conjugation reaction was promoted by raising the temperature from 40 to 60 degrees C, the WPI concentration from 2.5 to 10%, and the dextran concentration from 10 to 30% and lowering the pH from 8.5 to 6.5. The optimal conjugation conditions chosen from the experiments were 10% WPI-30% dextran and pH 6.5 at 60 degrees C for 24 h. WPI-dextran conjugates were stable under the conditions studied.

Properties of acid casein gels made by acidification with glucono-δ-lactone. 2. Syneresis, permeability and microstructural properties

Syneresis of casein gels made by acidification with glucono-δ-lactone (GDL) was studied in relation to gel structure as derived from permeametry and confocal scanning laser microscopy (CSLM). Gels made at 40 °C exhibited ‘spontaneous syneresis’ with wheying-off almost immediately after gelation, while those formed at 30 °C exhibited little syneresis and only after it was initiated by wetting the surface. Syneresis decreased with a reduction in the pH of gels (increasing amount of GDL added). Gels cooled to 5 °C (before initiating syneresis) and low pH gels exhibited ‘negative syneresis’, i.e. an increase in the height of the gel after wetting. Addition of NaCl had little effect on syneresis, except at pH values > 4.6, where gels with added NaCl exhibited stronger syneresis than those made without added NaCl. Higher gelation temperatures resulted in a far greater permeability coefficient (B), indicating the presence of large pores in these gels. Gels formed at low temperatures had a very low B. Addition of NaCl, at all gelation temperatures, markedly reduced B. Confocal scanning micrographs showed that gels made at high gelation temperatures had large pores, many > 20 μm. At low gelation temperatures, the pores were small, mostly < 5 μm. Fractal aggregation theory was used to explain some of the results, especially the rearrangement of aggregated particles at an early stage of the gelation process, i.e. over relatively short distances. It was concluded that gelation temperature had a large effect on this rearrangement. At low temperatures (e.g. 20 °C), rearrangement did not occur, whereas it was already extensive at 30 °C, implying that the ‘building blocks’ of the fractal gel consisted of dense aggregates of, say, 25 casein particles. This resulted in increased permeability. It was also concluded that the syneresis occurring at 30 °C is primarily due to consolidation of the gel network under its own weight, which is soon counteracted by the stress induced by the deformation of this network. Gels formed at higher temperatures may be unstable and show spontaneous syneresis, presumably because some of the strands in the gel network were weak enough to break. No conclusive explanation could be given for the effects of NaCl concentration and pH on the properties of acid casein gels.

Rheological properties of milk gels formed by a combination of rennet and glucono-δ-lactone

The effects of heat treatment of milk, and a range of rennet and glucono-δ-lactone (GDL) concentrations on the rheological properties, at small and large deformation, of milk gels were investigated. Gels were made from reconstituted skim milk at 30 °C, with two levels each of rennet and GDL. Together with controls this gave a total of sixteen gelation conditions, eight for unheated and eight for heated milk. Acid gels made from unheated milks had low storage moduli ( G ′) of < 20 Pa. Heating milks at 80 °C for 30 min resulted in a large increase in the G ′ value of acid gels. Rennet-induced gels made from unheated milk had G ′ values in the range ∼ 80–190 Pa. However, heat treatment severely impaired rennet coagulation: no gel was formed at low rennet levels and only a very weak gel was formed at high levels. In gels made with a combination of rennet and GDL unusual rheological behaviour was observed. After gelation, G ′ initially increased rapidly but then remained steady or even decreased, and at long ageing times G ′ values increased moderately or remained low. The loss tangent (tan δ) of acid gels made from heated milk increased after gelation to attain a maximum at pH ∼ 5·1 but no maximum was observed in gels made from unheated milk. Gels made by a combination of rennet and GDL also exhibited a maximum in tan δ, indicating increased relaxation behaviour of the protein–protein bonds. We suggest that this maximum in tan δ was caused by a loosening of the intermolecular forces in casein particles caused by solubilization of colloidal calcium phosphate. We also suggest that in combination gels made from unheated milk a low value for the fracture stress and a high tan δ during gelation indicated an increased susceptibility of the network to excessive large scale rearrangements. In contrast, combination gels made from heated milk formed firmer gels crosslinked by denatured whey proteins and underwent fewer large scale rearrangements.