Fermented milks, especially yogurt, are commonly associated with healthy foods, and as food vehicles to deliver probiotics and prebiotics to consumers (Costa et al. Reference Costa, Balthazar, Pinto, Cruz and Conte-Júnior2013). Goat milk yogurt has high digestibility and nutritional value, as well as claimed therapeutic and healthy dietary characteristics. However, goat milk yogurt presents a lower overall acceptance by the non-habitual consumer, when compared with cow milk yogurt (Costa et al. Reference Costa, Balthazar, Franco, Mársico, Cruz and Conte-Junior2014). This is due to its unpleasant ‘goaty’ taste and consistency, as it is perceived by consumers, even in goat milk yogurt with added strawberry pulp (Senaka Ranadheera et al. Reference Senaka Ranadheera, Evans, Adams and Baines2012). Regarding consistency, cupuassu pulp is considered an important technological strategy to improve the texture of goat milk yogurts (Costa et al. Reference Costa, Frasao, Silva, Freitas, Franco and Conte-Junior2015).
The intrinsic sensory characteristics of goat milk and derivatives, such as yogurt, are related to the presence of short-chain fatty acids such as caproic, caprylic, and capric acids (Ceballos et al. Reference Ceballos, Morales, de la Torre Adarve, Castro, Martínez and Sampelayo2009). An alternative to improve the taste of goat milk yogurts with fruit pulp is the use of skim milk in the preparation of these products (Costa et al. Reference Costa, Balthazar, Franco, Mársico, Cruz and Conte-Junior2014). However, the yogurt from skim goat milk can interfere in the physicochemical, apparent viscosity and texture. In this context, the aim of the research was to investigate the addition of inulin, maltodextrin, whey protein and skim milk powder on physicochemical, colour, apparent viscosity and texture parameters of low-fat cupuassu goat milk yogurts.
Materials and methods
Production of goat milk yogurts
Three batches of each cupuassu goat milk yogurt were prepared as described by Costa et al. (Reference Costa, Frasao, Silva, Freitas, Franco and Conte-Junior2015). In all treatments, thermophilic yogurt cultures (1% vol/vol; YF-L903®, Chr. Hansen, Valinhos, Brazil) and cupuassu pulp (10% w/vol; Polpa de Fruta®, Macapá, AP, Brazil) were added in UHT whole and skimmed goat milk (Caprilat®, Paraná, Brazil). The others ingredients were inulin (5% w/vol; Ingredients & Systems Biotechnology®, São Paulo, SP, Brazil), maltodextrin (5% w/vol; Max Titanium®, São Paulo, SP, Brazil), whey protein isolate (5% w/vol; Optimum Nutrition®, Meridian Lake, Aurora, USA) and skim milk powder (5% w/vol; Glória®, São Paulo, SP, Brazil). The whey protein was selected based on previous studies of this group (Almeida et al. Reference Almeida, Alvares, Costa and Conte-Junior2016). A total of 6 treatments of cupuassu goat milk yogurt were performed: whole (W); skimmed (S); skimmed with inulin (SI); skimmed with maltodextrin (SM); skimmed with whey protein (SW); skimmed with skim milk powder (SP). All ingredients were added before fermentation, and the samples were fermented in an oven at 43 °C. The fermentation was interrupted when the pH (AOAC, 2012) reached 4·5 ± 0·1. Finally, the product was refrigerated and stored at 4 ± 1 °C for 24 h. All analyses were performed on the 1st day of storage (D1). This experimental procedure was conducted in triplicate (n = 3). All analyses (of each replicate) were performed in triplicate.
Physicochemical analyses and instrumental colour
The cupuassu goat milk yogurts were analysed for pH by a digital potentiometer (model PG1800, Cap Lab, SP, Brazil), protein by the Kjeldahl method using a conversion factor of 6·38, fat content by the Gerber method and moisture by oven drying (AOAC, 2012). Syneresis was determined by weight difference of the supernatant and initial yogurt samples (10 g), after centrifugation at 1500 g for 10 min (Ramírez-Sucre & Vélez-Ruiz, Reference Ramírez-Sucre and Vélez-Ruiz2013).
Colour determinations were made at 5 °C by means of a Minolta CM-600D spectrophotometer (Minolta Camera Co., Osaka, Japan). The colorimeter was previously calibrated with illuminant D65 and a 2° standard observer (Costa et al. Reference Costa, Frasao, Silva, Freitas, Franco and Conte-Junior2015). Yogurt samples (50 ml) were stirred and placed in an aluminium cylinder (outside diameter 55 mm), with the surface optically flat before measuring, and the sensor was mounted directly on top of the cylinder to prevent ambient light noise.
Apparent viscosity and texture analyses
The apparent viscosities of the yogurts samples (300 ml) were measured at 5 °C using a Brookfield concentric cylinder viscometer (DV3T, Brookfield Engineering Laboratories Inc., Stoughton, MA, USA) equipped with rotor no. 63, mixing at 60 rpm. The apparent viscosity was measured in triplicate.
Firmness and consistency were measured using a texture analyser (TA-XT.Plus, Stable Micro Systems Ltd., Surrey, UK) equipped with a 5-kg.f load cell. The back extrusion cell plunger was 3·6 cm in diameter and set at 20 mm above the sample surface. The test cell penetrated with a distance of 2 cm into the sample (300 ml) at 5 °C. Firmness was defined as the maximum force (at the topmost point of the textural profile curve) and expressed in g. Consistency was defined as the area of the curve, calculated by the force value multiplied by the corresponding distance and expressed in g/s.
Statistical analysis
The data obtained for physicochemical, colour, texture and apparent viscosity were analysed by ANOVA and reported as means (±sd). All ANOVA were subjected to Tukey's test at P < 0·05 using XLSTAT version 2013.2.03 (Addinsoft, Paris, France). All of the experimental replication (n = 3) was done in triplicate.
Results and discussions
Proximate composition
The results of physicochemical proprieties are shown in Table 1. SW and SP had higher protein content (P < 0·05) than the others treatments (W, S, SI and SM), where milk-fat was substituted by non-protein milk solids. The W had a higher (P < 0·05) fat content than the others treatments (S, SI, SM, SW, and SP) (Table 1), which is expected, as it was elaborated from whole milk, whereas the reduced milk-fat yogurts were made from skimmed milk. Treatments with fat replacers (SI, SM, SW and SP) presented lower (P < 0·05) moisture than W and S yogurts.
Table 1. Physicochemical, apparent viscosity and texture analyses of the different cupuassu goat milk yogurts (W, S, SI, SM, SW, SP)
W, whole cupuassu goat milk yogurt; S, skimmed cupuassu goat milk yogurt; SI, skimmed with inulin cupuassu goat milk yogurt; SM, skimmed with maltodextrin cupuassu goat milk yogurt; SW, skimmed with whey protein cupuassu goat milk yogurt; SP, skimmed with milk powder cupuassu goat milk yogurt.
Values were expressed as mean ± sd.
a–eDifferent lower case letters in the same line represent significant differences (P < 0·05); n = 3.
L*, lightness; a*, redness; and b*, yellowness.
Syneresis and pH
Syneresis was affected (P < 0·05) by the addition of fat-substitutes, with exception of SM (Table 1). The syneresis is related to instability of the gel network resulting in the loss of the ability to entrap all the serum phase. Without the addition of whey protein and skimmed milk powder the syneresis in goat milk yogurts (W, S, SI, and SM) were higher than 5·65/100 g. Therefore, the higher water holding capacity observed for the SW and SP yogurts can be explained due to the greater protein content (Table 1).
The decrease of goat milk pH (6·58 ± 0·07), in all treatments, is in line with the growth of the yogurt bacteria. These microorganisms fermented the lactose with the final production of organic acid, mainly lactic acid, which reduced the pH during fermentation. Therefore, the pH decreases can indicate an increase in the number and/or metabolic activity of acid-producing starter culture (Costa & Conte-Junior, Reference Costa and Conte-Junior2015; Costa et al. Reference Costa, Frasao, Costa Lima, Rodrigues and Conte-Junior2016). In addition, the pH was also affected (P < 0·05) by the addition of fat substitutes. Although all yogurts were cooled at pH 4·6, the W and S goat milk yogurts presented lower values 4·46 and 4·45, respectively, than the yogurts containing inulin, maltodextrin, whey protein and skimmed milk powder (Table 1). This pH difference between the treatments may be related to the pH of each fat replacer. For example, Franck (Reference Franck2002) reported that inulin pH varies between 5 and 7, which depends on the average degree of polymerisation.
Instrumental colour
The L* is defined as lightness, in which 100 represents white, whereas zero represents the black. According to Costa et al. (Reference Costa, Frasao, Silva, Freitas, Franco and Conte-Junior2015) the goat milk yogurt with cupuassu pulp exhibits a lower lightness than a natural goat milk yogurt, due to this fruit pulp colour. In addition, in this study, the L* values were changed (P < 0·05) by skimmed milk (S, SI, SM, SW, and SP). Moreover, the addition of inulin, maltodextrin and whey protein decreased (P < 0·05) this parameter.
In relation to a* value (greenness-redness), the greenness colour of cupuassu goat milk yogurts is explained by the presence of natural pigments, originating from cupuassu pulp (Costa et al. Reference Costa, Frasao, Silva, Freitas, Franco and Conte-Junior2015). Furthermore, the W yogurt was higher (P < 0·05) than the treatments with skimmed milk (S, SI, SM, SW, and SP). The b* values (blueness-yellowness) was different between all treatments, and the S treatment was less yellow than the other treatments (W, SI, SM, SW and SP). The yellowness of treatments can be attributed to the addition of cupuassu pulp, depends on the type and level of fruit or fiber (Costa et al. Reference Costa, Frasao, Silva, Freitas, Franco and Conte-Junior2015).
Apparent viscosity
For apparent viscosity, the SP yogurt was higher than W, S, SI, SM and SW yogurts (P < 0·05). However, the addition of inulin, maltodextrin and whey protein also increased (P < 0·05) the apparent viscosity (Table 1). These behaviours can be explained by the aggregation of casein micelles and gel formation that is a consequence of biochemical and physicochemical changes during fermentation of milk (Karam et al. Reference Karam, Gaiani, Hosri, Burgain and Scher2013). However, further studies should be conducted to assess the effect of these fat substitutes on rheological properties of cupuassu goat milk yogurts.
Texture analyses
Regarding texture analyses (firmness and consistency), only cupuassu goat milk yogurt added with skimmed milk powder (SP) differed (P < 0·05) from other treatments (W, S, SI, SM and SW). The gel structure is the main responsible for texture properties, which results for casein aggregation. In addition, other parameters, such as milk base composition and total solids, also perform a determinative role in gel structure formation (Damin et al. Reference Damin, Alcântara, Nunes and Oliveira2009).
Conclusions
Skim milk powder has potential as a fat substitute, which could be a technological strategy for the dairy industry to produce low-fat goat milk yogurt manufactured with fruit pulp. In addition, inulin, maltodextrin, and whey protein can also be used to increase the viscosity of goat milk yogurts.
The authors wish to thank the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (grant no. E-26/201·185/2014 and E-26/010·001·911/2015, FAPERJ, Brazil), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (process no. 125, CAPES/Embrapa 2014, CAPES, Brazil) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (process no. 311361/2013-7, 400136/2014-7 and 166186/2015-5 CNPq, Brazil) for their financial support.
