Effects of Milling and Baking Processes on Bread Quality: A Case in Kamise, Ethiopia

Milling and baking are processes which determine bread quality. Proximate compositions of wheat grains and our samples from one central milling factory and bread samples from different bakeries in Kamise, Ethiopia were determined using standard methods in the laboratory. The fat and ash contents decreased by milling wheat grains; whereas, the moisture, protein and fat contents of the wheat ours increased in bread samples after baking. Highest amounts of Mg (243.25± 2.5 mg/Kg) and Ca (350.40 ± 2.2 mg/Kg) were found in the bread; while lowest concentrations of Mg (81.25 ± 0.15 mg/Kg) and a higher Ca concentrations (110.75 ± 0.2mg/Kg) were observed in our samples. Signicantly highest amount iron was obtained in the grains than in the our and bread samples; indicating loss of iron in milling and baking processes. Lead was found on bread samples than in the grain or our samples, indicative of the presence of contamination during baking processes. The variations between samples due to milling and differences in baking processes which could be as a result of differences in water used, additives and fermentation parameters. The evaluation of milling and baking processes in affecting nutrient composition are important to control the nutritional quality of bread.


Introduction
Cereals crops are one of the most important agricultural commodities in Ethiopia, where wheat is the second important cereal consumed next to maize (FAO, 2014). With regard to the human diet, the major cereals consumed include wheat (Triticum), rice (Oryza sativa), oats (Avena sativa), barley (Oldeumvulgare), rye (Secalecereale), corn (Zea mays) and millet (Panicummili-aceum). From these varieties, wheat is the most common and widely consumed cereal crops in the world (Tejera, Luis, Weller, Caballero, Gutierrez, Rubio, &Hardisson, 2013).
Wheat provides nutrients and essential elements (such as Fe, Se, Co, Cu and Zn)which are vital to our biological functions at optimal levels but also hazardous to our wellbeing in excess concentrations (Stefanovic,Filipovic & Jovanovic, 2008). It can also contain some inherently toxic heavy elements (such as As, Hg, Pb and Cd) which are rated in the priority lists from human exposure risk point of view (CERCLA, 2003).
The presence of trace essential and non-essential elements in wheat-based foods depend up on many factors, such as biological factors, environmental conditions, and methods of production and processing. Of theseprocesses such as milling, baking and process equipment such as machine types have considerable effects on the levels of some trace metals and hence nutritional quality of bread and our (Klaus and Robert, 1977).
Wheat milling involves processes of cleaning, tempering, or conditioning in water to toughen the bran in order to reduce fragmentation when it is removed (Bender, 2006).The processes of drying and conditioning with steam resulted in minerals migration into the endosperm which may increase the content of certain minerals in the our. In the process of milling, the our released from the endosperm can be separated into different grades or streams, according to neness which possesses different mineral and protein contents (Blakeney, Cracknell, Crosbie, Jefferies,&Miskelly, 2009). The implication is that milling is the critical process affecting the concentration of nutrients in wheat-derived food products (Genet, Gulelat, Ashagire, & Habtamu, 2016).
Bread making processes such as fermentation, mixing and baking also in uence the nutritional quality of bread, for example; physicochemical changes such as evaporation of water, volume expansion, and protein denaturation take place during baking affects bread quality (Arpita &Datta, 2008). Therefore, baking is also considered as an important step in bread making process which needs attention to control bread quality.
The quality of bread is in uenced not only by grain quality from the farm but also from the milling and baking processes in bread making which indicated that nutritional quality of the bread could be improved by way of constant monitoring and improving related processes. Based on these premises, this study make its concern in determining proximate composition and levels of some essential and toxic elements (such as Mg, Ca, Fe, Co, Cu, Cd and Pb)present in wheat grain, our and bread samples in Kamise Ethiopia to evaluate bread quality for human consumption and study the effect of milling and baking processes on bread quality.

Sampling and sample preparation
The grain and our samples were collected from a central wheat our provider factory in Kamise town, Ethiopia and the bread samples were purchased from Bakeries in the town, which used ours produced by this factory.
These samples were transported to the laboratory for immediate preliminary treatment and subsequent analysis using the method adapted from previous literature (Sintayehu&Merid, 2014), which was based on the wet digestion. To select an optimum condition for digestion procedure, parameters like digestion time, reagent volume and digestion temperature were optimized by varying one parameter at a time while keeping the others constant.
From the optimization procedures, an acid volume of 7 mL mixture of HNO 3 , HClO 4 and H 2 O 2 in 3:2:2 volume ratio at temperatures 112,110 and 104° C for grain, bread and our respectively for 3 hrs., were found to be the optimal condition to digest 1g of samples. Three replicate digestions were made for each sample together with blanks for every experiment. The spiked samples were also prepared in an identical way.

Sample measurement and analysis
The proximate analysis of the samples (moisture, protein and fat) was performed using the O cial Methods of Analysis of AOAC International (AOAC, 2000).The nitrogen content of the samples was determined by Kjeldhal method. The elemental analysis was measured by ICP-OES (Perkin Elmer Optima 8000). A calibration curve was constructed using a standard mix solution containing all analyzed elements. The element contents were calculated by using standard curves and the nal concentrations wereexpressed as mg/Kg.The mean and standard deviation were calculated for each experimental parameter. Differences among the samples were determined for each element by analysis of variance(ANOVA).

Method validation
The method of detection (MDL) and limits of quanti cation (LOQ)for all the metals have been calculated from the response of several replicates of the calibration reagent blank using a standard formula. The data revealed that the values for MDL have ranged 0.003 -0.063, while the corresponding LOQ value ranged from 0.01to 0.21.This indicated that the instrument used was in good sensitivity for analysis.

Protein, moisture, fat and ash content in samples of wheat grain, our and bread
The proximate composition of all samples is presented in Table1.The protein contents in samples are ranged from 11.03-13.52 %, both lowest and highest value were observed in bread samples B1 and B4 respectively. The fat content ranged from 0.50 -1.50 % where the maximum fat content, was found in wheat grain whereas, the lowest in bread samples. Table 1 The mean levels of proximate composition in the wheat whole grain and wheat re ned our samples from the factory and wheat re ned our bread samples from different bakeries in the bread supply system in Kamise in Ethiopia. Grain samples. Statistical analysis revealed signi cant differences between the bread samples. Table 2 The mean levels of minerals and toxic metals in the wheat whole grain and wheat re ned our samples from the factory and wheat re ned our bread samples from different bakeries in the bread supply system in Kamisee in Ethiopia.

Toxic metals (Cd and Pb)
The levels of toxic metals (Cd &Pb) have been investigated in all samples and the result is presented in Table2.  (Kerlies, Somers & Nelson,1984).In addition, the process of milling like; drying and conditioning also cause mineral migration of metals in to the endosperm which is the main component of our (Blakeney et al.,2009). This process critically affects the concentration of nutrients in wheat derived products and reduces the nutrient content of our by concentrating metals in residue. Usually the metal concentration in wheat grain is expected to be higher than its values in our samples by the factor of 5 to 10 times (Kerlies et al., 1984).On the other hand, the process of milling in the study sample increased the levels of Ca and Co from 103.75mg/K and 0.05 mg/Kg in grain to 110.75 mg/Kg and 0.13 mg/Kg in our respectively. However, this rising was not found statistical signi cant in Ca.
The proximate composition result showed a decrease of protein, fat and ash when grain processed to our. Milling is a process of converting grain to our, during this process bran and germ two main part of grain with high content of protein and fat will be separated from the grain (Gys, Gebruers, Sørensen, Courtin, & Delcour, 2004). In a study conducted by (Müge,Müberra, &Rabia; 2020) the ash content and protein reduced from 1.26 and 10.6 to 0.50and 9.52 respectively. Therefore, the lower value of ash and protein in the present study is expected and agrees with different studies. The moisture content was higher in our than in the grain; this increment on moisture content of our compared to grain could be due to the addition of water during milling (Genet et.al., 2016).

Baking effect
Baking processes vary in fermentation and baking condition such as time, temperature and additives in the dough mixture (Clare and Patel, 2004).As observed from the current study, process of baking affects the nal product. Even though all bakeries used similar our the result of bread analysis showed different results with a signi cant increase of Mg and Ca in all bread types. Signi cant increment was observed in magnesium content of bread where, our concentration (81.25mg/Kg) was increased with lower value 181.25mg/Kg (B3) and higher 243mg/Kg (B2) which showed an increase of two to three times. Correspondingly Ca level of bread increased from 110.75mg/Kg to 350.40mg/Kg and there was a difference not only between bread and our but within the bread too which varied from 235.75 mg/Kg to 350.40mg/Kg. Therefore, the results show baking increased the levels of Mg and Ca signi cantly. This could be the water and additives which all bakers used during baking.
The Fe content of bread varied with in breads from 12.55mg/Kg to 15.88mg/Kg, the results also reveals signi cant different between our and bread content of Fe. The level is expected to decrease in bread since baking process use water however, the increase result in some bread indicate that Fe content of bread is in uenced by baking process. In addition, content of Cu also showed different results between bread sample where value decreased and some bread content increased than the our and Co was also detected in two bread samples.
Toxic metals Cd didn't show any change during baking which remained 0.33 mg/Kgin all sample of bread and our this indicated that baking in this study didn't affect the Cd content. However, the other toxic metal in this study Pb showed a signi cance difference during baking and the amount increased from 1.48 mg/Kg ( our concentration) to 5.73 mg/Kg( B1) even the other four bread sample also show higher value than the our this indicated that baking materials and additive need serious evaluation.
Several factors like baking equipment and utilities used during the process may contribute to the metals level presence in bread samples. Hence, levels of metals usually increase in breads due to baking process than their levels in wheat our samples (Tejeraet al., 2013;Kerlies et al., 1984). From the results of this study it can be also concluded that most mineral contents compared in bread after baking was found high. Previous study (Genet et al., 2016;Magomya, Yebpella, Udiba, Amos, & Latayo 2013) argue metal introduction during Bread making may arise froma number of sources which include bioaccumulation in the raw materials used, water used by the bakeries, baking fuel types and metal pans used for baking. In the vicinity of current study, many bakeries observed using cultural method of making bread. The locally molded metal pan was commonly used by the bakeries. These used materials and poor safety in the process may have contributed to metals level in bread samples collected from these bakeries.
Proximate results also showed variation after baking compared with the our. Protein and ash value increased after baking whereas fat and moisture decreased. The increment on moisture content of our and bread compared to grain could be due to the addition of water during milling and baking (Genet et.al., 2016).

Comparison of proximate compositions and metal levels of wheat and wheat bread in literature
The levels of Ca concentration determined in bread were higher than the reported values (Eshetu et al., 2018;Anna &Malgorzata, 2011;Nermin&Senol, 2015) in Ethiopia, Turkey and Poland. But its concentration in our was lower compared to previous study Rennan et al (2008) and Tejera et al (2013). However the Mg result agreed with results found in Bangladish (Saeid ,Hoque, Kumar, Das, Muhammad, Rahman, Ahmed M, 2015). Similarly as can be seen from Table 3 the mean concentration of Mg content, in bread and our sample were lower than in (Nermin&Senol, 2015;Rennan et al .,2008 andSaeid et al.,2015) and higher than in (Anna &Malgorzata,2011).
Gahana The concentration of Fe in this study is signi cantly higher than the result obtained by (Wodaje, 2015;Tejera et.al., 2013;Anna et. al. 2011;and Eshetuet.al.2018) in wheat grains, wheat our, and wheat bread samples respectively (Table3). However, it is lower than the concentration obtained by (Gonzalo, Raquel, Rita,&Marsa,2011and Rennan et.al., 2008 in grain and our. The average result of Co metal obtained in wheat grain were lower compared to previous reported values (Wodaje et al, 2014) and agreed with the values by (Gonzalo et.al. 2011). On the other hand, the concentration of copper in this study was higher than previous studies in different countries (Gonzaloet.al.,2011 andWodaje, 2014) in grain (Rennan et.al., 2008) in our and (Oyekunle, Adekunle, Ogunfowokan, Olutona, Omolere,2014 andAnna &Malgorzata, ,2011) in bread.
Although processing does not alter its concentration, Cd level is found higher than the results of similar study as well as limit of standard values set by the Ethiopia standard (2017) and FAO/WHO (2006). In addition the values of Pb in wheat grain, our and bread were higher than the reported values (Doe, Awua ,Gyma , & Bentil ,2013;Bobere& Jacobson (2012).The study results were also higher than tolerable limit for toxicity of Pb set by Ethiopian standard (2017) The reported protein level in grain and our were higher than Bekele &Shimelis (2011) but within the range of results obtained by Genet et al., (2016) and Saeid et al.,(2015) as well as limit recommended by Ethiopian Standard (2005)for wheat our. On the other hand protein content in breads corresponds to previous reported levels (Eshetu, Atlabachew&Abebe,2018) and higher than reported value of (Odunlade et.al.,2017).
The value of fat in wheat grain, our and bread results in the current study lie in the range of previously reported work by (Genet et. al., 2016, Saeid et. al.,2015, and Eshetuet. al., 2018 respectively. The moisture content of bread was lower than the previous study (Eshetu et al., 2018) and level set by Ethiopian standard (2017) which was 12.16-28.38% and 13% respectively. However, the our moisture content was in the range of previously reported values (Genet et. al., 2016).The ash content of the our is slightly higher than the standard value of 0.60% which is recommended by Ethiopian Standards (2017).

Conclusions
Based on the above results, it could be concluded that milling and baking had a noticeable effect on the bread quality. The milling processes decreased contents of Fe, Cu, protein and fat levels, while the baking processes increased contents of Mg, Ca, and ash levels in bread making. The proximate composition and metal analysis results revealed differences after milling the grain and baking ; which could be due to water used, additives and fermentation parameters in the process of baking. The toxic metal assessment for Pb in bread found higher than the maximum permissible limit. Hence, special caution is necessary when selecting the ingredient used during the baking and milling process in order to maintain health and nutritional quality.