R E S E A R C H A R T I C L E Influence of heat treatment of goat milk on casein micelle size, rheological and textural properties of acid gels and set type yoghurts
Marina Hovjecki1 | Zorana Miloradovic1 | Vladislav Rac2
| Predrag Pudja1 | Jelena Miocinovic1 1Department of Animal Source Food
Technology, Faculty of Agriculture, University of Belgrade, Belgrade, Serbia
2Department of Chemistry and Biochemistry, Faculty of Agriculture, University of Belgrade,
Belgrade, Serbia Correspondence Marina Hovjecki, Department of Animal
Source Food Technology, Faculty of Agriculture, University of Belgrade, Nemanjina
6, Belgrade 11080, Serbia.
Email: marina.hovjecki@agrif.bg.ac.rs Funding information
Ministry of Education, Science and Technological development, Grant/Award
Number: 46009 Abstract Acid gels and yoghurts were made from goat milk that was heated at 72C/30 s,
85C/5 min, and 95C/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 tex- ture 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 85C/5 min. Also, the pH of gelation, the storage moduli (G0) and yield stress were higher for this yoghurt, compared with the other two. Textural properties of goat milk yoghurts such as firm- ness and consistency were strongly affected by milk heat treatment, and the highest values were recorded for yoghurt produced from milk preheated at 85C/5 min, as well. The largest casein micelles were measured after 85C/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.
K E Y W O R D S goat milk, heat treatment, micelle size, rheology, texture, yoghurt
1 | INTRODUCTION Fermented dairy products such as yoghurt are usually produced from cow's milk, but for the last decade goat's milk products are becoming increasingly popular (Tamime, Wszolek, Božanic, & Özer, 2011). How- ever, due to lack of knowledge, goat's milk products are usually produced based on the technological procedure for cow milks despite they are sim- ilar in gross composition, but quite different in technological properties (Miloradovic, Kljajevic, Jovanovic, Vucic, & Macej, 2015).
Milk heat treatment is important processing variable in the produc- tion of fermented dairy products such as yoghurt. High heat treatment of milk is usually applied for production of fermented dairy products due to its positive impact on the textural properties (Robinson
& Tamime, 1993). There is evidence that for a constant duration of heat treatment (10, 15, or 30 min), increasing the temperature of skim milk heat treatment in the range of 70–95C increases denaturation of whey proteins (Dannenberg & Kessler, 1988a, 1988b), formation of heat- induced serum and micelle-bound complexes, and increases the gelation pH and elasticity of acid gels as well (Lucey, van Vliet, Grolle, Geurts, &
Walstra, 1997; Vasbinder, Alting, & de Kruif, 2003). On the other hand,
Raynal and Remeuf (1998) reported that severe heat treatments above
This article was published on AA publication on: 12 April 2020
Received: 23 January 2020 Revised: 21 March 2020 Accepted: 6 April 2020
DOI: 10.1111/jtxs.12524 J Texture Stud. 2020;1–8. wileyonlinelibrary.com/journal/jtxs
© 2020 Wiley Periodicals LLC 1 90C cause rearrangement of casein components in the micellar struc- ture through aggregation and dissociation reactions, which could also lead to changes in the gelation processes and different texture properties of acid gels.
The rheology of acid gels and fermented dairy products made from cow's milk were studied in many researches (Guggisberg,
Cuthbert-Steven, Piccinali, Bütikofer, & Eberhard, 2009; Lee &
Lucey, 2003; Lucey, Munro, & Singh, 1998; Lucey, Teo, Munro, &
Singh, 1997; Lucey, van Vliet, et al., 1997; Peng, Serra, Horne, &
Lucey, 2009).
However, to the best of our knowledge, just a few reports con- cerning the study of the rheological properties of goat's milk and products were presented (Domagała, Sady, Grega, & Najgebauer- Lejko, 2007; Jumah, Shaker, & Abu-Jdayil, 2001; Vargas, Cháfer,
Albors, Chiralt, & González-Martínez, 2008).
The influence of high heat treatment of milk on its compo- nents, especially proteins, is quite different in goats' milk as com- pared with cows' milk. It has been reported that the extent of protein precipitation in goat milk due to heat treatment is several times higher compared with the cow milk treated under same con- ditions, which indicates lower colloidal stability of goat milk. Also, the nature of heat induced protein complexes differs at these two milks (Miloradovic et al., 2015; Pesic et al., 2012). Hence, the objec- tive of this study was to determine the effects of goat milk heat treatment on rheological properties of acid gels as well as textural properties of set type yoghurts obtained from milk treated on the different heat regime. In order to better understand the behavior of different heat-treated goat milk during acid gelation, casein micelle size and SDS PAGE electrophoresis were also determined. Data about the influence of goat milk heat treatment on the casein micelle size are quite rare in literature that gives an additional value of this study.
2 | MATERIALS AND METHODS 2.1 | Goat milk samples Raw milk was collected from a local farm of Saanen goats (250 goats).
Approximately 3 L of raw bulk milk was collected every week during
3 weeks from the local farm, and immediately after collection milk was poured into the stainless steel pot and heated on the stove by agitation until completing appropriate heat treatment. All three heat treatments and following experiments were performed every week.
Compositional analyses were performed immediately after the sample collection.
Saanen goat's milk (3.1 ± 0.2% milk fat, 2.64 ± 0.04% proteins,
7.22 ± 0.11% non-fat dry matter and pH 6.72 ± 0.06) was treated by different heat regimes (72C/30 s, 85C/5 min, and 95C/5 min) and was used for casein micelle size measurements, electrophoresis, acid gelation, and set type yoghurt properties determination. All experi- ments were done in triplicate while analysis was done at least duplicate.
2.2 | DLS measurement The sizes of casein micelles were determined by dynamic light scatter- ing method (DLS) that was performed using a Horiba NanoPartica SZ- 100 (Horiba, Japan). After heat treatments, goat milk samples were immediately cooled by immersing in cool water, and left in refrigerator overnight. Samples were reheated to 40C and cooled down to 20C to achieve samples equilibration. Raw and heat treated goat milk sam- ples were skimmed by centrifugation (centrifuge model 5430;
Eppendorf AG, Hamburg, Germany) at 20C for 15 min at 600g. This skimming regime was used as the best appropriate in order to avoid protein loss (Miloradovic et al., 2015). Samples were diluted 100-fold in distilled water (Day, Williams, Otter, & Augustin, 2015) and mea- surements were performed at 20C, using a scattering angle of 90, in photon correlation mode and the correlation functions were analyzed to calculate the average hydrodynamic diameters. Measurements were performed at least three times on each sample.
2.3 | SDS-PAGE Raw and heat treated goat milk samples were prepared as reported by
Miloradovic et al. (2015), which is previosly described for DLS mea- surements. After the centrifugation, skimmed milk samples were
10-fold diluted by mixing aliquots of samples with SDS-R-PAGE (reducing) and
SDS-NR-PAGE (non-reducing) sample buffers according to the method of Anema and Stanley (1998). SDS-R-PAGE sample buffer pH 6.8 contained 0.055 M Tris–HCl, 2% (wt/vol) SDS,
7% (vol/vol) glycerol, 5% (vol/vol) β-mercaptoethanol, and 0.0025% (wt/vol) bromophenol blue. SDS-NR-PAGE sample buffer was without β-mercaptoethanol, and that was the only difference.
SDS-PAGE was performed according to the Laemmli (1970) with
4% stacking gel of pH 6.80, and 15% resolving gel pH 8.85. Five microliters of prepared samples were loaded at each well. Electropho- resis was performed using TV200YK twin-plate electrophoresis unit (Consort, Belgium), the power supply was set at constant current of
80 mA and the maximum voltage of 300 V for 1.5 hr. Gels were fixed and stained for 1 hr at 45C in Coomassie blue dye solution with
0.23% (wt/vol) Coomassie brilliant blue R250, 3.9% (wt/vol) trichlor- oacetic acid, 6% (vol/vol) acetic acid and 17% (vol/vol) methanol.
Destaining was done with a solution containing 8% (vol/vol) acetic acid and 18% (vol/vol) methanol, and the gels were scanned.
2.4 | Gel formation and yoghurt preparation Saanen goat's milk after different heat regimes (72C/30 s [A];
85C/5 min [B]; 95C/5 min [C]) was cooled to 43C and inoculated with 0.2% wt/wt “Yoflex 812” starter culture (Chr Hansen, The Neth- erlands). Small amount was used for rheological measurements, while fermentation was set at 43C until pH 4.6 was reached. Yoghurts were cooled during 24 hours in fridge temperature (4–7C) and then ana- lyzed. All experimental trials were repeated three times.
2 HOVJECKI ET AL.
2.5 | Rheological measurements Viscoelastic properties of acid gels and yoghurts were monitored by dynamic oscillatory shear measurements, performed on a Kinexus Pro
+ Rheometer (Malvern, Worcestershire, United Kingdom) with the four blade vane as a tool. The milk samples, after addition of starter culture were stirred for 2 min and then transferred to the rheometer, where were oscillated at a frequency of 0.1 Hz (0.1% shear strain) until the pH reached 4.6. Gelation time and pH were measured at the point when gels reached the storage modulus value (G0) ≥1 Pa (Lucey, van Vliet, et al., 1997), as well as G0 at pH 4.6, as a measure of gel firmness after fermentation.
When fermentation and coagulation were finished, gels were cooled to 5C (1C/min) and held at this temperature for 30 min before they were subjected to a frequency sweep (0.01–10 Hz; 0.1% strain).
Storage (G0) and loss moduli (G00) were calculated at the 1 Hz as a measure of gel properties after cooling.
Strain sweeps were performed (0.001–500%) at constant fre- quency of 1 Hz to determine the linear viscoelastic range (LVE). The yield stress value and the complex viscosity (η*) were calculated from cross over point (G0 = G00) outside of the LVE (Guggisberg et al., 2009).
All measurements were performed in triplicate.
2.6 | Textural properties Texture analysis was performed in plastic containers where fermenta- tion and gel formation were done. Due to achieving repeatability,
100 g of inoculate previously were measured into containers where the fermentation and cooling of yoghurt took place. Textural proper- ties of yoghurts such as firmness, consistency, cohesiveness and index of viscosity, were analyzed by TA.XT Plus Texture analyzer (Stable
Micro System, Godalming, Surrey, United Kingdom) through a single compression test, using a back extrusion cell disc (A/BE; diameter
35 mm; distance 30 mm; speed 0.001 m/s) and an extension bar, with
5 kg load cell at 5C. Textural parameters were automatically calcu- lated from the graph, by the Exponent Software (Stable Micro System,
Surrey, UK). Four parameters were measured: (a) firmness (N) was defined as the peak of the maximum force; (b) consistency (Ns) was taken as the area of the curve up to the point of firmness: (c) cohesiveness (N) was defined as the maximum negative force dur- ing the returning of the probe; (d) index of viscosity (Ns) was defined as the total negative area obtained when the probe returns. Six mea- surements were carried out for each sample. Experiment was per- formed in triplicate.
2.7 | Statistical analysis A one-way analysis of variance (ANOVA) was done using Statistica
10.0 software (Stat Sof. Inc., Tulsa) in order to evaluate the effects of milk heat treatment on the rheological and textural properties of goat's acid milk and yoghurts as well as casein micelle size. Mean comparisons of the parameters were performed by Fisher LSD-test, with the level of significance of .05.
3 | RESULTS AND DISCUSSION 3.1 | Mean casein micelle diameter
Results of DLS measurements showed that there were no modifi- cation of micelles size when milk was treated at 72C/30 com- pared with raw samples (~253 nm). At higher temperatures, the size of micelles increased significantly to ~360 nm and ~321 nm after 85C/5 min and 95C/5 min treatments, respectively (Table 1.).
Micelles diameter increase due to heat treatment are the result of several factors, primarily intensive formation of micelle/whey protein aggregates, and increase of micelle voluminosity caused by heat induced phosphate precipitation (Jeurnink & De Kruif, 1993;
Raynal & Remeuf, 1998). As Anema and Li (2003) reported, the majority of the changes in casein micelle size are due to the associ- ation of the denatured whey proteins with the casein micelles.
However, it is still not possible to determine whether the size changes observed are due exclusively to the association of dena- tured whey proteins with the casein micelle surface or due to the partial aggregation of casein micelles that occurs at the same time and is proportional to the levels of whey proteins that have associ- ated with the casein micelles.
Raynal and Remeuf (1998) reported that micelle size in goat milk remained unchanged after heating at 75C and increased at 85C remaining the same at 90C. Our results showed that heating at
95C/5 min led to a decrease in the micelle diameter, as compared to
85C/5 min treatment.
We assume that this phenomena probably occur because of casein dissociation after severe heating, especially of κ-casein (Singh & Creamer, 1991) which interacts with denatured whey pro- teins in the serum, or another explanation would be that κ-casein in the form of complexes with whey proteins is released from the micelle in the serum phase. Also, other studies suggested that κ-casein disso- ciated at temperatures below that of denaturation of the whey pro- teins and reached its maximum at an earlier stage than denaturation of the whey proteins (Anema,
2008b; Anema, Kim Lee, & Klostermeyer, 2007). Moreover, heat precipitated calcium phosphate
TABLE 1 Casein micelle size as affected by different heat treatments of goat milk
Heat treatment Casein micelle size (nm) Raw milk 253.2 ± 14.78a
72C/30 s 253 ± 11.34a 85C/5 min 360 ± 14.46b 95C/5 min
321 ± 3.09c Note: Values in the table represent means of three replicated trials ± SD;
Values with different letter are significantly different (p < .05).
HOVJECKI ET AL.
3 is not able to keep integrity of the native micelle, thus favoring casein dissociation (Anema & Klostermeyer, 1997), or cause the micelle to shrink (Jeurnink & De Kruif, 1993).
3.2 | SDS-PAGE analysis Observing the electrophoretogram of raw and heat-treated goat milks, we noted existence of high molecular weight disulfide-linked protein complexes in the samples treated at 85C and 95C/5 min, that could not diffuse into the gel. Significant irreversible covalent aggregation of whey proteins could be observed in samples obtained after treatments above 72C/30 s. From the Figure 1 it is evident greater degree of denaturation of β-lactoglobulin after
95C/5 min treatment.
In non-reducing conditions, significant reduction of the band intensity of major whey proteins and polymers of κ-CN were observed for samples treated above 72C/30 s treatments. Also by analysis in non-reducing conditions, it could be noted that α-lactalbumin was more heat stable than β-lactoglobulin that is in agreement with Pesic et al. (2012).
3.3 | Rheological properties Significant differences were observed among the acid gels made from goat milk treated on different heat regime considering the gelation time and the time to reach pH 4.6 (Table 2). The shortest time was recorded for the B (85C/5 min) and the longest one for the A sample (72C/30 s). The pH at gelation point for the A sample (4.67) was sig- nificant lower (p < .05) from the pH of B and C gels (4.91 ± 0.10 and
4.88 ± 0.10, respectively). Reducing the gelation time and increasing pH of gelation due to heat treatment was also found for cow milk (Lucey et al., 1998; Lucey, van Vliet, et al., 1997). In accordance with our results, Lucey, van Vliet, et al. (1997) observed an increase in pH at gelation point with increasing heat temperature up to 85C after which there was little further change. This heat induced change is consequence of the higher isoelectric point of whey proteins (5.2–5.3) that denature and form complexes with κ-casein contributing that aggregation starts earlier (Lucey, van Vliet, et al., 1997).
The significant variations (p < .05) in G0 values at the end of fer- mentation (pH 4.6) among the yoghurts were found. The highest G0 was recorded for B yoghurt (85C/5 min) while the lowest one for A sample (72/30 s). This indicates that the strength and number of
FIGURE 1 SDS PAGE patterns of raw (S) and heat treated (72, 85 and
95 corresponding to 72C/30 s, 85C/5 min and 95C/5 min heat treatments) goat milk samples prepared in reducing (R) and non- reducing (N) conditions
4 HOVJECKI ET AL. bonds in the network obtained from milk heated at 85C/5 min is much higher than in the other gels (Roefs, De Groot-Mostert, & Van
Vliet, 1990; van Vliet, van Dijk, Zoon, & Walstra, 1991).
All examined acid gels showed a significant increase in G0 after cooling probably due to swelling of casein particles and an increase in the contact area between particles (Lucey, van Vliet, et al., 1997) as well as nature of protein network. However, acid gels made from severely heated goat milk (85C/5 min and 95C/5 min) had signifi- cantly higher G0, G00, and the phase angle (tan δ) values than samples treated at lower temperature (72C/30 s).
The yield stress values were significant various (p < .05) between acid gels obtained from milks heated on different regime. The highest yield stress value for gel made from milk treated at 85C/5 min was found, indicated that the structure is less susceptible to rearrangements and fracture, compared with other two gels. For com- parison, Lucey, van Vliet, et al. (1997) found that heating of cow milk at temperatures higher than 75C resulted in an increase of the shear stress at fracture of acid gels produced, and maximum was recorded in samples obtained from milk that was heated at 85C for 15 min, after which shear stress decreased. Generally, fracture properties of gels depend on the number of bonds per cross section of the strand as well as the strength of each bond (van Vliet et al., 1991), therefore lower yield stress value recorded for acid gels obtained from
95C/5 min heated milk, indicate that this treatment could led to weakening of bonds.
Differences in the rheological properties of acid gels analyzed in this study are a consequence of different heat treatment of goat milk used.
It is known that milk heat treatment causes denaturation of whey proteins, some of which associate with casein micelles and during acidification these aggregates act as bridges between casein micelles and make strength of bonds within a protein network more stronger resulted in higher G0 (Lucey, van Vliet, et al., 1997). Moreover, the concentration of protein would be increased because of active partici- pation of denatured whey proteins in the gel structure. Both these factors are responsible for the increased G0 of acid gels made from heated milks compared to raw milk.
The most studies done for cow milk acid coagulation also show positive influence of heat treatment on the rheological properties of acid gels (Dannenberg & Kessler, 1988a, 1988b; Lee & Lucey, 2003;
Lucey, van Vliet, et al., 1997). However, rheological properties of acid gels obtained from cow milk such as G0 is much higher compared to goat milk gels because of difference between properties of compo- nents, especially proteins, as well as micelle composition, size, mineral- ization and hydration (Park, Juárez, Ramos, & Haenlein, 2007).
Additionally, as reported by Martín-Diana, Fraga, and Fontecha (2002), κ- casein in goat milk is far less glycosilated than in bovine milk. This glycosylation degree negatively correlates with the casein micelle size, which corresponds to our results, as we recorded significantly larger micelles in caprine milk, as compared to the literature data for cow milk (Day et al., 2015).
Our results showed that better rheological properties were found for acid gel obtained from goat milk heated at 85C/5 min than at
95C/5 min. Lucey, van Vliet, et al. (1997) observed a similar trend in regard to very severe heat treatments (90C/30 min) on cow milk, that resulted in reduction in the G0 of acid gels, compared with milks that were subjected to milder heat treatments. Authors consider that reason for this behavior could be very high level of whey protein denaturation and formation of large aggregates that contribute to decrease of G0 (Lucey, van Vliet, et al., 1997). However, smaller casein micelle size found in our study did not support this opinion indicating that it should be look for some other explanation.
In many studies, the higher level of whey proteins denaturation contributes to better rheological properties of acid gels obtained from cow milk (Dannenberg & Kessler, 1988a, 1988b; Lucey, van Vliet, et al., 1997; Vasbinder et al., 2003). However, in our study, as can be seen at electrophoretogram (Figure 1), the level of whey protein dena- turation after 95C/5 min treatment is higher than after 85C/5 min that also could not give explanation for lower G0.
Anema (2008a) reported that changes in acid gel properties were not dependent only to whey protein denaturation level in the milk.
Mentioned research showed that treatment temperatures higher than
70C produced acid gels with markedly higher final G0, with the final G0 increasing with treatment temperature to a maximum at about 85C, and then decreasing at more severe heat regimes. Denaturation of the whey proteins is insufficient to predict the final properties of acid gels, and the interactions of the denatured whey proteins with other pro- teins in the milk as well as the changes in the casein micelle structure are more significant for the acid gel properties than the level of whey protein denaturation (Anema, 2008a), which in the light of our results obtained for the casein micelle size better describes this phenomenon.
Moreover, internal micellar integrity and the rates of change of structure and composition with pH play a role in defining the visco- elastic properties of the resulting gels (Horne, 2003).
TABLE 2 Rheological properties of goat milk acid gels affected by different heat treatment
Parameter Yoghurt 72C/30 s 85C/5 min 95C/5 min Gelation time (min)
247 ± 13a 118 ± 6b 168 ± 22c Gelation pH 4.67 ± 0.04a
4.91 ± 0.10b 4.88 ± 0.10b Time to pH 4.6 (min) 310 ± 36a
241 ± 21b 277 ± 20c G0 at pH 4.6 (Pa) 1.12 ± 0.11a
3.7 ± 0.5b 2.0 ± 0.15c Elastic modulus, G0 at 1 Hz (Pa)
9.51 ± 1.90a 27.47 ± 9.30b 25.5 ± 1.73b Viscous modulus,
G00 at 1 Hz (Pa) 2.23 ± 0.38a 6.65 ± 2.34b 6.52 ± 0.4b
Tan delta (loss tangent) 0.235 ± 0.006a 0.241 ± 0.005b 0.256 ± 0.003b
Yield stress (Pa) 12.31 ± 2.14a 19.54 ± 1.67b 17.06 ± 1.52c
Note: Values in the table represent means of three replicated trials ± SD;
Values with different letter within the same row are significantly differ- ent (p < .05).
HOVJECKI ET AL.
5 Additionally, Raynal and Remeuf (1998) concluded that heat treatments above 90C cause rearrangement of casein components in the micellar structure through a series of aggregation and dissociation reactions which also impairs the micelles ability to form strong pro- tein/protein bonds through gelation process. Van Hooydonk, De
Koster, and Boerrigter (1987) pointed out that many physico-chemical modifications are induced by heating (changes in milk salt equilibria, micelle size and hydration), that might also interfere with processes during aggregation and gelation.
Although we recorded smaller mean micelles diameter in milks treated at 95C/5 min compared to 85C/5 min, we suggest that such high heat treatment induced changes in the casein micelle structure, so that the capacity to form strong protein chains was reduced and that led to weaker gel structure, compared to B yoghurt.
3.4 | Textural properties Yoghurt textural characteristics are an important criterion for quality assessment.
Generally, firmness describes moderate resistance of product to deformation. Consistency relates to the “firmness,” “thickness,” or
“viscosity” of a liquid or fluid semi-solid. Stirring a fluid or semi fluid food with a spoon or a finger is frequently used by consumers to give an indication of the viscosity or consistency. Cohesiveness is the ten- dency of a product to cohere or stick together. The intermolecular attractions by which the elements of a body or mass of material are held together determine its cohesiveness. It is related to the internal stickiness of a product and is usually determined by measurement of the amount of force to remove an item from the product mass (Gunasekaran
& Mehmet Ak, 2003).
However, some authors (Nishinari, Fang, & Rosenthal, 2019; Peleg, 2019) discussed the limita- tions of the instrumental texture profile analysis (TPA) and they highlighted an importance of consistency in test conditions (tempera- ture, humidity), size and shape of specimen, probe's geometries and the set deformation level that all significantly affect the TPA parame- ters' magnitudes.
In this research, firmness and consistency of yoghurts were strongly affected by goat milk heat treatment applied, while statistical difference for cohesiveness and an index of viscosity among samples were not found. Regarding firmness and consistency, yoghurt pro- duced by preheating milk at 85C/5 min had the highest values (Table 3).
Heat treatment of milk has an important impact on milk proteins, and enhances the texture of yoghurt. Presence of denatured whey proteins associated with casein micelles causes an increase in gel firm- ness, which has been well documented (Dannenberg
& Kessler, 1988a, 1988b; Lucey, Munro, & Singh, 1999; Lucey, van Vliet, et al., 1997). A relation between firmness and viscosity of yoghurt and the extent of denaturation of β-lactoglobulin during heat treatment has been reported (Dannenberg & Kessler, 1988a; Dannenberg &
Kessler, 1988b). However, our research suggests that acid gel firm- ness depends, beside the level of protein denaturation, on the micelle structure changes as well, that has been indicated by the mean micelle size measured.
Oliveira, Sodini, Remeuf, and Corrieu (2001) reported that increase of acid gel firmness depends on the total solids as well as on protein content and type. However, firmness of goat milk coagulum was about half that of cow's milk even with similar total solids con- tent, indicating that texture depends not only on the total solids con- tent, but also on the casein content as well as micelle structure between these two milk types (Vegarud et al., 1999).
In yoghurt manufacture cooling the gel after fermentation is com- plete, is considered to be an important factor in improving the texture of the final product (Robinson & Tamime, 1993), which is also indi- cated by the increased G0 values for all of our acid milk gels measured after cooling the samples.
Guggisberg et al. (2009) showed a good correlation between the sensory evaluated firmness and the yield stress values determined by vane method. Our study showed that there was a significant differ- ence between yield stress values as well as instrumentally measured firmness between samples. Also, it has been observed that in case of yoghurt prepared from milk treated at 85C for 5 min, the yield stress value correlates with the firmness results.
Textural properties of examined yoghurts were significantly lower compared to literature data obtained for cow milk yoghurts. Low casein content and other characteristics such as αs- casein proportions and micellular size are believed to be responsible for the weak texture of goat milk yoghurt (Park et al., 2007).
Choosing the right process parameters and the addition of various fortifying agents can improve textural quality of yoghurt, which has been thoroughly investigated on cow's milk. The addition of milk pro- tein isolates can significantly improve textural properties of goat's yoghurt, but obtained gel would still be less firm compared to cow milk yoghurt (Miocinovic et al., 2016). Herrero and Requena (2006) found that supplementation of goat's milk with whey protein concen- trate, followed by heat treatment, increased yoghurt firmness, adhe- siveness, fracturability and provided the product with similar values to that of yoghurt made from cow's milk. Firmness of acid cow milk gels was impacted by the pasteurization scale of milk, but the composition of milk proteins (casein/ whey protein ratio, heat-induced whey pro- tein aggregates) were highly significant as well (Nguyen et al., 2018).
TABLE 3 Textural properties of goat milk yoghurts affected by different heat treatments
Parameter Yoghurt 72C/30 s 85C/5 min 95C/5 min Firmness (N)
0.30 ± 0.04a 0.39 ± 0.01b 0.25 ± 0.06c Consistency (N s)
7.35 ± 0.89a 10.14 ± 0.29b 6.46 ± 1.32c Cohesiveness (N)
−0.13 ± 0.00a −0.20 ± 0.00b −0.13 ± 0.01a Index of viscosity (N s)
−0.18 ± 0.01a −0.24 ± 0.03b −0.21 ± 0.01a Note: Values in the table represent means of three replicated trials ± SD;
Values with different letter within the same row are significantly differ- ent (p < .05).
6 HOVJECKI ET AL.
4 | CONCLUSION The results showed that different heat treatments of goat milk prior acidification produce gels with significantly different rheological and textural properties of yoghurt. As unexpected, acid milk gels and yoghurts obtained from milk treated at 85C/5 min were character- ized by the best properties regarding rheological and textural attri- butes, compared with milder and more severe heat regime.
Also, casein micelle size of goat milk after this treatment was the largest compared to other two regimes, despite higher level of whey protein denaturation after 95C/5 min.
Poorer rheological and texture properties of gels obtained from highly heated goat milk (95C/5 min) may have been due to changes in the casein micelle structure and the release of the aggregates into the serum phase that alter the process of gel formation during acidifi- cation of heated milk, but to confirm this assumptions, further investi- gation is needed.
Our DLS measurements led to the conclusion that the micelles experience structure changes in the 85–95C interval, that cause worsening in the rheological and textural properties of acid gel, and that 85C/5 min treatment of goat milk would be the most appropri- ate in the yoghurt manufacture.
These findings would improve technological processes in the manufacturing of goat milk yoghurts as well as yield more energeti- cally efficient production procedures.
ACKNOWLEDGMENTS This study was supported with project No. 46009 financed by the
Ministry of Education, Science and Technological development of
Republic of Serbia.
AUTHOR CONTRIBUTIONS M.H. designed and performed experiment. M.H. wrote the manuscript and analyzed the data with support from Z.M. and J.M. V.R. and
M.H. conducted measurements and analyzed data about casein micelle size P.P. supervised the research. J.M. co-wrote the manu- script and conceived the original idea.
ETHICAL STATEMENTS Conflict of Interest: The authors declare that they do not have any conflict of interest.
Ethical Review: This study does not involve any human or animal testing.
ORCID Marina Hovjecki https://orcid.org/0000-0002-1594-7112
Zorana Miloradovic https://orcid.org/0000-0003-1703-4656
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S0958-6946(99)00090-4 How to cite this article: Hovjecki M, Miloradovic Z, Rac V,
Pudja P, Miocinovic J. Influence of heat treatment of goat milk on casein micelle size, rheological and textural properties of acid gels and set type yoghurts. J Texture Stud. 2020;1–8. https://doi.org/10.1111/jtxs.12524
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