The effect of lactic acid bacteria and co‐culture on structural, rheological, and textural profile of corn dough

Abstract This study is aimed at assessing the effect of lactic acid bacteria (LAB) on corn flour using dynamic characterization methods including RVA, TPA, Rheometer, SEM, and DSC along with co‐culture technique in order to enhance its applicability by evaluating the variations in rheological, textural, morphological, thermal, and structural properties. Our findings suggested that bacterial incorporation both individually and in combination (co‐culture) revealed an improved corn dough profile with better properties. SEM showed irregular shape of particles having more grooves, indentations, and cracks. RVA demonstrated different pasting behavior on the dough. Bacterial inoculation in flour attributed to increase the TO (68.61–71.18), TP (73.74–78.42), TC (78.78–85.36), melting temperature (10.17–15.19), and ΔH (2.72–5.40). The hardness of corn was found approximately 75% of native dough. In treated corn, an increase was noted in both loss and storage modulus in correspondence with changes in the starch configuration and leaching of constituents. The results from DSC presented an increased melting temperature range and gelatinization enthalpy owing to bacterial treatment accredited to diversified morphological characteristics. The outcomes concluded in demonstration of a novel influence on structural, thermal, morphological, and rheological capabilities and capacities of corn dough. Lactic acid bacteria hydrolyzed part of the corn and flour had smaller, irregularly shaped particles with more holes in them, resulting in a reduced water retaining capacity. Textural, thermal, and pasting profile has also been improved due to degradation of macromolecules. Furthermore, the insight alterations induce various changes leading to improved corn flour. It may also develop the associations about the upright insurgence in the corn dough profile and its potential usage in industry and homes.


| INTRODUC TI ON
During the last epoch, microbial biotechnology has attained phenomenal attention due to its huge role in production of commercially valuable products. Among them, lactic acid bacteria (LAB) are of particular interest owing to its widespread applications especially in food industry such as yogurt and cheese. It can also be applied as emulsifier, improving agents, and esters due to its hygroscopic nature (Castillo Martinez et al., 2013). Lactic acid fermentation can provide a lot of benefits, such as lower energy consumption, cheap renewable substrate, and production of pure lactic acid. It is also considered as a precursor of several large and small compounds (Abdel-Rahman et al., 2013). Food and drug administration announced lactic acid and its salts as generally recognized as safe (GRAS) (Arshadi et al., 2016).
Co-culture is a recent fermentation technique wherein two or more populations are cultured together to stimulate growth and to produce a determined product (Eş et al., 2017). Microbial technology is very delicate branch of science because of the production of large amount of by-products such as ethanol and acetic acid which required further separation and purification and ultimately increase overall cost. Mostly the homofermentative LAB are unable to covert sugars into lactic acid while heterofermentative LAB can easily convert sugars into lactic acid. These heterofermentative strains also produce a lot of ethanol and acetic acid. The third most pronounced form is facultative heterofermentative, they use glucose to produce lactic acid via Embden-Meyerhof pathway. Therefore, co-culture that can outcompete with other bacteria or strain will reduce byproduct accumulation. Zhang and Vadlani, (Zhang & Vadlani, 2015) used L. plantarum and L. brevis as co-culture and their results concluded high yield while lower by-product accumulation.
Cereals are the most extensively used crops of the world and corn ranked third among them. Corn is full of nutrients and wisely used in food production or direct consumption. Only 5% of the produced corn is used for human consumption while rest of it (95%) is transformed into fodder and alcohol. Owing to certain limitations like low shear and thermal resistance, mucin deficiency, higher retrogradation and thermal decomposition, which limits its functional properties in industry.
However, several modifications such as esterification, acidic hydrolysis, carbonation, etherification, polymerization, extrusion, addition of gums, and enzymolysis have been used to improve the applicability of corn flour (Reyes et al., 2016). The chemical modification is easy to achieve but it worsens the taste and also unhealthy for humans. Extrusion is a prevalent technique, but its cost is very high. Biotechnological modification changes the molecular structure and gives high specificity by improving the corn flour (Uthumporn et al., 2012).
In this study, LAB (L. casei, L. plantarum, L. fermentum) were used to improve corn flour properties. The objective of the present investigation was to characterize rheological, morphological, structural, textural, and thermal changes in corn flour due to the inoculation of multiple bacteria (coculture) and single bacteria. The obtained outcomes of the study may provide innovative insights about the corn granules and its deterioration, thereby providing a comprehensive data to improve the quality of a final product.

| MATERIAL S AND ME THODS
For this study, corn grains (Jingke 968) were obtained from Jilin Agricultural University, China. Milling of the grains was carried out using laboratory miller (TAISITE, FW100) the flour was sieved and the fraction retained (0.180 mm) was used. Lactobacillus casei (L. casei), Lactobacillus plantarum (L. plantarum) and Lactobacillus fermentum (L. fermentum) were propagated on MRS medium at 37°C and stored at 4°C. Active dry yeast (Angel brand), sugar and salt were purchased from local market. All other chemicals were of analytical grade.

| Fermentation conditions
Isolated L. casei, L. plantarum, and L. fermentum were grown in MRS medium (liquid) in an incubator (HZQ-F160) to a bacterial concentration of 10 7 -10 8 cfu/mL. Twenty percent bacterial suspension was added in corn flour and placed in incubator for 1-5 days at 37°C. Dough was then prepared by mixing all ingredients thoroughly

| Pasting properties of flour
The pasting properties were ascertained by using Rapid Visco Analyzer (Perten) as per the procedure described by Wani (Wani et al., 2012). Three grams (

| Micromorphology of corn
Scanning electron micrographs (Phenom TM ) of each sample were obtained at magnifications of ×5000. Flour samples were fixed on the holders with double spread and gold layer was sputtered on it and then scanned in a vacuum of 5kv potential difference.

| Textural analysis of corn flour
Fifty grams (50 g) of corn flour was kneaded with 50 mL of water in order to make a standard dough by giving 60 min of fermentation at room temperature. The textural analysis comprises hardness, adhesiveness, springiness, cohesiveness, chewiness, gumminess, and resilience were measured by a textural analyzer (TA.XT plus).
Maximum force applied can be considered as hardness. The conditions were as follows: test distance 10 mm, probe P/0.5, and test velocity were 0.5 mm.

| Statistical design
The obtained data for each parameter were subjected to appropriate statistical analysis through Statistical Package Origin-Pro 8.5 software. The analysis was performed using completely randomized design (CRD) and the analysis of variance was applied to determine the level of significance.

| Textural analysis of corn flour
A remarkable variation in textural analysis between simple corn flour and bacterial treated corn flour were observed in this study (Table 2).
Bacterial incorporation in corn flour improved the textural properties by reducing hardness and more cohesiveness, gumminess, chewiness, and springiness which enables easy processing of flour into dough. While LC, LF, and LC-LP results in more hardness, which may be due to denaturation and gelation of proteins and loss of water. It is reported that the acid-induced gelation due to LC and LF causes rigidity in flour and results in more hardness (Hu et al., 2007). Yang (Yang et al., 2017) worked on the fermented amylase-rich fortified

| Thermal characterization of flour
Results depicted in Table 3 showed the thermodynamic properties including the onset-T O , peak-T P , conclusion-T C , melting temperature ing transition temperatures and gelatinization enthalpy reflected more ordered granules and higher degree of crystallinity, which ultimately results in higher temperatures and leading to delayed gelatinization, which is in agreement with previous studies by Tao who concluded that the higher gelatinization temperature results in delayed gelatinization; ultimately, the granules become more resistant to be gelatinized (Tao et al., 2015). Kaur (Kaur et al., 2004) reported that phosphorus content, amylose to amylopectin ratio, degree of branching, molecular conformation, and its length directly influenced the gelatinization temperature and enthalpy.

| Rheological characterization of dough
The governing factor for proper dough manufacturing is its rheology. A frequency sweep test was conducted to study the quality and processability of dough ingredients. Comprehensive understanding of dough rheology is imperative for the production of a high quality end product (Letang et al., 1999). The storage modulus, loss modulus, and tan delta comprise the parameters for rheology of dough as shown in Figure 1. Both Storage modulus and loss modulus are directly proportional to frequency. Loss modulus is always inferior to storage modulus, indicating that dough was more elastic than viscous (Narsimhan, 1994). Bacterially fermented corn flour exhibited slightly higher storage modulus and loss modulus while a little lower tan delta than native corn flour. During gelatinization, amylose, amylopectin, and starch-binding proteins and lipids leached out from the granules resulted in increased storage and loss modulus and less tan delta (Leon et al., 2006). LAB disrupt the physicochemical properties of flour and mainly hydrolyzed amylopectin into amylose which affect the rheological properties of flour (Yu et al., 2016). Enzymatic hydrolysis particularly the amylolytic enzymes led to the partial fragmentation of flour, that in turn affects the dough rheology.

| Pasting properties of flour
Pasting parameters of native and bacterial fermented samples were summarized in Table 4 (Ilowefah et al., 2015). They explicated the fact that acidification leads to breakage and more fragility of the starch granules. The pasting properties results are also closely consistent with the previous researchers, who stated that breakdown and setback of rice flour reduced after lactic acid fermentation (Yang & Tao, 2008).
The key reason for these variations was the collapse of crystal structure in the starch granules during fermentation. Amylopectin swelling leads to formation of crystalline that results in more PV. Hence,   (Chiang & Yeh, 2002). It is anticipated that viscoelastic properties are affected by some major moieties such as lipid, protein, and amylose contents. Amylopectin contributes in starch swelling however amylose is responsible for maintaining the swollen starch integrity and also suppresses the starch swelling (Jane et al., 1999). Higher amylose content is positively correlated to lower BV and PV. That is why, fermented flour swells more and degraded with substantial decrease in viscosity (Chung et al., 2011). On the other hand, the amylose leached from the system is rapidly aggregated when cooled.

F I G U R E 1 (a) Storage modulus [G'], (b) Loss modulus [G''], (c) Tan Delta
These amylose junction zones were responsible for the development of SV and FV (Barrera et al., 2013). Therefore, the extent of pasting properties of fermented flour was mainly (negatively/positively) correlated with the leaching of materials, damaged starch, and inner morphology. These aspects facilitated the interactions between molecules which involved in hydration.  Stasio et al., 2007). Smaller and irregular particles in bacterial treated flour lead to increased water absorbing properties, lower water retention capacity, that eventually leads to compact dough with better rheological properties (Shrestha et al., 2015).

| Micromorphology of corn
Bacterial treatment directly affects and alters the flour morphological characteristics. The surface of treated samples with co-culture and individual bacteria displayed more grooves and shallow indentations. It may be due to phase transformation and hydrolyzation that lead to suppressing and crumbling of granules and acquisition of irregular granules. Oh (Oh et al., 2008) reported irregular, cracked, and disrupted granules in modified corn starch compared to native starch.

| CON CLUS IONS
The study concluded that bacterial treatments, individually and as a coculture modified the quality of flour and has potential applicability. These modifications positively impacted the structural, morphological, rheological, and thermal properties of corn dough.
The dough morphology showed more grooves, irregular cracks, and

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

E TH I C A L A PPROVA L
The manuscript does not involve any human or animal studies and it is according to the guidelines.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.