Abstract
Preparation of curd at home is quite challenging as it requires skills like addition of proper amount of starter culture, maintenance and storage of inoculum for the preparation of good and consistent quality curd. The present work utilized bacterial attachment property of PVDF (Polyvinylidene fluoride) membrane to prepare a strip which can be dipped into milk for the preparation of consistent quality curd. Shelf-life of the strip is around 100 days. The strip prepared curd was well comparable with the curd prepared by the commercial inoculum based on their pH, % lactic acid, % syneresis and bacterial load. Strip of size 5 × 5 cm2 was enough for preparation of 500 mL curd. It was proved by different analytical techniques like AFM, SEM and FTIR that PVDF was not having any leaching property during curd preparation. It can also be used in repeated contact with food products, as it is FDA (Food and Drug Administration) compliant and non-toxic. The curd strip has significant industrial relevance as it is a cost-effective alternative of any commercial inoculum (very expensive) and also meets the demand of consumers with the rising health awareness and busy lifestyles. Further, it is spillage proof, portable, ready-to-use.
Supplementary Information
The online version contains supplementary material available at 10.1007/s13197-021-05339-3.
Keywords: Polyvinylidene fluoride (PVDF), Strip, Curd, Inoculum, Lactobacillus
Introduction
Yogurt is popularly known as Curd (Dahi) in India, which is one of the most common fermented milk products in the world and produced commercially or at majority of home regularly (Willey et al. 2008). Traditional method of yogurt preparation is carried out from long past throughout the world. In Nepal, traditional yogurt is prepared by transferring hot milk in a special mud pot called theki, without adding any extra inoculum (Bhattarai et al. 2013). The West Sumatran yogurt called Dadih is prepared traditionally from water buffalo milk, fermented in bamboo tubes (Wirawati et al. 2019). In India also, red stems of chili pepper is used to induce curdling of milk when starter culture runs out (Sharma et al. 2013); (Sarkar and Alam 2018). However, all the above methods require either special utensil, bacteria or other ingredients which may not be available readily in modern days’ home. Moreover, it is a challenge to prepare curd at home having the same consistency as the starter used is either sub-cultured or the original starter is preserved. In both the cases the viability of starter, pH and sensory properties are affected (Priadi et al. 2020). Many times, consumers forget to save the inoculum due to which they have to purchase curd from the market. Therefore, the present work explains the development of a technique for the preparation of curd with the help of a strip which is made up of PVDF membrane. The curd strip can be directly dipped into the processed milk for the preparation of curd. The strip will be removed before consuming or further processing the curd. This simple method of using the strip for the curd preparation makes it advantageous as they are ready-to-use anytime and is free of hassles like keeping inoculums. Also, the curd prepared is fresh and will have the same consistency every time.
The curd starter cultures are prepared in liquid, frozen and dried forms. Among them, the famous method is the freeze-drying as it is reliable in terms of cell viability, and long storage time (Gong et al. 2014). Many companies are also manufacturing ready-to- use dried starter culture for the preparation of curd at home e.g., Yogourmet®. However, the strip prepared here is different from the freeze-dried starter as it uses a substrate to carry the starter bacteria. Secondly, compared to commercially available starters like Yogourmet® the strip-based inoculum is cost-effective. Because, the time and energy required to dry the membrane is relatively less than drying large amount of bacterial concentrates. Also, the size of strip can be varied according to the requirement of consumers e. g. small size of strip can be sold for the preparation of a lesser amount of curd.
Again, the curd strip is different from microencapsulated starter culture used for yoghurt manufacturing. Alginate and chitosan were used for the encapsulation which resulted in slower milk acidification (De Prisco et al. 2017). Also, encapsulation can negatively affect the textural and the sensorial properties of the product e. g. yoghurts containing encapsulated bacteria have grainy texture and grittiness (Afzaal et al. 2019; Mitropoulou et al. 2013; Kailasapathy 2006). But unlike the microencapsulation, the strip has to be removed from the curd after preparation. Therefore, the strip prepared here cannot alter the textural or sensorial properties of yogurt as it doesn’t have any extra additives.
Further, the materials/polymers used for membrane preparation are thermally, mechanically and chemically very stable. They have high glass transition temperature and their biodegradability is also very low. These membranes are also common in use due to its simplicity in the preparation and cost-effectiveness.
Apart from filtration properties, these membranes have an innate characteristic of adhering bacteria on its surface (Mallevialle et al. 1996) due to various bacterial and membrane characteristics (Habimana et al. 2014), accumulation of trace organic nutrients at the solid–liquid interface (Ridgway et al. 1984) and the attachment does not necessitate any stereospecific macromolecular binding sites (Mallevialle et al. 1996). This leads to the attachment of numerous kinds of bacteria on different polymeric membrane surfaces as it does not require any distinct interaction.
Again, PVDF is a highly non-reactive thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride. PVDF is a specialty plastic which has resistance to solvents, acids, bases, and heat (Yousefi and Karimi 2013). It can also be used in repeated contact with food products, as it is FDA compliant and non-toxic (Kosar and Morris 2007).
Hence, due to the above-mentioned properties of PVDF and its enhanced bacterial affinity as explained in our previous publications (Kumar et al. 2017a, b) membrane was used to prepare curd strips. Up to our knowledge, there is no such kit available in the market anywhere.
Therefore, the main objectives of this study are (1) development of simple, user-friendly, affordable kit which can be used for the preparation of consistent quality curd from milk. (2) optimize the shelf life of the strip to outline the time duration of its storage. (3) analyse chemical leaching if any from the curd strip to the prepared curd.
Materials and methods
Materials
PVDF (573 KDa, Solef, Solvay, France), polyester fabric (Filtration Sciences Corporation, USA), N, N dimethyl formamide (Qualigen, India), were used to prepare the asymmetric membranes. Reverse osmosis treated water was used for membrane preparation.
Lactobacillus MRS Broth (MRS Broth) and Streptococcus Thermophilus Isolation Agar (HiMedia, India) were used for the study of bacterial load in the curd. MacConkey Agar (HiMedia, India) and Potato Dextrose Agar (HiMedia, India) were used for checking the contamination present in the prepared curd. Processed milk (Amul Gold, India) with total fat- 4.5 g, saturated fat- 2.8 g, cholesterol- 12 mg, total carbohydrate- 5 g and selected minerals in 100 mL were used for the curd preparation. Commercial curd (Amul Dahi, India) was used for the isolation of curd bacteria and skimmed milk (Amul, India) as a cryoprotectant were used in the study. Detailed information for all the used products is available at www.amul.com.
Phenolphthalein indicator (RANKEM, India) was used for the titration of the prepared curd.
Preparation of 20% W/W PVDF membrane
PVDF membrane was prepared as described in our previous paper with following modification (Kumar et al. 2017a, b). In detail, PVDF of 20% (W/W) concentration in dimethylformamide was prepared in stoppered conical flask through slow dissolution by heating at 45 °C for 4 h. Subsequently, the viscous polymeric solution was cast on a sheet of non-woven polyester fabric taped onto a glass plate using a 203 mm (8-mil) gate size casting bar and the membrane was immediately immersed in non-solvent medium (water). Then after 15 min the membrane was allowed to air dry for an hour. The prepared membrane was then cut into the pieces of varying size viz. 2 × 2 cm2, 2 × 4 cm2, 4 × 4 cm2 and 5 × 5 cm2 and sterilized at 121 °C for 15 min. Following this, the membranes were properly dried in an oven for its further use.
Preparation of curd strip
Fresh curd (Amul Dahi) was purchased from the local market for the experiment. It was then centrifuged at 1000 rpm. The supernatant (1 mL) of the centrifuged curd was inoculated into MRS broth and kept it (on shaking) for overnight for the growth of bacteria present in the curd. The grown curd bacteria were then washed with PBS (2–3 times) to remove all the traces of growth media. The washed bacteria were resuspended in 10% skimmed milk. The pieces of sterile 20% PVDF membrane was then added to the bacterial suspension as prepared above. The bacteria were allowed to attach to the membrane for 24 h on shaking (100 rpm) at 37 °C and 42 °C separately. The bacterial attached membrane was finally lyophilized (FreeZone 18 Liter Console Freeze Dry System, LABCONCO) to make the curd strips and stored at 4 °C.
Characterization of the curd strip
To evaluate the bacterial attachment of curd bacteria on the membrane surface, following experiments were done:
Scanning electron microscopy (SEM)
The lyophilized bacterial strip was fixed before analysis. For that, it was first dipped in 2% glutaraldehyde for 30 min and washed with phosphate buffer (pH 7.2) two times for 5 min each. It was then gradually transferred to six ascending concentrations of ethanol from 10 to 90% and rectified spirit. All the dipping duration was of 30 min and it was of twice in rectified spirit. Finally, the strip was dried in the desiccators for micrograph. It was then gold-coated by sputter coater for generating electrical conductivity. SEM images were recorded using LEO 1430VP scanning electron microscope at 15 kV accelerating voltage to have a view of phase morphology of the membranes.
Total bacterial count on curd strip at different time intervals
The method was followed as per our previous paper with following modifications (S.B. Kumar et al., 2017a, b). In brief, bacteria were recovered from the area of 1 cm2 of the lyophilized membrane with the help of cotton swabs in N-saline. Subsequently, 100 µL of the recovered bacterial solution was spread on the Streptococcus Thermophilus Isolation Agar plates. Plates were incubated at 37 °C for overnight. After 24 h of incubation the colonies obtained were counted and represented as CFU/mL. All the experiments were done in triplicates and the average of three values was considered.
Sequencing analysis of bacteria isolated from curd strip
The bacteria from the curd strip were isolated as described in the above section and its partial 16S rRNA gene was amplified, sequenced and compared to NCBI database for their molecular identification as described in our previous study (Raval et al. 2017).
Optimization of strip size for curd preparation
For the preparation of curd, processed milk was purchased from the local market. The milk was then boiled completely and allowed to cool till 40–42 °C. Different sized curd strip (2 × 2 cm2, 2 × 4 cm2, 4 × 4 cm2, and 5 × 5 cm2) was added in different numbers (1–5) for various milk concentrations (250–500 mL) for standardizing the membrane area and the optimum milk volume for efficient curd preparation and kept inside the incubator for overnight at 42 °C.
Characterization of curd prepared from curd strips
To check the quality of the curd prepared with the help of curd strips, different tests were performed in comparison with the positive control (curd prepared with the commercial inoculum) and negative control (without inoculum). The different tests are explained in the following sections.
Characteristics (pH, lactic acid content and bacterial concentration) of curd obtained from curd strip
The pH of all the prepared curd samples was measured by the digital pH meter (Eutech). Lactic acid content (%) was estimated by titration method taking 0.1 N NaOH as a titration solution and phenolphthalein as an indicator which is described in the reported method (http://www.agriinfo.in/default.aspx?page=topic&superid=9&topicid=716) and also in APHA manual (RB and Barraquio 2015).
Acidity was expressed as percent lactic acid and was computed using the following formula:
For the determination of bacterial concentration, each curd samples were diluted many-fold (106–1010) and spread on Streptococcus Thermophilus Isolation Agar plates. The colonies obtained were counted and represented as CFU/mL.
Statistical analysis
Post hoc comparison of means was carried out using Fisher’s test at significant level of p < 0.05 for %lactic acid and pH. In Table 3, the values represented by same alphabets in the column are not significantly different and the different alphabets indicated significant difference at p < 0.005 by Fisher’s test.
Table 3.
Post hoc comparison of means using Fisher’s test at significant level of p < 0.05 for both lactic acid and pH
| Curd type | % Lactic acid | average | SD | ||
|---|---|---|---|---|---|
| Curd strip | 1.2 | ||||
| Curd strip | 1.3 | ||||
| Curd strip | 1.4 | 1.3 | 0.1 | a | |
| Commercial inoculum | 1.6 | ||||
| Commercial inoculum | 1.2 | p value 0.0013 | |||
| Commercial inoculum | 1.4 | 1.4 | 0.2 | a | |
| Without inoculum | 0.3 | ||||
| Without inoculum | 0.7 | ||||
| Without inoculum | 0.5 | 0.5 | 0.2 | b |
| Curd type | pH | average | SD | ||
|---|---|---|---|---|---|
| Curd strip | 3 | ||||
| Curd strip | 3 | ||||
| Curd strip | 3 | 3 | 0 | b | |
| Commercial inoculum | 2.9 | ||||
| Commercial inoculum | 3.1 | p value < 0.0001 | |||
| Commercial inoculum | 3.3 | 3.1 | 0.2 | b | |
| Without inoculum | 5 | ||||
| Without inoculum | 4.9 | ||||
| Without inoculum | 4.5 | 4.8 | 0.264575 | a |
Determination of syneresis by centrifugation method
Curd prepared from commercial inoculum, from the strip, and without inoculum were taken out from refrigerator and stirred 20 times clockwise and anticlockwise with a glass rod. Approximately 30 g of the stirred curd was then transferred into a 50-mL falcon tube and left at 4 °C for 2 h for stabilization. The stirred samples were then centrifuged (Sorvall RT 7) at 3313 × g for 15 min at 10 °C. The separated whey was weighed. The syneresis was expressed as the percentage weight of the whey separated from the gel over the initial weight of the gel (Amatayakul et al. 2006).
Contamination detection of coliforms and yeast and molds
All the prepared curd was spread on MacConkey agar and Potato Dextrose agar for the enumeration of Escherichia coli and yeast & molds respectively. MacConkey agar plates were kept at 37 °C for 2 days and Potato Dextrose agar plates were kept at 28 °C for 5 days.
Analysis of leaching property of curd strip
As the strip was going to be used in a food product, the leaching of PVDF in the curd was checked with the help of following analytical techniques. For this, 20% PVDF membrane was considered as control and the used curd strip was compared with it.
SEM analysis
Again, the SEM analysis was done for the used curd strip and 20% PVDF membrane. For this, the used curd strip was properly washed with tap water before analysis to remove the curd remains on it and air dried subsequently. SEM analysis of both the samples was then done as described in Sect. “Scanning electron microscopy (SEM)”.
Atomic force microscopy (AFM) analysis
Active surfaces of 20% PVDF membrane and used curd strip were characterized by atomic force microscopy (AFM). AFM images of the membrane samples were taken by using NTMDT, Russia AFM instrument. The analysis of the image was done by “NOVA” software. The membrane samples were dried in vacuum at 40 °C and stored in CaCl2 desiccators before taking images. Images were recorded from different areas of each sample.
Fourier-transform infrared spectroscopy (FTIR) studies
The IR spectra of 20% PVDF membrane and used curd strip was compared. Also, the IR spectra of only PVDF polymer, curd prepared with commercial inoculum and curd prepared with the strip was determined. A PerkinElmer spectrum GX FTIR spectrometer was used with spectral range, 4000–400 cm−1 to record the IR spectra.
Results and discussion
Preparation of the curd strip
The curd strip was prepared successfully as shown in the Figure S1. The thickness of the strip was 30–35 µm approximately. Different size of the membrane can be prepared according to the requirement.
In our previous study, it was already shown that polymeric membranes attach bacteria on their surface, especially the 20% PVDF membrane adheres relatively more bacteria (Kumar et al. 2017a, b). Hence, it was selected for the preparation of the curd strips. For complete bacterial attachment on the membrane, the membrane was kept at shaking for 24 h. Further, to preserve the bacteria in a viable condition, the bacterial attached membrane was lyophilized and sealed in small plastic bags with the help of heat sealer. To protect the cells from freezing damage (ice formation) during lyophilization, and maintaining their viability through freezing, water removal, and subsequent storage; 10% skim milk was used as a cryoprotectant.
Initially, 37 °C temperature was used for the bacterial attachment. But the curd bacteria grow more optimally at 42 °C (Mchiouer et al. 2017) therefore, this temperature was selected for further study.
Characterization of curd strip
SEM
The crystal-like structure on the surface of membrane showed lyophilized bacteria which were completely attached on the membrane surface uniformly (Fig. 1a). Also, compared to the Fig. 1b, c, where no bacterial attachment was observed for 20% PVDF membrane and washed curd strip; it can be interpreted that the bacteria was attached to the curd strip. Hence, the image ensured that there was enough and absolute adherence of the bacteria on the membrane surface which can be efficiently used as inoculum for the curd preparation.
Fig. 1.
a SEM micrograph of Curd strip b SEM micrograph of 20% PVDF membrane and c Used and subsequently washed curd strip
Total bacterial count on curd strip at different time intervals
To check the shelf-life of the curd strip, the total viable bacterial count was done at different time intervals. Table S1 shows that there was a decrease in bacterial count on curd strip after 30th day. But after that, the bacterial count on the strip remained constant. Moreover, the curd prepared with the help of the strip was comparable to the commercial curd till 100th day (with respect to the curd characteristics explained in Sect. “Comparative study of curd prepared with commercial inoculum and with curd strip”). Therefore, the shelf-life of the strip was at least 3 months or more. All the curd strips were prepared as single batch and the strips of same batch were used for the present study to improve the fidelity and consistency of the experiment.
Identification of the curd strip bacteria
The partial 16S rRNA sequence of the bacteria isolated from the curd strip was matched in the NCBI database by blast search (Figure S2). The sequencing result showed that the bacteria attached on the curd strip showed maximum homology with the strain Lactobacillus fermentum with 100% query coverage and 99% identity.
Preparation of curd taking curd strip as inoculum
Curd strips of size 2 × 2 cm2, 2 × 4 cm2, and 4 × 4 cm2 were used in different numbers for 250 mL of milk for the preparation of curd. All the formed curd showed inconsistent quality with respect of curd prepared with commercial inoculum (data not shown). After that, a different number of 5 × 5 cm2 sized curd strips (prepared at 37 °C and 42 °C) were checked for 250 mL of milk concentration. The curd formed was comparable to the positive control curd and only one strip was enough for the preparation of 250 mL curd (Table 1).
Table 1.
Characteristics of curd obtained from curd strips in 250 mL of milk. The values of % lactic acid and pH are represented in mean values ± (standard deviation).
| Inoculum type | % Lactic acid | pH | CFU/mL (approx.) |
|---|---|---|---|
| Strip prepared at 37 °C (5 × 5 cm2) | |||
| 4 strips | 1.2 ± 0.12 | 3.55 ± 0.05 | 1012 |
| 3 strips | 1.7 ± 0.1 | 3.95 ± 0.05 | 1013 |
| 2 strips | 1.45 ± 0.05 | 3.9 ± 0.1 | 1013 |
| 1 strip | 1.42 ± 0.07 | 3.95 ± 0.05 | 1013 |
| Commercial inoculum | 1.4 ± 0.45 | 3.8 ± 0.1 | 1015 |
| Without inoculum | 0.55 ± 0.1 | 5.1 ± 0.3 | Lawn growth* |
| Strip prepared at 42 °C (5 × 5 cm2) | |||
| 4 strips | 1.1 ± 0.08 h | 4.0 ± 0.1 | 1012 |
| 3 strips | 1.8 ± 0.15 | 4.2 ± 0.1 | 1013 |
| 2 strips | 1.4 ± 0.2 | 3.4 ± 0.2 | 1014 |
| 1 strip | 1.55 ± 0.05 | 3.3 ± 0.2 | 1013 |
| Commercial inoculum | 1.4 ± 0.45 | 3.8 ± 0.1 | 1015 |
| Without inoculum | 0.55 ± 0.1 | 5.1 ± 0.3 | Lawn growth* |
*Mixture of colonies with different shapes, sizes and foul odour
Therefore, with the help of strip, curd was prepared successfully at different ambient temperatures i.e., at 37 and 42 °C. The ideal curd characteristics like % lactic acid, pH etc. are explained in the next section.
Comparative study of curd prepared with commercial inoculum and with curd strip
The curd formed with both the types of strips (prepared at 37 °C and 42 °C) showed similar quality as the curd prepared by commercial inoculum (Sect. “Preparation of curd taking curd strip as inoculum”). Hence, both the temperatures are optimum for bacterial attachment on the membrane.
Furthermore, as the single strip was sufficient for 250 mL of milk, an attempt was made to use it for 500 mL of milk, which was successful with respect to the quality of curd prepared from commercial inoculum. Therefore, all the further characterizations were done for the curd prepared from 500 mL milk taking only one strip as inoculum (Table 2).
Table 2.
Characteristics of curd obtained from curd strip (42 °C, 5 × 5 cm2) in 500 mL of milk. The values of % lactic acid and pH are represented in mean values ± (standard deviation)
| Curd type | % Lactic acid | pH | CFU/mL (approx.) |
|---|---|---|---|
| Curd strip | 1.2 ± 0.1 | 3.0 ± 0.0 | 1015 |
| Commercial inoculum | 1.4 ± 0.2 | 3.1 ± 0.2 | 1015 |
| Without inoculum | 0.5 ± 0.2 | 4.8 ± 0.26 | Lawn growth* |
*Mixture of colonies with different shapes, sizes and foul odour
Comparative study of pH, lactic acid content and bacterial concentration
Lactic acid is produced from the fermentation of lactose which is responsible for the sour taste of curd by decreasing its pH and it also gives the curd its characteristic texture by acting on the milk proteins (Zourari et al. 1992). Further, the low pH is toxic for many microorganisms as well as prolongs the shelf life of the curd (Hui et al. 2004). Again, the minimum viable count of bacteria should be present in the fermented milk so that it can provide the claimed health benefits to humans (RB and Barraquio 2015). Therefore, determination of the lactic acid content, pH and viable CFU count in curd is important for testing the curd quality.
Statistical analysis
For both lactic acid and pH (Table 3), Post hoc comparison of means was carried out using Fisher’s test at significant level of p < 0.05. There were no significant differences between the % lactic acid and pH of the curd prepared with the help of strip and commercial inoculum. But compared to curd without inoculum the % lactic acid and pH of the curd prepared with the help of strip and commercial inoculum were significantly different. Therefore, the strip was capable of preparing curd with the same efficiency as the commercial inoculum.
Comparison of syneresis
Syneresis or natural whey separation on the surface of set curd is considered as a defect (Amatayakul et al. 2006). Hence, the syneresis observed in curd prepared with the aid of the strip was compared with the curd prepared with commercial inoculum. It was observed that the % syneresis of the strip prepared curd was 56.6 which was comparable to the % syneresis of the curd prepared from commercial inoculum i.e., 50.6. Moreover, the % syneresis of the curd formed without inoculum was 74 which was much higher than the strip prepared curd. Therefore, the strip can be perfectly used for quality curd preparation.
Testing of coliforms and yeast and molds
Table S2 showed that there is no contamination of coliforms in any type of prepared curd. Only the milk product without inoculum showed contamination on potato dextrose agar plate. This confirms that there is an absence of slow acid development and the curd prepared is of good quality. Hence, the strip in the form starter culture is as efficient as the commercial inoculum.
Comparative analysis of 20% PVDF membrane and used curd strip
SEM analysis of used curd strip in comparison with 20% PVDF membrane
Figure 1b depicts control 20% PVDF membrane used for the preparation of curd strip. The used strip after curd preparation (c) is comparable to control membrane; except the smoothness which represents the insoluble curd particles blocking the porosity (c) since the membrane was washed with water only. Hence, the membrane was intact during the curd formation. The duplicate images of other used strips illustrated the same interpretation and hence, not shown here.
AFM analysis of used curd strip in comparison with 20% PVDF membrane
Atomic force microscopy was used to analyze feature height distribution (the height of the features, to compare topography) of 20% PVDF membrane and used strip to check if any damage occurred on the membrane surface or not. As shown in Fig. 2, both the AFM images are visibly comparable with respect to any shearing on the surface of the membrane. The used curd strip has little higher surface roughness because of the adhered curd particles and bacteria on the membrane surface (Marka and Anand 2017). Hence, no membrane damage during the curd preparation was established from the AFM results.
Fig. 2.
a AFM images of 20% PVDF membrane b Used and subsequently washed curd strip. Each image outlines are on same scale
Comparative study using FTIR
Figure 3a clearly revealed that the FTIR spectra of 20% PVDF membranes and used curd strip contained similar peaks attributed to PVDF membrane (Jaleh et al. 2015). Therefore, it can be interpreted that no damage and/or breakage of the membrane occurred during the curd preparation. Moreover, the FTIR spectra of the curd prepared with commercial inoculum and prepared with the strip were similar. In addition to that, no peaks attributed to the PVDF (bands located at 3022 and 2980 cm−1 corresponded to the CH2 asymmetric and symmetric vibration of PVDF, the absorption peak at 1403 cm−1 attributed to CH2 wagging vibration, the C–C band of PVDF at 1185 cm−1, the peaks at 878 and 840 cm−1 related to C–C–C asymmetrical stretching vibration and CF stretching vibration of PVDF (Bai et al. 2012 and Ahmed et al. 2013) appeared in the spectrum of both control and test curd (Fig. 3b). Hence, after considering all the results, theoretically there is no chance of leaching of the PVDF polymer inside the curd during its preparation and the curd prepared with the aid of strips are of same characteristics like the curd prepared with the commercial inoculum.
Fig. 3.
a The FTIR spectrum of PVDF membrane (red) and used curd strip (blue) respectively. b The FTIR spectrum of PVDF polymer (red), curd prepared with commercial inoculum (blue) and curd prepared with curd strip (yellow) respectively
Conclusion
The study showed the use of polymeric membrane as the carrier of starter culture for the curd preparation. The starter bacteria for the curd can be attached on the PVDF membrane and subsequently used for the preparation of curd same as the commercial curd. Further, the analytical studies suggested that there is negligible chance of leaching of PVDF membrane in the curd. Therefore, the curd strip has immense market value as the industry continuously looks for improvement and strives for the use of latest technologies to make it more user friendly. It has advantages like it can be sold as sachets which are cost-effective, the perfect size to trial a product, easy and cheap to distribute. In future, various types of bacteria/probiotics (depending on the requirement) can be attached on this membrane which will help to prepare curd with required supplements to address mass nutritional requirements of the society. Also, many other studies are required to improve its efficiency, e. g. optimization of cryoprotectants, reusability of strips, improving the shelf-life of strips, acidification rate and checking the effects of different starter bacteria on membrane etc.
Supplementary Information
Below is the link to the electronic supplementary material.
Acknowledgements
Authors sincerely acknowledge AESD & CIF of CSIR-CSMCRI for providing all the instrument facilities. SBK acknowledges DBT for her SRF financial support and MJB acknowledge CSIR for financial support. We also acknowledge Gopal Bhojani for his help in lyophilization process. The manuscript has been assigned CSIR-CSMCRI –179/2018 registration number.
Author contributions
SBK conceived, carried out the experiments and wrote the MS; MJB and SC has evaluated the quality of prepared curd from the curd strip and involved in study to transport the curd strip in ambient temperature; SH conceptualize and overall supervised the work and edited the manuscript.
Funding
The work has been carried out by institute internal funding. Financial assistance received by authors are mentioned in the acknowledgement section.
Availability of data and material
All supportive data are included under the supplementary materials.
Declarations
Conflict of interest
There is no conflict of interest or competing interest among the authors.
Consent to participate
All authors have given consent to participate in the manuscript.
Consent for publication
All authors have given consent to publish the manuscript. Written consent has been given in the authorship form submitted by Springer.
Footnotes
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