Material & methods

Analytical methods

Moisture content

Moisture contents of flavoured curd powder were determined gravimetrically by oven drying at 105 °C for 3 h.

$${\rm MC} = \; \displaystyle{{{\rm Intial}\,{\rm Weight} - {\rm Final}\,{\rm Weight}} \over {{\rm Final}\,{\rm Weight}}}\; \times 100\% $$

Water activity (a _w)

The water activity was determined using a water activity meter (LabSwift-aw; Novasina, SRM University, India).

Dispersibility

Dispersibility measurements of flavoured curd powder were performed in water by little stirring and the time of dispersion was noted down.

Bulk density and tapped density

The bulk density (ρ_b) of the flavoured curd powder was determined by taking the weight of the powder and the corresponding volume. For the tapped density (ρ_t), the cylinder was tapped strenuously by hand until no further change in volume took place (Nijdam & Langrish, Reference Nijdam and Langrish2005).

Porosity

The porosity (ε) of the powder samples was evaluated using the relationship between the tapped (ρ_t) and bulk (ρ_b) densities of the powders (Nijdam & Langrish, Reference Nijdam and Langrish2005):

$${\rm \epsilon} = \; \displaystyle{{{\rm \rho} _{\rm t} - {\rm \rho} _{\rm b}} \over {{\rm \rho} _{\rm t}}}$$

Particle morphology

The morphological properties of flavoured curd powder (the appearance and the particle shape) identified by using images which are taken via a scanning electron microscope (SEM, FESEM–NRC, India).

Colour analysis

The colour of mango flavoured curd powder was measured using a Hunter Lab Colour quest XE-Hunter colour meter based on L* (black to white), a* (red to green), and b* (yellow to blue) values (Behboudi-Jobbehdar et al. Reference Behboudi-Jobbehdar, Soukoulis, Yonekura and Fisk2013).

pH

pH of mango flavoured curd powder was determined after reconstitution using pH meter (Chegini et al. Reference Chegini, HamidiSepehr, Dizaji and Mirnezami2014).

Functional group identification

FTIR is done to identify the functional groups present in the mango flavoured curd powder (Mitali & Baruah, Reference Mitali and Baruah2011).

Identification of the pure culture

Isolated pure culture on MRS agar petri plates was recognised with the help of biochemical tests like endospore test and catalase test.

% Lactic acid

Acidity is indicated as per cent lactic acid. Since 1 ml of 0·1 N lactic acid carries 0·009 g of lactic acid, the number of ml. of 0·1 N NaOH needed to neutralise the lactic acid in the sample, multiplied by 0·009 will produce the quantity of lactic acid (grams) in the sample, when the result is divided by weight of sample and multiplied by 100 the per cent lactic acid will be obtained. Phenolphthalein indicator is used and colour changed to pink which remained constant for 10–15 s. The titration is completed within 20 s.

$$\eqalign{&{\rm \%} \,{\rm Lacti}\,{\rm acid} \cr & \quad {\rm No}\,{\rm of}\,{\rm ml}\,{\rm of}\,0 \cdot 1 {\rm N}\,{\rm NaOH}\,{\rm solution}\,{\rm required}\,{\rm for} \cr & \quad ={{\rm nutrilization} \times 0 \cdot 009 \over \quad \quad \quad {\rm Weight}\,{\rm of}\,{\rm Sample}} \; \times 100}$$

(Weight of sample = Volume of sample × specific gravity).

Statistical analysis

All measurements were made in triplicate and results are expressed as mean ± standard deviation. The analysis of variance (ANOVA) at a confidence level of 95% was performed. All the results that were obtained were analysed using IBM SPSS Statistics 20·0 software (SRM University, India).

Results and discussion

As reported in Table 1 the moisture content of the fresh curd sample was determined as 8·58% (150 °C) and 3·51% (170 °C) wet basis. The moisture content (%, wet basis) of mango flavoured curd powders obtained at the 2 spray drying conditions with 3 different ratios (1:0·5, 1:1, 0·5:1) are also shown in Table 1, where it is observed that samples with more amount of curd have higher amount of moisture content and water activity. The moisture content as well as water activity decreased with higher inlet temperature. For yogurt powder moisture content varies from 3·98% to 7·17% based on different inlet and outlet temperature (Koc et al. Reference Koc, Sakin-Yılmazer, Kaymak-Ertekin and Balkır2014). From Table 1 it can be obtained the reconstitution property (dispersibility) (P < 0·05) was higher with higher inlet and outlet temperature. The dispersibility of flavoured curd powder is affected by the outlet air temperature, but the trend of this effect cannot be figured out clearly (Koc et al. Reference Koc, Sakin-Yılmazer, Kaymak-Ertekin and Balkır2014). Bulk and tapped density decreased with higher inlet and outlet temperature but porosity increased, hence there was an inverse proportionality between the two characteristics. Spray-dried products are usually non-homogeneous. Milk powder which is dried in low temperature provides relatively high bulk density due to lower porosity, whereas milk powder dried in high temperature gives low bulk density because of porous individual particles (Nijdam & Langrish, Reference Nijdam and Langrish2005).

Table 1.

Physical & Chemical Properties (mean ± sd) of spray dried curd and mango flavoured curd powder

* P < 0·05

The scanning electron microscopy images of yoghurt powder, produced at 2 different spray drying condition and 4 different ratios, are given in Fig. 1 with a level of magnification of 800× to 4000×. The first four pictures were for 150 °C sample 1:0·5, 1:1, 0·5:1 and control curd sample, respectively. The other four pictures were for 170 °C as the above stated order of ratios.

Fig. 1.

The scanning electron microscopy images of Mango flavoured curd powder, produced at the optimum spray drying condition (inlet air temperature of 150 °C & 170 °C, outlet air temperature of 80 ± 10 °C & 105 ± 10 °C and feed temperature of 15 ± 5 °C)

The pH was increased with higher inlet and outlet temperature samples. Some circumstances led to an increase in the density of the concentrated liquid samples, which resulted in an increase in the pH values (Chegini et al. Reference Chegini, HamidiSepehr, Dizaji and Mirnezami2014). Sample with equal ratio of curd and mango had higher pH than the rest of the samples. Curd sample had a greater amount of lactic acid than flavoured sample powders. Lactic acid percentage decreased with higher inlet and outlet temperature. In previous experiment on yogurt-ice cream mix the percentage of lactic acid was found to be between 0·60 and 0·63 (Layusa et al. Reference Layusa, Barraquion, Dizon, Elegado and Sevilla2013). The colour analysis showed that the curd sample was white in colour and all the other three ratios were yellow in colour (reported in online Supplementary Tables S1 and S2). All the flavored sample powders had a lower luminosity (L*) and higher yellow colour intensity (b*) compared to the curd-only sample. The luminosity (L*) decreased and intensity of yellow colour (b*) and intensity of red colour (a*) was increased with higher inlet temperature.

The FTIR results showed that with different inlet spray drying temperature and ratios the functional groups present in the sample powder changed (reported in online Supplementary Table S3). In both the cases ratio of mango 0·5: curd 1 had the almost similar functional group against control curd sample (online Supplementary Figs. S1–S8). The number of functional groups decreased with higher inlet temperature. Alkanes were present as same number in flavoured curd powder at both 150 °C and 170 °C inlet temperature. From the result we can see alkenes groups were not present in only curd sample at both 150 °C and 170 °C inlet temperature.

To identify the lactobacillus strain present in the curd powder we obtained an almost-complete 16S rRNA gene amplification with primer combinations 27F and 1492R. The phylogeny tree S9 (reported in Supplementary Fig. S9) shows the strain present in the sample is Lactobacillus_curieae which is a gram-positive, non-motile, facultative anaerobic, non-spore-forming, rod-shaped bacteria which did not exhibit catalase activity (Lei et al. Reference Lei, Sun, Xie and Wei2013).