Co-fermentation process strongly affect the nutritional, texture, syneresis, fatty acids and aromatic compounds of dromedary UF-yogurt

Abstract

This work intended to compare dromedary yogurt’s characteristics obtained by a co-fermentation process with plant (carob powder) or autochthonous bacteria (Enterococcus faecium and Streptococcus macedonicus). For this reason, the ultrafiltration process (UF) is applied to increase the rate of total solids in dromedary milk within the margin needed to prepare a yogurt. Carob powder or autochthonous bacteria were incorporated at the level of 2% in UF milk. Then mixtures were fermented with the strains Lactobacillus bulgaricus and Streptococcus thermophiles, and the obtained products are named CFC (yogurt with carob), CFS (yogurt with autochthonous strains) and control (yogurt with only L. bulgaricus and S. thermophilus) respectively. All along of 3 weeks at cold, CFC and CFS maintained Streptococcus at appropriate levels (>8 log CFU/g). Moreover, CFC showed the lowest syneresis, highest cohesiveness and springiness values, and oleic acid (C18:1n9; 26.315%). However, CFS yogurt resulted in higher volatile compound formation than CFC and control, where isobornyl propionate was the major one.

Introduction

Dromedary milk occupies an increasingly important place in the human diet thanks to its therapeutic and nutritional virtues. In fact, this milk possesses polyunsaturated fatty acids, a considerable level of iron and vitamin C, and several bio-activities including anti-diabetic, antioxidant, anti-hypertensive, anti-microbial effects (Al haj and Al Kanhal 2010; Ayoub et al. 2018). Furthermore, dromedary milk is devoid of β-lactoglobulin and has a limited amount of α_S2- casein, which make it a desirable food for allergy sufferers (Mati et al. 2017). Therefore, dromedary milk products become an alternative to cow milk (El-Hatmi et al. 2018). However, the technical difficulty of dromedary milk to process in-derived products is commonly claimed. Hence, the attention of some research, in the last few years, is paid to improving the weak gel structure of dromedary yogurt (Chen et al. 2019; Jrad et al. 2019).

Away proposed for dromedary yogurt processing is to concentrate the milk by ultrafiltration (UF) (Jrad et al. 2019). This process increased the total solids content of milk to 12–14 g/100 g, which is the required content to obtain a yogurt with adequate viscosity and texture. In previous work, the capacity of UF to produce a concentrate with increased nutritional and functional properties than those generated by chemical and thermal treatments was demonstrated (Ng et al. 2018).

Several types of research, over the past ten years, have geared on producing varieties of yogurts with additional functional properties and thus in dealing with the consciousness of the consumers of the health advantages of such products (Fazilah et al. 2018). Dromedary milk includes attractive properties, which are benefits to human health and many functional foods can be produced from this milk. What is more, the aptitude of dromedary milk to provide useful products may be well deployed by the co-fermentation process of yogurt starter with probiotic strains or plant material. Such co-fermentation can liberate active metabolites or improve the functionality of bioactive compounds. Indeed, plant and lactic acid bacteria, especially probiotics, confer a health benefit to the consumer (Granato et al. 2017). Dromedary milk can be an excellent vehicle for probiotic bacteria, which are identified as Enterococcus faecium strains (Ayyash et al. 2018). In our previous work, two strains -with technological and antioxidant abilities- have been isolated from dromedary milk fermented spontaneously and identified as E. faecium and S. macedonicus. These strains are judged as promoting the formulation of functional products (El Hatmi et al. 2018). To date, many attempts have been made to use different bovine cultures co-fermented with a conventional starter to produce dromedary yogurt (Shori and Baba 2014; Varga et al. 2014), but no study was interested in textural and functional qualities of dromedary yogurt using autochthonous culture.

On the other hand, it has been reported that so many vegetables and fruits are evaluated as prebiotics and may be incorporated in dairy products as functional substances (Granato et al. 2017). Carob (Ceratonia siliqua L.) has traditionally been used in human food. Recently, there has been an interest in using carob as a raw material in the generation of food supplement and functional foods regarding its strong bioactive compounds (Rico et al. 2019).

Thereby, the purpose of the current work was to develop yogurt from ultrafiltrated dromedary milk and to compare the effect of the co-fermentation process with plant (carob powder) or autochthonous bacteria (E. faecium and S. macedonicus) on lactic acid bacteria viability, physical–chemical, textural, aroma, fatty acids and antioxidant properties of yogurts.

Materials and methods

Materials and chemicals

Dromedary milk was obtained from the station of Chenchou in Hamma-Gabes. Carob was gathered from the market, dried and the powder was obtained after squashing in a crusher (Type D56, Moulinex, Seb Group, France).

The preparation of yogurt starter culture (DVS, Chr. Hansen, Horsholm, Denmark) was compiled in keeping with the instructions of the manufacturer. The chemical products used in the analysis were acquired from Sigma-Aldrich.

Ultrafiltration bioprocess

Raw dromedary milk was skimmed and ultrafiltrated with a pilot UF (MP350B, DeltaLab, Carcassone, France) contained a module in ceramic (Pall Membralox Inc., 1P19-40 GL; 0.24 m² of surface; 50 nm of cut-off). Before starting the UF of milk, the water flux was determined. The transmembrane pressure and the factor water permeability were 0.25 bar and 150 L/h.m².bar, respectively. Then the skimmed camel milk was concentrated two and a half times (VCF = 2.5). VCF is defined as the volume concentration factor and expressed as follows:

$\text{VCF}=\frac{\mathit{Vi}}{Vi-Vp}$, with Vi is the initial volume of the milk and Vp is the permeate volume.

Finally, the cleaning of membrane was carried as is explained by the manufacturer with NaOCl (0.5%) and NaOH (1%) at temperature of 50 °C for 15 min and followed by an acidic treatment with HNO3 (0.5–1%, T = 60 °C, operating time = 15 min), until the original water flux was restored.

Determination of protein retention (Rt%) was calculated in that way:

$$\text{RT}\left(\%\right)=1-\frac{Yp/\left(Wp-Yp\right)}{Yr/\left(Wr-Yr\right)}\times 100.$$

where Y = the proportion of protein in percent found in retentate (r) or permeate (p).

Wp = percentage of water permeate.W

r = percentage of water retentate.

Autochthonous culture preparation

Streptococcus macedonicus and Enterococcus faecium are two strains isolated and identified in our previous work (El Hatmi et al. 2018). Identified strains were stored at − 20 °C in sterilized dromedary skim milk. Then, the inoculation of an overnight culture (100 μL) into sterile MRS broth (10 mL) and their incubation at 37 °C for 24 h, have been realized. Biomass, obtained by centrifugation of cultures (5000 × g, 15 min, 4 °C), were rinsed doubly with sterile water and inoculated in sterile skim milk (10 mL) before incubation for 24 h at 37 °C until obtaining inoculums of about 10⁷ CFU/mL.

Yogurt production

Three types of yogurt are prepared: control, yogurt co-fermented with carob powder (CFC) or autochthonous strains (CFS). Dromedary cream was added to ultrafiltrated milk, the mix was heated (85 °C; 15 min) and cooled to 42 °C. After that, 2% (w/v) of carob powder or autochthonous starter culture was incorporated into the mix. At a later stage, mixtures were incubated at 42 °C after adding 0.1% (w/v) of the yogurt starter (L. bulgaricus and S. thermophiles). The control contained only L. bulgaricus and S. thermophilus strains.

When the pH achieved the value of 4.5, the fermentation was stopped by refrigeration the yogurt samples, which are then stored for 28 days at 4 °C.

Physical–chemical analysis

Analysis of titrable acidity and total solids was established by AFNOR (1993) method. The pH and viscosity values of yogurts were determined at 20 °C by the assistance of respectively a pH-meter (Jenway, Staffordshire, United Kingdom) and a viscometer (Brookfield, model DV-E, MA, USA).

Konica Minolta Chroma Meter CR-400/410 (Minolta Co., Ltd., Japan) was used to measure the color parameters: lightness (L*, 100 = white, 0 = black), redness (a*, + red, − green), and yellowness (b*, + yellow, − blue). At first, the instrument was calibrated using standard white plates with illuminant D65. The yogurt samples (100 g) were taken directly from the refrigerator at 4 °C and placed in an aluminum cylinder (outside diameter 55 mm), with the surface optically flat before measuring. The sensor was mounted directly on top of the cylinder to prevent ambient light and an average value was determined by taking measurements on five points of each yogurt sample.

Syneresis of different yogurts were assessed by centrifugation of 50 g of yogurt at 1000 × g for 15 min and expressed as the mass of the whey related to the total weight of the gel, multiplied by 100.

Microbiological counts

S. thermophilus and L. bulgaricus counts were quantified as described by Jrad et al (2019) and the results were calculated as log colony-forming units per gram yogurt.

Texture Analysis

The texture analyzer (Texture Analyser, TA Plus, LLOYD instruments, England) was used to perform the textural parameters of different yogurts. Samples were placed in a cylindrical cell, (4 cm high × 4 cm diameter), and tested using a 12-mm back extrusion probe. Then, the samples were compressed with a rate of 40 mm/min to 50% of their original volume in a duplicate cycle. The texture profile parameters, namely firmness (N), springiness (mm), and cohesiveness were computed from the resulting force–deformation curves. Three different cups were measured for each sample. All measurements were tested at ambient temperature with product temperature near 4 °C. The samples (100 g) were stored at least for 24 h at 4 °C.

DPPH radical scavenging activity

The radical scavenging activity of different yogurt was determined using the method described by Jrad et al. (2019) and expressed as a rate of DPPH· inhibition activity in percent.

Fatty acids analysis

Total lipids from yogurt were subjected to methylation using a KOH (2 N) solution prepared in methanol, mixed thoroughly with hexane to extract the fatty acid methyl esters (FAME) which are analyzed by using gas chromatography QP2010 Shimadzu (Tokyo, Japan) coupled to mass spectrometry. A fused silica capillary column Supelcowax-TM10 (30 m length × 0.25 mm i.d. and 0.25 μm film thickness) (Bellefonte, PA, USA) was used. The injection port temperature was 250 °C, and the split ratio was 10.0. The carrier gas used was helium (99.99% purity), the flow rate was of 1.20 ml/min.

Fatty acid composition are identified and estimated by GC MS solution program and Wiley 275 mass spectra libraries (software, D.03.00, PA, California, USA).

Aromatic Profile

Aromatic compounds were analyzed based on the protocol described by El-Hatmi et al. (2018) by using a gas chromatography QP2010 Shimadzu using an RTX-5MS capillary column (30 m × 0.25 mm i.d. and 0.25 μm film thickness) (Bellefonte, PA, USA).

Principal components analysis analyses were also performed for aromatic compounds, the data were set 64 × 3 matrix, whereby 64 rows represented the aromatic components and 3 columns depicted the samples of yogurt. Hierarchical cluster analysis, considering Euclidean distances (dissimilarity), was performed to identify groups of samples with different volatile compounds.

Statistical analysis

A one way analysis of variance (ANOVA) was used followed by Tukey-test was employed to highlight significant differences among the yogurt samples. All statistical analyses were carried out with XLSTAT (version 2015.5; Addinsoft, Paris, France).

Results and discussion

Concentration of dromedary milk by UF

The optimization of the ultrafiltration of skim dromedary milk needs a comprehension of the mechanisms of flux permeate and volume concentration factor with regard to the physicochemical properties of milk after UF. Very few studies have been investigating dromedary milk UF (Mehaia 1996; Jrad et al. 2019). When skim milk is ultrafiltrated, two phases are obtained: the retentate in which proteins and colloidal minerals are concentrated, and the permeate containing lactose and soluble minerals. In fact, for discussing how to permeate flux decline, some knowledge of dromedary milk chemistry and its properties, which changes after UF, is necessary. Skim dromedary milk consisted of 8.49 g/100 g of dry matter, 2.68 g/100 g of proteins and 0.85 g/100 g of ash content. The processing of skim dromedary milk by UF at different VCR (1; 1.5; 2 and 2.5), allowed to obtain a retentate with a total solids level between 11.33 and 12.50 g/100 g, a crude protein content ranged from 5.56 to almost 6.98 g/100 g and an ash levels of 1.56–1.26 g/100 g. This study showed that 98.74% of proteins are retained in the different concentrates, this means that 1.26% of proteins passed through the membrane. Mehaia (1996) reported higher protein retention (100%) in the UF-dromedary milk, this could be explained by the nature of used membrane during UF (Mehaia 1996, used fibre UF module with a polysulphone membrane, but in this study, we used ceramic membrane). Ng et al. (2017) reported that in the ceramic membrane (such is the case in this study), there are a preferential fouling by whey proteins and a low quantity of casein can be bound to the membrane via salt-bridge. Hence, the interest of UF applications in which high-protein retention is necessary. The UF was stopped then at a VCF = 2.5 with a view to concentrating dromedary milk on a suitable level for the yogurt production.

Changes in yogurt characteristics during refrigeration storage

Physical–chemical composition and counts of viable microorganisms

The physical–chemical characteristics and count of viable lactic acid bacteria of different yogurts were studied during 28 days of cold storage and the results were presented in Table 1.

Table 1.

Effect of dromedary yogurt co-fermentation with carob powder or autochthonous strains on pH, titratable acidity (g lactic acid/100 g), total solids (g/L), proteins (g/L) apparent viscosity (cP) and viability of Streptococci and Lactobacilli (CFU/g) and during 28 days of storage at 4 °C

Type of yogurt	Period of storage (days)
	1	7	14	21	28
pH
control	4.600 ± 0.002aA	4.39 ± 0.01aB	4.37 ± 0.04aB	4.160 ± 0.002aC	4.042 ± 0.005aC
CFC	4.723 ± 0.626abA	4.600 ± 0.167abA	4.466 ± 0.011bA	4.195 ± 0.005bB	4.091 ± 0.008bB
CFS	4.580 ± 0.009bA	4.389 ± 0.005bB	4.227 ± 0.021cC	4.110 ± 0.05abCD	3.990 ± 0.007cD
Titrable acidity
control	0.76 ± 0.08aA	0.98 ± 0.25aABC	1.03 ± 0.02aB	1.14 ± 0.05aC	1.25 ± 0.025aC
CFC	0.81 ± 0.07bA	1.134 ± 0.06aAB	1.271 ± 0.05bABC	1.355 ± 0.01bBC	1.388 ± 0.04bC
CFS	1.125 ± 0.100cA	1.148 ± 0.152aA	1.392 ± 0.230bB	1.434 ± 0.115cC	1.551 ± 0.040bC
Total Solids
control	177.85 ± 0.74aA	175.14 ± 8.18aA	173.10 ± 5.04aA	168.71 ± 2.84aB	161.12 ± 1.70aAB
CFC	243.9 ± 1.68bA	240.42 ± 0.74bAB	230.65 ± 0.79bABC	218.16 ± 4.09bBC	202.01 ± 5.60bC
CFS	183.93 ± 28.49cA	183.48 ± 10.62cA	181.97 ± 4.96abAB	180.54 ± 3.03cB	173.88 ± 1.39cB
Proteins
control	73.8 ± 2.3 aA	73.82 ± 2.22 aA	73.8 ± 2.2 aA	73.6 ± 2.3 aA	73.6 ± 2.0 aA
CFC	74.1 ± 2.1 aA	74.2 ± 2.1 aA	74.1 ± 2.3 aA	74.0 ± 2.1 aA	74.0 ± 1.8 aA
CFS	73.7 ± 2.5 aA	73.8 ± 2.2 aA	73.8 ± 2.4 aA	73.7 ± 2.0 aA	73.6 ± 2.1 aA
Apparent viscosity
control	252.66 ± 52.20aA	242 ± 13.89aB	200 ± 2.58aC	179 ± 2aD	133 ± 1aE
CFC	462.66 ± 2.51bA	458 ± 1.00bAB	455.50 ± 0.100bABC	453.00 ± 3.00bABC	450.00 ± 1.00bC
CFS	275.33 ± 36.93cA	264.00 ± 33.15cAB	258.33 ± 21.93cB	198.33 ± 0.57cC	190.66 ± 1.52cD
Viability of Lactobacillus
control	7.64 ± 0.1aA	7.68 ± 0.25aA	7.57 ± 0.33aA	7.55 ± 0.52aA	7.16 ± 0.4aB
CFC	7.75 ± 0.3abA	7.76 ± 0.2bA	7.68 ± 0.24bA	7.66 ± 0.13bA	7.38 ± 0.29bA
CFS	7.78 ± 0.11bA	7.79 ± 0.12bA	7.70 ± 0.3bA	7.66 ± 0.26bA	7.40 ± 0.55bA
Viability of Stroptococcus
control	8.63 ± 0.31aAB	8.50 ± 0.51aA	8.40 ± 0.10aB	8.21 ± 0.15aB	7.94 ± 0.10aC
CFC	8.65 ± 0.21aA	8.61 ± 0.35bA	8.55 ± 0.40bAB	8.42 ± 0.25bB	8.28 ± 0.45bB
CFS	8.66 ± 0.13aA	8.68 ± 0.12cA	8.65 ± 0.21cA	8.63 ± 0.11cA	8.60 ± 0.15cA

Means with different small letters in the same column are significantly different (p < 0.05).

Means with different uppercase letters in the same line are significantly different (p < 0.05).

CFC, yogurt co-fermented with carob powder;

CFS, yogurt co-fermented with autochthonous strains of dromedary milk (S. macedonicus and E. faecium)

The pH values of dromedary yogurts reduced substantially during the storage time. CFS and control samples demonstrated a significant difference in the decrease of pH value till the 7th day, while CFC samples until day 21 having a longer time to pH reducing. Similar results were reported for dromedary Greek yogurt fortified with date powder (Jrad et al. 2019). The stability in pH values is owing to the buffer action caused by carob proteins. In fact, the buffering capacity of the carob is known from the literature (Al-Dabbas et al. 2010). In the last week of the storage time, values of pH decreased depending strongly with the co-fermentation process, in which CFC showed the highest pH value of 4.091 ± 0.008 as a result of the increase of solids caused by carob powder supplementation. These findings accord with preceding studies demonstrated an increase in pH values of yogurts due to the increase of solids after Moringa seed extract addition (Kiros et al. 2016). However, CFS had the lowest pH value (3.990 ± 0.007) after 3 weeks of cold storage due to an augment in the count of lactic acid bacteria in those samples (Table 1), and as a consequence, more lactic acid was produced. These results match with prior research, where the pH drop to 3.98 after 28 days of storage in case of yogurt fortified with 3% of honey (Mercan and Akin 2017) and some probiotic fermented milk (Medina and Jordano 1995).

During storage, titrable acidity showed a tendency to increase in all samples of yogurts. Samples co-fermented with autochthonous strains (CFS) showed the highest lactic acid amounts during storage followed by the CFC samples and presented a significant difference (p < 0.05) relative to the control in various assessed time. This is due to the potent acidifying power of Streptococcus macedonicus and Enterococcus faecium strains (El-Hatmi et al. 2018). Throughout the cold storage period, titrable acidity values of different yogurts varied from 0.76 and 1.551 g lactic acid/100 g of sample, which is in accordance with legal standard. In fact, according to the Codex Alimentarius Commission (2010), titrable acidity of the yogurt must range from 0.6–1.5%. However, many factors affect the changes of titrable acidity of yogurts like storage temperature strongly, the initial acidity value of yogurt and the acidifying capacity of culture.

In terms of total solids, CFC showed significantly higher values than those of control and CFS yogurts. Total solid contents did not demonstrate notable changes (p < 0.05) all along 14 days of preservation time for all yogurt samples.

All yogurt samples have higher protein contents (73.6–74.2 g/l) than regular yogurt (non-concentrated yogurt), which is in line with most data on high-protein yogurts made from cow milk (Costa et al. 2019; Tamime et al. 2014). The great amounts of proteins induce a higher cross-link density of the gel network, which cause a greater firmness of yogurt (Hinrichs and Keim, 2007). The co-fermentation process had no influence (p > 0.05) on protein content. In the same way, storage time has not a significant effect (p > 0.05) on protein content for all yogurt samples. Similar behavior was observed by another researcher in regular dromedary yogurt during cold storage (Abou-Soliman et al. 2017).

During the whole storage time, there were different trends regarding the changes in viscosity values for all yogurt samples (Table 1). There was a significant (p < 0.05) decrease in the apparent viscosity of control and CFS yogurt during storage, whereas the viscosity of CFC yogurt remained stable during 21 days of cold storage. Similar to our results, the supplementation of yogurt with plant substances or fruit usually improves the viscosity during storage (Costa et al. 2015; Zhang et al. 2019). Additionally, carob powder is considered a good source of soluble fiber, consisting essentially of galactomannans, which acts as a hydrocolloid and possesses a very good gelling property, which contributes to a good viscosity of the product (Barak and Mudgil 2014).

Changes in viable counts of control and yogurts obtained by co-fermentation through the refrigeration storage time are depicted in Table 1. As observed, the viability of Lactobacilli showed constant trends during 21 days of storage in different samples. Lactobacilli counts exhibited a rise, but it was not statistically significant (p > 0.05) between the 1st and 7th day of storage in different yogurts. Prolonged storage for 28 days resulted in a reduction in viability of Lactobacillus for all yogurts and similar counts were observed in CFS and CFC samples. These findings are an agreement with the results of Paseephol and Sherkat (2009), who indicated a decline-of-Lactobacillus viability during stocking of probiotic yogurt enriched by Jerusalem artichoke inulins.

It is important to note that the general result observed in CFS samples on which the viable counts of Streptococci showed no significant decrease during the total storage time. Comparable trends were observed by Varga et al. (2014), who reported that the viable counts of Stroptococci showed no significant decrease in probiotic fermented dromedary milk all along with the storage duration. In contrast, Streptococci counts in CFC decreased significantly after 21 days at 4 °C. Dromedary yogurt produced by co-fermentation enhanced bacterial viability (p < 0.05) when compared with control on the 7th day of the cold storage period. This could be explained by the prebiotic effect of carob powder in one hand and the capacity of S. thermophilus to survive with probiotic strains, on the other hand. The same trend was observed in probiotic yogurt enriched with banana puree (Sert et al. 2017). The overall loss of Stroptococci viability throughout the 28 days of refrigerated storage was more pronounced in control.

DPPH radical scavenging activity

The results of the DPPH· test for the first and last storage days of control, CFS and CFC samples are presented in Fig. 1a. Both CFS and control exhibited considerable DPPH· inhibition (59.226–57.52%) and (56.89–53.51%) for both times analyzed, respectively. This finding was in agreement with that of El-Hatmi et al. (2018), who reported that the DPPH· inhibition of dromedary milk enhanced after being fermented by the two autochthonous strains S. macedonicus and E. faecium. On the other hand, the CFC sample exhibited significantly higher antioxidant activity than control and CFS during the storage period. The capacity of carob powder to scavenge free radicals was previously attributed to phenolic compounds (Rico et al. 2019).

Fig. 1.

Changes in DPPH·-radical scavenging activity a and water exudation b of different dromedary yogurts during storage at 4 °C

The antioxidant activity of dromedary yogurts decreased with the duration of storage. This issue is possibly related to the decrease in viable counts at the last storage day, which ultimately results in a decrease of proteolytic activity and then of antioxidant peptide generation.

Syneresis

Syneresis of different yogurts was quantified at the beginning and at the end of cold storage time and the results are presented in Fig. 1b. As it can be viewed in this figure, the low syneresis values were shown by CFC, followed by CFS and then control. Previous studies on dromedary yogurt fortified with date powder and xanthan from orange waste, showed a similar pattern to what appears in this finding (Jrad et al. 2019; Mohsin et al. 2019). During the storage time, syneresis tended to increase for control and CFS yogurts, while CFC presented a stable whey exudation. This may be explained by the presence of soluble fibers and polyphenols in the carob. Barak and Mudgil (2014) reported that the specific structure and the huge number of hydroxyl groups of the carob soluble fibers allows more hydrogen linking with water than starch and gluten. Fiber is considered as a physical stabilizer which plays a crucial function as a bio-thickening agent by improving water retention and limiting syneresis. In addition, it has been reported that polyphenol compounds present in plants possess a considerable affinity to the casein of milk by establishing a stable complex with the strong internal liaison. As a result, the rearrangement of proteins is reduced through preservation time, which offer more equilibrium to casein networks and thus contribute to the lower occurrence of syneresis (Vital et al. 2015). Some studies informed that the incorporation of fruits/vegetables in yogurt, reduce the syneresis (Zhang et al. 2019; Vital et al. 2015).

Effects of co-fermentation on dromedary yogurt characteristics

Color measurement

The color parameters of different yogurt samples were presented in Fig. 2a, according to the CIE color scale. The carob powder strongly influenced the yogurt color properties and caused a reduction in the lightness (L, 102) and intensify in red (a, − 1.67) and yellow (b, 13.26). In contrast, co-fermentation of dromedary yogurt with autochthonous strains had no changes in color parameters: Similar L, a and b values were shown by CFS and control.

Fig. 2.

Effect of co-fermentation on color a and textural parameters b of dromedary yogurts

Texture profile analysis

Figure 2b presents the results of firmness, cohesiveness and springiness of different dromedary yogurts.

Firmness is deemed as a substantial variable for yogurt texture. In this study, the highest firmness value was assigned to CFS (0.26 N). This may be due to a higher protein rearrangement in CFS. Enhancement of CFS firmness compared to control may be associated with the texturing ability of one of the used strains or both. This result is in line with the work of El-Hatmi et al. (2018), who fermented dromedary and cow milk with S. macedonicus and E. faecium strains, and observed significant enhancements in the consistency of products.

Present results showed that carob powder affected the firmness and a lower value (0.23 N) was observed in CFC samples. Therefore, carob powder increased the softness of the gel, which was due to augmented water in the coagulum due to lowered syneresis. These results corroborate with previous studies that observed a lower firmness of yogurts made with different fruit powder (Perina et al. 2015).

The highest values in cohesiveness and springiness were observed in CFC samples. The increase of springiness in CFC could be owed to the high quantity of water in the gel, which make it softer and with a strong network. As the addition of carob powder contributed to a greater strength of internal bonds, cohesiveness values were then higher in CFC than the other yogurt samples. A similar trend was noted in yogurt added with Pleurotus ostreatus aqueous extract (Vital et al. 2015).

Fatty acids composition

The average relative fatty acid compositions of control and yogurts developed by co-fermentation are summarized in Table 2. Saturated fatty acids (SFA) corresponded at least to 50% of the total fatty acid composition. SFA were at a higher level in control samples due to the increased amount of palmitic (C16:0) and myristic (C14:0) acids than those obtained by the co-fermentation technique. On the contrary, monounsaturated fatty acids (MUFA), constituted by oleic (C18:1 cis 9) and palmitoleic (C16:1 cis 9) acids were higher in CFS and CFC yogurts. Finally, bioactive fatty acids like butyric acid, MUFA for instance oleic acid and polyunsaturated fatty acid rate, what contained linoleic acid (C18:2; cis 9, 12), α-linolenic acid (C18:3, cis 9,12,15) and docosahexaenoic acid (C22:6; cis 4,7,10,13,16,19), were higher in the co-fermentation process especially CFS.

Table 2.

Fatty acid composition (%) of dromedary yogurt co-fermented with carob powder (CFC) and autochthonous strains (CFS)

Fatty acids	Type of yogurt
Individual	Control	CFC	CFS
Butyric acid, C4:0	1.29	0.15	2.80
Caproic acid, C6:0	0.72	0.21	1.53
Caprylic acid, C8:0	0.61	0.17	1.04
Capric acid, C10:0	1.06	0.26	1.46
Decenoic Acid, C10:1	0.13	0.04	0.21
Undecanoic Acid, C11:0	0.03	0.03	0.06
Lauric Acid, C12:0	2.25	1.27	2.12
Tridecanoic Acid, C13:0	0.11	0.15	0.10
Myristic Acid, C14:0	11.07	9.84	9.27
Myristoleic Acid, C14:1	0.87	0.98	0.86
Pentadecanoic Acid, C15:0	2.09	3.03	1.62
Cis-10-Pentadecanoic Acid, C15:1	0.01	0.08	0.48
Palmitic Acid, C16:0	30	23.11	23.54
Palmitoleic Acid, C16:1 cis9	6.9	10.48	6.44
Heptadecanoic Acid, C17:0	1.01	1.73	0.75
Cis-10-Heptadecanoic Acid, C17:1	0.71	1.44	0.72
Stearic Acid, C18:0	12.22	14.07	10.02
Oleic Acid, C18:1cis9	23.99	26.52	23.41
Linoleic Acid, C18: 2 cis 9,12	2.87	2.95	3.81
γ-Linolenic Acid, C18:3n6	0.21	0.53	0.53
Linolenic Acid, C18:3n3	0.43	0.69	0.98
Arachidic Acid, C20:0	0.28	0.64	n.d
Cis-11-Eicosenoic Acid, C20:1	0.19	0.31	n.d
Eicosadienoic, C20:2	0.19	0.26	n.d
Heneicosanoic Acid, C21:0	n.d	0.09	2.56
Arachidonic Acid, C20:4n6	n.d	0.24	n.d
Behenic Acid, C22:0	0.31	0.23	0.98
Docosadienoic, C22: 2n6	n.d	0.2	n.d
Docosahexaenoic C22:6n3	0.47	0.31	4.70
Sums of fatty acids
SFA	59.34	54.16	50.96
PUSFA	32.8	39.85	32.12
MUSFA	3.98	4.92	10.02
USFA	36.78	44.77	42.14
∑ n6	3.08	3.92	4.34
∑ n3	0.68	0.84	5.23
Index of atherogenicity	1.88	1.28	1.21

These findings signaled that the strains could significantly modify the composition of fatty acid of fermented dairy products. As observed, CFS could be considered as a functional food because of its elevated content in n-3 fatty acids (Linoleic acid and docosahexaenoic acid). Indeed, Aluko (2012) reported that the higher ratio of n-3 fatty acids in the diet is more desirable as a mean of improving human health by reducing damaging inflammatory conditions that are responsible for chronic diseases such as cancer, kidney malfunction, diabetes and cardiovascular disorder. This was confirmed by the index of the atherogenicity of the CFS, which is the lowest compared to other yogurts.

Aroma compounds

A total of 64 volatiles compounds was detected in dromedary yogurts, namely 10 carbonyl compounds, 18 alcohols, 3 organic acids, 1 heterocyclic carbon, 18 esters, 1 nitrogen compounds, 11 hydrocarbons, 1 terpene and 1 phenol. Forty-eight from the totality of identified volatiles were aroma-active compounds (Table 3). Alcohols and/or esters are the two chemical families with high levels in different dromedary yogurt samples. The same result was found in fermented milk by Ning et al. (2011). The co-fermentation with carob powder increased the content of esters, acids and phenol levels, whereas the co-fermentation with autochthonous bacteria improved carbonyl compounds and alcohol contents.

Table 3.

Aroma compounds (%) of dromedary yogurt co-fermented with carob powder (CFC) and autochthonous strains (CFS)

Aroma compounds	Type of yogurt
	Control	CFC	CFS	Odour Description*
Carbonyl compounds
2-propanone	n.d	0.2	1.04	Sweet, fruity (Cheng, 2010)
Diacetyl	1.82	0.98	1.83	Butter, creamy, vanilla (Cheng, 2010)
2-heptanone	0.36	0.13	0.84	Butter
Acetoin	2.3	3.38	8.32	Yogurt odour and a fatty creamy « tub» butter taste
2-nonanone	n.d	0.04	0.3	Fruity, musty (Cheng, 2010)
Benzaldehyde	0.17	n.d	0.15	Butter, Burnt sugar
1,3 Butandiol	0.28	n.d	n.d	Fruit
Acetone	0.89	n.d	n.d	Pungent
2-butanone	0.17	n.d	n.d	Sweet, fruity Varnish-like,
(Z)-3-hexenal	n.d	0.16	n.d	Fatty, green, fruit, apple
Alcools
Ethanol	22.31	2.08	17.02	Mild, ether (Cheng, 2010)
1-butanol	0.82	0.33	1.52	A dry, burning taste
3-methyl, 1 butanol	5.5	0.74	9.16	A fusel oil, pungent odour and repulsive taste
2-methyl, 2 butanol	n.d	0.06	n.d	A floral note (Pino and Marbot, 2001)
1-hexanol	0.39	0.12	1.2	An herbaceous, woody, sweet and green fruity
cis-3-hexanol	n.d	0.05	3.57	Grass, green fruit
2-hexen-1-ol	n.d	1.03	n.d	Fruity, grass (https://www.taytonn.com/index.php/products/)
1-nonanol	0.12	0.05	n.d	A characteristic rose-orange odour and slightly fatty
Isobutyl alcool	0.81	n.d	1.48	Apple, butter, cheese
1-pentanol	n.d	n.d	0.13	Alcoholic, iodoform-like (Cheng, 2010)
2 methyl cyclopentanol	n.d	n.d	0.08	–
2-norbornanol	0.13	n.d	0.12	Fruity
3,7-dimethyl-1,6-octadien-3-ol	n.d	n.d	0.19	Floral, spicy
2,7-dimethyl-4,5-octandiol	n.d	n.d	0.72	–
3-butynol	n.d	n.d	0.07	–
Benzenethanol	n.d	n.d	0.28	Sweet, floral
1-dodecanol	0.11	n.d	n.d	Fatty, honey, coconut
Acids
Isobutyric acid	n.d	4.62	n.d	Sweet, mild, carob
Valeric acid	0.32	n.d	n.d	Sweet, rancid
Acetic acid	0.91	0.08	0.57	Vinegar, pungent, sharp, sour (https://www.taytonn.com/index.php/products.all)
Heterocyclic compounds
2-pentyl furan	n.d	0.09	n.d	Butter, floral, fruit
Esters
Ethyl acetate	n.d	1.46	3.74	Fruit, brandy, grape
Ethyl butyrate	n.d	16.95	8.0	Butter, cheese, pineapple
Butyle acetate	n.d	0.45	n.d	Apple, pineapple
Butyl butyrate	n.d	1.74	n.d	Floral
Iso-amyl Butyrate	n.d	6.91	n.d	Fruit
Methyl benzoate	0.8	0.19	0.96	Fruit
Terpenyl propionate	0.53	0.21	0.51	Fruit
Isobornyl propionate	24.98	29.18	25.14	Fruit
Ethyl 2-methyl butyrate	n.d	n.d	4.6	Apple, ester, green apple, kiwi
3-methyl-1-butanol acetate (Isoamyl acetate)	0.53	24.39	0.82	Apple, banana, glue, pear
Isoamyl Isovalerate	n.d	n.d	0.44	Green
Hexyl acetate	n.d	n.d	0.97	Fruit
Bornyl acetate	n.d	n.d	0.13	Floral
Butanoic acid, 2-methylpropyl ester	n.d	2.55	n.d	Acidic, fruity
Ethyl Hexanoate	n.d	n.d	2.39	Fruity, pineapple–banana note
1,2benzendicarboxylic acid, ethyl ester	29.37	n.d	1.04	–
Isobornyl acetate	0.08	n.d	n.d	Camphoraceous, sweet, citrus
Butanoic acid, 3-methylbutyl ester	n.d	n.d	0.08	Fruity
Nitrogen compounds
1,2 propanediamine	3.1	0.7	n.d	Floral
Hydrocarbons
Butyl cyclopropane	n.d	0.07	n.d	–
Tetramethyl pentadecane	0.25	0.12	0.28	–
Hexadecane	0.16	0.05	0.2	–
Heneicosane	0.33	0.09	n.d	–
Heptadecane	n.d	0.03	n.d	–
Octadecane	n.d	0.12	n.d	–
pentamethyl-heptane	0.8	n.d	0.13	–
5-propyldecane	n.d	n.d	0.07	–
Hexadecamethyl heptasiloxane	0.48	n.d	0.11	–
Tetradecamethyl cycloheptasiloxane	0.09	n.d	n.d	–
Tetradecane	0.07	n.d	0.32	–
Terpene
Borneol	0.66	0.21	0.66	Green, camphor
Phenols	0.36	0.44	n.d	Off-flavor

*all the description except the specially labelled are cited from George, (2009)

Carbonyl compounds. CFS yogurt is the richest one on carbonyl compounds (12.48%). Carbonyl compounds enclose the principal aromatic constituents of yogurt, in fact, some carbonyl compounds were found only in the volatile fraction of co-fermented yogurts like 2-propanone and 2-nonanone. These two compounds are known as the main aroma that donates the “Savory, fresh” flavor of fermented milk (Dan et al. 2017). Benzaldehyde was detected with comparable levels in control and CFS yogurts. This aldehyde is found in certain dairy products (e.g. Camembert cheese) and contributes to bitter almonds (George, 2009). Acetoin, 2-heptanone and 2,3-Butanedione (diacetyl) were detected in the three types of yogurts and with different proportions. Acetoin and diacetyl are important aroma compounds that at high levels are able to ameliorate yogurt flavor qualities. Cheng (2010) reported that acetoin gives an aromatic note moderately sweet butter-like flavor that is identical to that of diacetyl. The aroma compound 2-heptanone has a "Creamy, fresh” odor of fermented milk (Pionnier and Hugelshofer 2006).

Ester. This chemical family is a typical flavor of dairy products such as yogurt and gives the “fruity and floral” aroma (Guler and Stupp 2007). The aromatic compounds isobornyl propionate and 1.2-Benzendicarboxylic acid ethyl ester were the most abundant esters in co-fermented yogurt (CFS and CFC) and control, respectively. 3-methyl-1-butanol acetate, contributing to banana and apple aroma, is found in high content in CFC (24.39%). The co-fermentation with plant material and autochthonous strains compensated for some ester compounds: new volatile ester compounds, like ethyl acetate and ethyl butyrate, were observed in CFS and CFC. A chemical interaction between alcohol and free acids could result in an ester compound (Marilley and Casey 2004).

Alcohols. Samples of control and CFS also included considerable content of alcohol, and the main one found in these yogurts is ethanol. Ethanol is the ordinary product of lactic acid bacteria fermentation and is the result of acetaldehyde reduction (Gezginc et al. 2015). The co-fermentation with autochthonous strains compensates for many alcohol compounds that were absent in control, but the co-fermentation with carob powder compensated for only the 2-methyl, 2-butanol compound. The compound cis-3-hexanol, which contributes to the "Green fruit and herbal" flavor, was detected in both co-fermented yogurt but with higher intensity in CFS samples.

Acids. Acetic acid is the volatile acid detected in all yogurt samples (control, CFS and CFC) and it contributes to the sour and acidic flavor of fermented milk. The intensity of this acid decreased with the co-fermentation process and the lowest levels are found in CFC. It can be produced from fatty acids (C4–C12), from citrate metabolism and also from amino acid catabolism (Curioni and Bosset 2002). Isobutyric acid, found on the free state in carob, is detected only in CFC and it contributes to the sweet aroma of carob fruits.

Principal component analysis (PCA) was executed for each aroma compound of different samples to discriminate differences in flavor. The dimensionality reduction analysis displayed that the first two dimensions of the principal component analysis accounted for 91.79% of the total variance, at 70.52% and 21.27% in the first (PC1) and second (PC2) dimensions, respectively (Fig. 3a). CFC, CFS and control were markedly different from each other by PC2. CFS was nearer to control than CFC. The dendrogram generated by hierarchical cluster analysis divided the samples into two groups, and CFS and control grouped together. CFC was classified as a single cluster, which was comparable to the PCA result (Fig. 3b).

Fig. 3.

Projection of different dromedary yogurt samples on a surface of the first two principal components a, representation of the dendrogram obtained from the hierarchical cluster analysis b and 3D plots of the first two principal components of the PCA of relative abundance of aromatic compounds in control yogurt, yogurt produced by co-fermentation technique with carob powder (CFC) or autochthonous strains (CFS) c (color figure online)

As shown in Fig. 3c, PCA1 correlated positively with 7 aroma compounds at high content in these yogurt samples. CFC correlated with 4 volatile compounds (Iso-amyl acetate, ethyl butyrate, isobornyl propionate and iso-amyl butyrate) while the control and CFS correlated with 3 compounds (ethanol, 3-methyl, 1 butanol and 1,2, benzene dicarboxylic acid ethyl ester).

Conclusion

Ultrafiltration process was used to concentrate dromedary milk, fit for yogurt manufacturing. The co-fermentation technique effectively improved the quality of dromedary yogurts. In fact, co-fermentation with plant material (carob powder) displayed a remarkable radical scavenging potential and improved texture characteristics, whereas co-fermented dromedary yogurt with autochthonous strains enhanced the lipid profile and aroma compounds considerably. Moreover, during the cold storage period, both yogurts by co-fermentation with plant or autochthonous strains maintained lactic acid bacteria at relevant counts (>7 log CFU/g), thereby, classified as a potent probiotic product. Further research, like sensory evaluation and other biological activities, are undergoing.