Accepted Manuscript Analytical Methods Analysis of Bakery Products by Laser-Induced Breakdown Spectroscopy Gonca Bilge, Ismail Hakki Boyaci, Kemal Efe Eseller, Ugur Tamer, Serhat Cakir PII: DOI: Reference:
S0308-8146(15)00280-0 http://dx.doi.org/10.1016/j.foodchem.2015.02.090 FOCH 17189
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
16 October 2014 14 January 2015 17 February 2015
Please cite this article as: Bilge, G., Boyaci, I.H., Eseller, K.E., Tamer, U., Cakir, S., Analysis of Bakery Products by Laser-Induced Breakdown Spectroscopy, Food Chemistry (2015), doi: http://dx.doi.org/10.1016/j.foodchem. 2015.02.090
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Title: Analysis of Bakery Products by Laser-Induced Breakdown Spectroscopy
Name of the Authors: Gonca Bilgea, Ismail Hakki Boyacia,b,*, Kemal Efe Esellerc, Ugur Tamerd, Serhat Cakire
Affiliation of the Authors: a
Department of Food Engineering, Hacettepe University, Beytepe 06800, Ankara, Turkey
a
[email protected]
b
Food Research Center, Hacettepe University, Beytepe, 06800 Ankara, Turkey
b
[email protected]
c
Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara,
Turkey c
[email protected]
d
Department of Analytical Chemistry, Faculty of Pharmacy, Gazi University, 06330, Ankara,
Turkey d
[email protected]
e
Department of Physics, Middle East Technical University, 06800, Ankara, Turkey
e
[email protected]
*Corresponding Author: Prof. Dr. Ismail Hakki Boyaci Food Research Center, Hacettepe University, Beytepe,06800 Ankara, Turkey Phone: +90 312 297 61 46
1
Fax: +90 312 299 21 23 e-mail: [email protected]
ABSTRACT
In this study, we focused on the detection of Na in bakery products by using laser-induced breakdown spectroscopy (LIBS) as a quick and simple method. LIBS experiments were performed to examine the Na at 589 nm to quantify NaCl. A series of standard bread sample pellets containing various concentrations of NaCl (0.025–3.5%) were used to construct the calibration curves and to determine the detection limits of the measurements. Calibration graphs were drawn to indicate functions of NaCl and Na concentrations, which showed good linearity in the range of 0.025–3.5% NaCl and 0.01–1.4% Na concentrations with correlation coefficients (R2) values greater than 0.98 and 0.96. The obtained detection limits for NaCl and Na were 175 ppm and 69 ppm, respectively. Performed experimental studies showed that LIBS is a convenient method for commercial bakery products to quantify NaCl concentrations as a rapid and in situ technique.
Keywords: Laser-induced breakdown spectroscopy (LIBS); Bakery products; Detection of NaCl; Detection of Na; Titration; Atomic absorption spectroscopy (AAS)
2
1. Introduction
70-75 % of the total sodium chloride (NaCl) intake in the human diet is obtained from processed foods (James et al., 1987), out of which cereal and cereal products constitute approximately 30% (FSAI, 2005). Hence, reducing the NaCl content in bakery products could decrease the overall NaCl intake in human diet. Since a large number of deaths related to excessive salt intake are due to the consumption of bread with high NaCl contents, this quantity must be reduced (He & MacGregor, 2007). Moreover, studies have shown that excessive dietary Na intake increases blood pressure (Elliott et al., 1996) and may cause strokes (Xie et al., 1992) and coronary heart diseases (Tuomilehto et al., 2001). However, many bakers are reluctant to reduce the salt content in bread because it may lead to insufficient dough strength and undesirable sensory profiles. NaCl is an important ingredient in bakery products as it contributes to the dough-making process by regulating the fermentation rate, strengthening the dough and adding to the taste of the bread (Salovaara, 1982).
In addition, NaCl is a quality control indicator in bakery products. The NaCl level in foods should adhere to Codex Alimentarius standards. Therefore, NaCl detection is critical in bakery products. Several methods are used to determine the NaCl content in bakery products. Some of these methods are based on the presence of Na ions, whereas others are based on that of Cl ions (Plácido et al., 2012). The potentiometric method, which utilizes ion selective Na or Cl electrodes, is most widely used to determine the amount of salt in food products. Other
3
commonly used methods include titration to determine Cl levels and flame atomic absorption spectrometry to determine Na content (Capuano et al., 2013, Smith & Haider, 2014). These methods are time consuming, and their application procedures are difficult. They also require sample preparation, which makes them unsuitable for in situ and point detection analyses. Therefore, laser-induced breakdown spectroscopy (LIBS) has emerged as a valuable tool for quick and in situ analyses (Choi et al., 2013; Singh et al., 2008).
LIBS is an optical emission spectroscopy technique based on laser-produced plasma, in which a laser beam excites and intensively heats a small volume of the sample. The heated sample is taken to a gaseous plasma state and is broken down into atoms, which produces a characteristic light. This light is analysed spectrally; and through calibration, the intensity of the spectra indicates the concentration of the elements in the sample (St-Onge et al., 2004).
Most materials, such as solids, liquids and gases, can be analysed quantitatively and qualitatively by using LIBS (Miziolek et al., 2006; Pandhija & Rai, 2008; Pandhija & Rai, 2009; Rai & Rai, 2008; Singh et al., 2009). This technology has many advantages including quick and real-time analysis through in situ field operations without the need for sample preparation (Pandhija et al., 2010). Moreover, its application has expanded to the fields such as metallurgy, mining, environmental analysis and pharmacology (Radziemski, 1994; Rusak et al., 1997; Sneddon & Lee, 1999; St-Onge et al., 2002; Tognoni et al., 2002). However, there are few LIBS applications in food technology, some of which are in food safety applications such as detection of pesticides in powdered spinach and rice pellets (Kim et al., 2012) and identification of Escherichia coli O157:H7 and Salmonella enterica in foods and on surfaces (Multari et al., 2013). Other food applications include the examination of quality properties in foods, such as the determination of macro and micronutrients in sugarcane
4
leaves (Nunes et al., 2010), and direct determination of trace elements in starch-based flours (Cho et al., 2001).
In this study, detection of Na in bakery products is conducted by using a simple and inexpensive LIBS system for quantifying NaCl concentration to demonstrate the applicability of LIBS for the analysis of NaCl in bakery products. To this end, standard bread samples are chosen as the model system. In addition, commercial bakery products (crackers, biscuits and different types of breads) are included in this study. Further, titration and atomic absorption spectroscopy methods were used to confirm NaCl and Na concentrations in the samples.
2. Experimental Methods
2.1.
Reagents
Silver nitrate (AgNO3), potassium chromate (K2CrO4), nitric acid (HNO3) and NaCl were purchased from Sigma Aldrich (Steinheim, Germany). Bread flour and bread additive yeast were purchased from a local market. Standard bread samples were produced according to American Association of Cereal Chemists (AACC) Optimized Straight-Dough Bread-Baking Method No. 10-10.03 (AACCI, 2010). Twelve standard bread samples were produced by this method at various salt concentrations ranged between 0.025–3.5%. The bread dough, comprising 100 g flour, 0.2 g bread additive, 25 ml of 8% yeast solution, 25 ml salt solution at various concentrations and 10 ml water, was mixed by hand for 15 min. Dough pieces were rounded and incubated for 30 min during the first fermentation. After 30 min, the dough was punched and incubated for 30 min during the second fermentation. Then, the dough was
5
formed, placed into tins for the final fermentation and incubated for 55 min at 30 °C. Subsequently, the bread was baked for 30 min at 210 °C, taken out of the oven and cooled.
2.2.
LIBS Instrumentation
LIBS spectra were recorded using a Quantel-Big Sky Nd:YAG-laser (Bozeman, MT, USA), HR 2000 Oceanoptics Spectrograph (Dunedin, FL, USA) and Stanford Research System Delay Generator SRS DG535 (Cleveland, OH, USA). The experimental setup was presented in Fig. 1. The laser was operated at a fundamental wavelength of 532 nm and used for sample ablation. It was then run in the Q-switched mode at a repetition rate of 1 Hz. External gated detection was performed at 0.5µs gate delay and 20µs gate width. Spectrograph was externally triggered from the laser at every pulse with the delay generator. The laser energy was adjusted at 14 mJ/pulse. Samples were measured by the LIBS technique by scanning five different locations and four excitations per location. LIBS experiments were performed in order to examine the Na content at 589 nm in bakery products to quantify NaCl, which was necessary to obtain calibration curves. This was achieved by preparing standard bread samples with various known concentrations of NaCl ranged between 0.025% and 3.5%. For the LIBS measurements, bread samples were dried at 105 °C for 2 h and cooled in a desiccator. Then, 400 mg of dried powdered bread samples were formed as a pellet under 10 tons of pressure using a pellet press machine. Samples were measured by using LIBS technique in triplicate, scanning five different locations and five excitations per location. The calibration curves for the Na line at 589 nm were obtained by plotting its intensity versus NaCl and Na concentrations in each sample.
6
2.3.
NaCl detection in bakery products via titration (Mohr Method)
NaCl content of standard bread samples and commercial samples were measured by Mohr Method (reference method for NaCl measurements), which is a conventional method for NaCl detection in bakery samples (Serna-Saldivar, 2012). It is based on the determination of chloride ion concentration by titration with AgNO3. In this method, samples were dried at 105 °C for 2 h, and 10 g of the dried samples were balanced into a conical flask. Following this process, hot water was added to the flask, and the flask was shaken thoroughly. The solution was sealed in a 250 ml conical flask using filter paper. Then, the conical flask was washed with hot water, which was poured onto the filter paper. As a result, salt on the filter paper was passed to the flask. After the solution in the flask was cooled, it was filled with deionized water till the level reached the 100 ml volume line. Because the reaction is better in alkaline media, a NaOH solution was used to make the solution alkaline. K2CrO4 (2–3 drops) was added, and the solution was titrated with AgNO3. When all chloride ions were precipitated, the titration was terminated. Then, additional AgNO3 ions reacted with the chromate ions of the K2CrO4, forming a red–brown precipitate of silver chromate (Serna-Saldivar, 2012). The percentage of NaCl in the samples was calculated according to the following equation:
% =
2.4.
× ×. × ! " #$
× 2.5
(1)
Na detection in bakery products by atomic absorption spectroscopy
Na content of standard bread samples and commercial samples were measured using atomic absorption spectroscopy (the reference method for Na measurements). Samples were prepared according to the EPA Method 3051A by microwave-assisted digestion for atomic absorption 7
spectroscopy measurements (EPA, 1994). First, 0.3 g of the dried sample and 10 ml concentrated HNO3 were placed in a fluorocarbon polymer vessel. The samples were extracted by heating with a laboratory microwave unit. Then, the vessel was sealed and heated in the microwave unit. After cooling, the vessel contents were filtered with Whatman No. 1 filter paper and diluted in 100 ml of deionized water. The atomic absorption spectra for Na were recorded by the ATI-UNICAM 939 AA Spectrometer (Cambridge, UK) at 589 nm.
3. Results and discussion
In situ LIBS spectra of the bakery products were obtained by means of focusing the laser beam on the sample surface. In this study, our attention was centred on the variations of Na concentration only at 589 nm. In the LIBS method, the intensity of the spectra is related to the concentration of elements. Fig. 2 demonstrates the LIBS spectra of standard bread samples with various NaCl concentrations, which shows that increasing the NaCl concentration led to an increase in the Na emission at 589 nm. Narrow and intense analytical atomic Na lines appeared even in the NaCl-free bread. The spectral line at 765 and 769 nm is related to the atomic lines of magnesium (Mg) and potassium (K). The other low intensity Na lines are seen at 568 nm and 818 nm (Fig. 2) (NIST, 1995).
LIBS spectra of 12 different standard samples of known concentrations of NaCl (0.025–3.5%) were recorded. The samples were analysed in triplicate, scanning five different locations and five excitations per location to obtain the best signal-to-noise ratio. NaCl and Na concentrations in the standard bread samples were confirmed by titration (Mohr method) and atomic absorption methods, both of which are used as reference methods. Na and NaCl content in standard bread samples were measured as higher than the added values since water
8
was vaporized by thermal process, and Na and Cl ions became more concentrated. After that, the calibration curves were drawn for Na at 589 nm by plotting the emission intensity of Na in the LIBS spectra vs the NaCl and Na concentrations of the standard bread samples (Fig. 3).
The LIBS spectra of samples, including 25 shots for each sample (five locations and five excitations per location), were fitted to place the spectra to origin. Then, the peak intensities of Na emission at 589 nm were calculated by averaging the values according to the normal distribution graph in which ±5% deviations were not taken into account. Good linear calibration curves were obtained with correlation coefficients (R2) values of 0.981 for the NaCl calibration graph and 0.962 for the Na calibration graph at 0.025–3.5% NaCl and 0.01– 1.4% Na concentrations, respectively. As indicated in the individual error bars in the figures, these values were determined according to the distribution of three measurements for each concentration. The reproducibility for the three measurements yielded approximately 10% relative standard deviation (RSD). The limit of detection (LOD) for NaCl and Na was calculated to evaluate the sensitivity of the LIBS system according to the equation (Gondal et al., 2014):
'() = 2 ×
*.+. *
(2)
LOD: limit of detection S.D. : Standard deviation of the response S: Slope of the calibration curve
LOD values for NaCl and Na were calculated as 175 ppm and 69 ppm, respectively In some studies, authors obtained lower detection limits, as low as 5 ppm (Hussain & Gondal, 2013), and even 0.1 ppb by using dual-pulse and crossed-beam Nd:YAG lasers for Na on a 9
water film (Kuwako et al., 2003). The detection limits for the present study are not as low as the ones in other techniques because of the matrix effect of the bread samples. In addition, the variations of LIBS spectra from shot to shot negatively affected the detection limits due to laser energy fluctuations. However, obtained results for Na and NaCl measurements in bakery products were found to be in accordance with the regulations on salt contents in food products, which is1.5% NaCl for breads in Turkey. The detection limits of LIBS for Na and NaCl are adequate for detection of even dietary products.
The reliability of the LIBS technique was confirmed through comparison with the reference methods. Eight commercial samples of biscuits, crackers and different types of bread with various NaCl concentrations were obtained from a market to evaluate the reliability of LIBS technique and to compare with the results of the reference methods. The measured Na intensities for the commercial samples were calculated as NaCl and Na concentrations according to calibration graphs in Fig. 3. The comparison results are presented in Table1. The standard deviation was based on two or more replicate analyses of each sample. The calibration curves for NaCl and Na concentrations can be used for accuracy assessment. However, the calibration curve method depends on the known accuracies of the NaCl concentrations in the standard bread samples and on how close the matrices of standard bread samples are to those of the commercial samples. For minimizing the errors of the matrix effect, a standard addition method was performed. As it can be inferred from Table1, the LIBS technique is reliable for determination of NaCl and Na concentrations in the commercial samples. The accuracy of LIBS compared to AAS and titration for bakery products was evaluated with relative accuracy (RA) value. RA is calculated as fallows (Gondal et al., 2010):
,- =
|/|0*.+.× 1.234 /√7 8
(3) 10
d: Difference between LIBS measurement and reference method S.D. : Standard deviation of LIBS measurement n: The number of measurements t0.975: t value at 2.5% error confidence
The atomic absorption results of the commercial samples as a function of Na level showed low RA values at about 0.19, which means that LIBS is a quite acceptable technique for Na measurements in bakery products (Table1) (Gondal et al., 2009).
The calculated NaCl concentrations of the commercial samples showed low correlation with the titration results due to some coloured products. Coloured products, such as biscuits, caused erroneous results in the titration method because the colour of the K2CrO4 indicator resembles that of the biscuits, and it wasn’t possible to observe the real turning point. To overcome this drawback of Mohr’s titration method, de-coloration pretreatment is required for deep coloured foods (Skoog et al., 2000). Therefore, these products were de-colored as pretreatment of titration method. 10 g of sample was treated by stirring with 1 g of activated carbon and distilled water under boiling for 10 min. After that, samples were filtered to remove activated carbon, diluted and titrated. The titration results of the commercial samples as a function of NaCl level showed low RA values at about 0.21, which means that LIBS is an applicabletechnique for NaCl measurements in bakery products (Table1).
The ratio of NaCl to Na concentrations in the commercial samples showed that the Na concentration originated mainly from NaCl in the ingredients. Therefore, following the Na emission intensities by LIBS and correlating the results as a function of NaCl concentration is convenient for analyzing Na content in bakery products. Because the difference between the
11
reference methods and LIBS is not significant, it can be concluded that LIBS provides accurate results and is an adequate alternative for determination of Na and NaCl in the analyzed samples. Working with LIBS has great advantage over the atomic absorption spectroscopy and titration methods; and thus, analysis can be performed much easier, faster, cheaper and in situ.
4. Conclusion
In many countries salt (NaCl) has been considered as the main cause of several health damages such as kidney and heart diseases, and hypertension. Thus, the World Health Organization (WHO) has set a maximum salt intake regulation to reduce daily salt intake of adults. In accordance with these recommendations, many countries have developed their own guidelines and controlled salt content in different kinds of food. Bakery products, especially bread, have been identified as the main contributor to the daily salt intake. Despite the limitations of salt content, some products contain exceeding amounts of NaCl. Salt measurements in bakery products are important not only for health, but also for quality control in food industry. Using LIBS is an alternative approach for faster control of bakery products. It could become a valuable tool to quantify NaCl by following Na emission at 589 nm. The results of the present study demonstrated that LIBS can be used to analyze Na and NaCl contents in bakery products such as some kinds of breads, biscuits and crackers. Reliability of LIBS was confirmed by reference methods, namely the titration and atomic emission spectroscopy methods for commercial samples. Unlike the reference methods, LIBS does not require sample preparation and can analyse samples in less than 1 min. Furthermore, the coloured products such as biscuits which cause false results in titration method can be analyzed by LIBS accurately. Detection limits of LIBS is convenient even for low salty
12
bakery products, therefore, it can be used safely as a rapid and real-time monitoring, and an in situ technique for Na and NaCl measurements.
Acknowledgements
This research is partly supported by TUBITAK 2232 research grant Project No. 113C034. In addition, authors would like to thank Assoc. Prof. Dr. Aysel Berkkan for performing atomic absorption spectroscopy analysis.
References
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James, W. P., Ralph, A., Sanchez Castillo, C. P. (1987). The dominance of salt in manufactured food in the sodium intake of affluent societies. Lancet, 8530, 426–429. Kim, G., Kwak,
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Figure captions Figure 1. Experimental setup of laser-induced breakdown spectroscopy (LIBS). Figure 2. LIBS spectra of standard bread samples. Figure 3. Calibration curves of standard bread samples for the atomic Na signal as (a) a function of NaCl concentration, which was determined by titration, and (b) as a function of Na concentration, which was determined by atomic absorption method.
Table Captions Table1. Comparative results between reference methods (AAS, titration) and LIBS for commercial products.
18
19
20
21
Highlights • • • •
We detected Na and NaCl in bakery products using LIBS LIBS is a rapid and in situ method for Na and NaCl measurements of bakery products LIBS’ results were confirmed with reference methods LIBS owns low detection limits in bakery products
22
S0308-8146(15)00280-0 http://dx.doi.org/10.1016/j.foodchem.2015.02.090 FOCH 17189
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
16 October 2014 14 January 2015 17 February 2015
Please cite this article as: Bilge, G., Boyaci, I.H., Eseller, K.E., Tamer, U., Cakir, S., Analysis of Bakery Products by Laser-Induced Breakdown Spectroscopy, Food Chemistry (2015), doi: http://dx.doi.org/10.1016/j.foodchem. 2015.02.090
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Title: Analysis of Bakery Products by Laser-Induced Breakdown Spectroscopy
Name of the Authors: Gonca Bilgea, Ismail Hakki Boyacia,b,*, Kemal Efe Esellerc, Ugur Tamerd, Serhat Cakire
Affiliation of the Authors: a
Department of Food Engineering, Hacettepe University, Beytepe 06800, Ankara, Turkey
a
[email protected]
b
Food Research Center, Hacettepe University, Beytepe, 06800 Ankara, Turkey
b
[email protected]
c
Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara,
Turkey c
[email protected]
d
Department of Analytical Chemistry, Faculty of Pharmacy, Gazi University, 06330, Ankara,
Turkey d
[email protected]
e
Department of Physics, Middle East Technical University, 06800, Ankara, Turkey
e
[email protected]
*Corresponding Author: Prof. Dr. Ismail Hakki Boyaci Food Research Center, Hacettepe University, Beytepe,06800 Ankara, Turkey Phone: +90 312 297 61 46
1
Fax: +90 312 299 21 23 e-mail: [email protected]
ABSTRACT
In this study, we focused on the detection of Na in bakery products by using laser-induced breakdown spectroscopy (LIBS) as a quick and simple method. LIBS experiments were performed to examine the Na at 589 nm to quantify NaCl. A series of standard bread sample pellets containing various concentrations of NaCl (0.025–3.5%) were used to construct the calibration curves and to determine the detection limits of the measurements. Calibration graphs were drawn to indicate functions of NaCl and Na concentrations, which showed good linearity in the range of 0.025–3.5% NaCl and 0.01–1.4% Na concentrations with correlation coefficients (R2) values greater than 0.98 and 0.96. The obtained detection limits for NaCl and Na were 175 ppm and 69 ppm, respectively. Performed experimental studies showed that LIBS is a convenient method for commercial bakery products to quantify NaCl concentrations as a rapid and in situ technique.
Keywords: Laser-induced breakdown spectroscopy (LIBS); Bakery products; Detection of NaCl; Detection of Na; Titration; Atomic absorption spectroscopy (AAS)
2
1. Introduction
70-75 % of the total sodium chloride (NaCl) intake in the human diet is obtained from processed foods (James et al., 1987), out of which cereal and cereal products constitute approximately 30% (FSAI, 2005). Hence, reducing the NaCl content in bakery products could decrease the overall NaCl intake in human diet. Since a large number of deaths related to excessive salt intake are due to the consumption of bread with high NaCl contents, this quantity must be reduced (He & MacGregor, 2007). Moreover, studies have shown that excessive dietary Na intake increases blood pressure (Elliott et al., 1996) and may cause strokes (Xie et al., 1992) and coronary heart diseases (Tuomilehto et al., 2001). However, many bakers are reluctant to reduce the salt content in bread because it may lead to insufficient dough strength and undesirable sensory profiles. NaCl is an important ingredient in bakery products as it contributes to the dough-making process by regulating the fermentation rate, strengthening the dough and adding to the taste of the bread (Salovaara, 1982).
In addition, NaCl is a quality control indicator in bakery products. The NaCl level in foods should adhere to Codex Alimentarius standards. Therefore, NaCl detection is critical in bakery products. Several methods are used to determine the NaCl content in bakery products. Some of these methods are based on the presence of Na ions, whereas others are based on that of Cl ions (Plácido et al., 2012). The potentiometric method, which utilizes ion selective Na or Cl electrodes, is most widely used to determine the amount of salt in food products. Other
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commonly used methods include titration to determine Cl levels and flame atomic absorption spectrometry to determine Na content (Capuano et al., 2013, Smith & Haider, 2014). These methods are time consuming, and their application procedures are difficult. They also require sample preparation, which makes them unsuitable for in situ and point detection analyses. Therefore, laser-induced breakdown spectroscopy (LIBS) has emerged as a valuable tool for quick and in situ analyses (Choi et al., 2013; Singh et al., 2008).
LIBS is an optical emission spectroscopy technique based on laser-produced plasma, in which a laser beam excites and intensively heats a small volume of the sample. The heated sample is taken to a gaseous plasma state and is broken down into atoms, which produces a characteristic light. This light is analysed spectrally; and through calibration, the intensity of the spectra indicates the concentration of the elements in the sample (St-Onge et al., 2004).
Most materials, such as solids, liquids and gases, can be analysed quantitatively and qualitatively by using LIBS (Miziolek et al., 2006; Pandhija & Rai, 2008; Pandhija & Rai, 2009; Rai & Rai, 2008; Singh et al., 2009). This technology has many advantages including quick and real-time analysis through in situ field operations without the need for sample preparation (Pandhija et al., 2010). Moreover, its application has expanded to the fields such as metallurgy, mining, environmental analysis and pharmacology (Radziemski, 1994; Rusak et al., 1997; Sneddon & Lee, 1999; St-Onge et al., 2002; Tognoni et al., 2002). However, there are few LIBS applications in food technology, some of which are in food safety applications such as detection of pesticides in powdered spinach and rice pellets (Kim et al., 2012) and identification of Escherichia coli O157:H7 and Salmonella enterica in foods and on surfaces (Multari et al., 2013). Other food applications include the examination of quality properties in foods, such as the determination of macro and micronutrients in sugarcane
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leaves (Nunes et al., 2010), and direct determination of trace elements in starch-based flours (Cho et al., 2001).
In this study, detection of Na in bakery products is conducted by using a simple and inexpensive LIBS system for quantifying NaCl concentration to demonstrate the applicability of LIBS for the analysis of NaCl in bakery products. To this end, standard bread samples are chosen as the model system. In addition, commercial bakery products (crackers, biscuits and different types of breads) are included in this study. Further, titration and atomic absorption spectroscopy methods were used to confirm NaCl and Na concentrations in the samples.
2. Experimental Methods
2.1.
Reagents
Silver nitrate (AgNO3), potassium chromate (K2CrO4), nitric acid (HNO3) and NaCl were purchased from Sigma Aldrich (Steinheim, Germany). Bread flour and bread additive yeast were purchased from a local market. Standard bread samples were produced according to American Association of Cereal Chemists (AACC) Optimized Straight-Dough Bread-Baking Method No. 10-10.03 (AACCI, 2010). Twelve standard bread samples were produced by this method at various salt concentrations ranged between 0.025–3.5%. The bread dough, comprising 100 g flour, 0.2 g bread additive, 25 ml of 8% yeast solution, 25 ml salt solution at various concentrations and 10 ml water, was mixed by hand for 15 min. Dough pieces were rounded and incubated for 30 min during the first fermentation. After 30 min, the dough was punched and incubated for 30 min during the second fermentation. Then, the dough was
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formed, placed into tins for the final fermentation and incubated for 55 min at 30 °C. Subsequently, the bread was baked for 30 min at 210 °C, taken out of the oven and cooled.
2.2.
LIBS Instrumentation
LIBS spectra were recorded using a Quantel-Big Sky Nd:YAG-laser (Bozeman, MT, USA), HR 2000 Oceanoptics Spectrograph (Dunedin, FL, USA) and Stanford Research System Delay Generator SRS DG535 (Cleveland, OH, USA). The experimental setup was presented in Fig. 1. The laser was operated at a fundamental wavelength of 532 nm and used for sample ablation. It was then run in the Q-switched mode at a repetition rate of 1 Hz. External gated detection was performed at 0.5µs gate delay and 20µs gate width. Spectrograph was externally triggered from the laser at every pulse with the delay generator. The laser energy was adjusted at 14 mJ/pulse. Samples were measured by the LIBS technique by scanning five different locations and four excitations per location. LIBS experiments were performed in order to examine the Na content at 589 nm in bakery products to quantify NaCl, which was necessary to obtain calibration curves. This was achieved by preparing standard bread samples with various known concentrations of NaCl ranged between 0.025% and 3.5%. For the LIBS measurements, bread samples were dried at 105 °C for 2 h and cooled in a desiccator. Then, 400 mg of dried powdered bread samples were formed as a pellet under 10 tons of pressure using a pellet press machine. Samples were measured by using LIBS technique in triplicate, scanning five different locations and five excitations per location. The calibration curves for the Na line at 589 nm were obtained by plotting its intensity versus NaCl and Na concentrations in each sample.
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2.3.
NaCl detection in bakery products via titration (Mohr Method)
NaCl content of standard bread samples and commercial samples were measured by Mohr Method (reference method for NaCl measurements), which is a conventional method for NaCl detection in bakery samples (Serna-Saldivar, 2012). It is based on the determination of chloride ion concentration by titration with AgNO3. In this method, samples were dried at 105 °C for 2 h, and 10 g of the dried samples were balanced into a conical flask. Following this process, hot water was added to the flask, and the flask was shaken thoroughly. The solution was sealed in a 250 ml conical flask using filter paper. Then, the conical flask was washed with hot water, which was poured onto the filter paper. As a result, salt on the filter paper was passed to the flask. After the solution in the flask was cooled, it was filled with deionized water till the level reached the 100 ml volume line. Because the reaction is better in alkaline media, a NaOH solution was used to make the solution alkaline. K2CrO4 (2–3 drops) was added, and the solution was titrated with AgNO3. When all chloride ions were precipitated, the titration was terminated. Then, additional AgNO3 ions reacted with the chromate ions of the K2CrO4, forming a red–brown precipitate of silver chromate (Serna-Saldivar, 2012). The percentage of NaCl in the samples was calculated according to the following equation:
% =
2.4.
× ×. × ! " #$
× 2.5
(1)
Na detection in bakery products by atomic absorption spectroscopy
Na content of standard bread samples and commercial samples were measured using atomic absorption spectroscopy (the reference method for Na measurements). Samples were prepared according to the EPA Method 3051A by microwave-assisted digestion for atomic absorption 7
spectroscopy measurements (EPA, 1994). First, 0.3 g of the dried sample and 10 ml concentrated HNO3 were placed in a fluorocarbon polymer vessel. The samples were extracted by heating with a laboratory microwave unit. Then, the vessel was sealed and heated in the microwave unit. After cooling, the vessel contents were filtered with Whatman No. 1 filter paper and diluted in 100 ml of deionized water. The atomic absorption spectra for Na were recorded by the ATI-UNICAM 939 AA Spectrometer (Cambridge, UK) at 589 nm.
3. Results and discussion
In situ LIBS spectra of the bakery products were obtained by means of focusing the laser beam on the sample surface. In this study, our attention was centred on the variations of Na concentration only at 589 nm. In the LIBS method, the intensity of the spectra is related to the concentration of elements. Fig. 2 demonstrates the LIBS spectra of standard bread samples with various NaCl concentrations, which shows that increasing the NaCl concentration led to an increase in the Na emission at 589 nm. Narrow and intense analytical atomic Na lines appeared even in the NaCl-free bread. The spectral line at 765 and 769 nm is related to the atomic lines of magnesium (Mg) and potassium (K). The other low intensity Na lines are seen at 568 nm and 818 nm (Fig. 2) (NIST, 1995).
LIBS spectra of 12 different standard samples of known concentrations of NaCl (0.025–3.5%) were recorded. The samples were analysed in triplicate, scanning five different locations and five excitations per location to obtain the best signal-to-noise ratio. NaCl and Na concentrations in the standard bread samples were confirmed by titration (Mohr method) and atomic absorption methods, both of which are used as reference methods. Na and NaCl content in standard bread samples were measured as higher than the added values since water
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was vaporized by thermal process, and Na and Cl ions became more concentrated. After that, the calibration curves were drawn for Na at 589 nm by plotting the emission intensity of Na in the LIBS spectra vs the NaCl and Na concentrations of the standard bread samples (Fig. 3).
The LIBS spectra of samples, including 25 shots for each sample (five locations and five excitations per location), were fitted to place the spectra to origin. Then, the peak intensities of Na emission at 589 nm were calculated by averaging the values according to the normal distribution graph in which ±5% deviations were not taken into account. Good linear calibration curves were obtained with correlation coefficients (R2) values of 0.981 for the NaCl calibration graph and 0.962 for the Na calibration graph at 0.025–3.5% NaCl and 0.01– 1.4% Na concentrations, respectively. As indicated in the individual error bars in the figures, these values were determined according to the distribution of three measurements for each concentration. The reproducibility for the three measurements yielded approximately 10% relative standard deviation (RSD). The limit of detection (LOD) for NaCl and Na was calculated to evaluate the sensitivity of the LIBS system according to the equation (Gondal et al., 2014):
'() = 2 ×
*.+. *
(2)
LOD: limit of detection S.D. : Standard deviation of the response S: Slope of the calibration curve
LOD values for NaCl and Na were calculated as 175 ppm and 69 ppm, respectively In some studies, authors obtained lower detection limits, as low as 5 ppm (Hussain & Gondal, 2013), and even 0.1 ppb by using dual-pulse and crossed-beam Nd:YAG lasers for Na on a 9
water film (Kuwako et al., 2003). The detection limits for the present study are not as low as the ones in other techniques because of the matrix effect of the bread samples. In addition, the variations of LIBS spectra from shot to shot negatively affected the detection limits due to laser energy fluctuations. However, obtained results for Na and NaCl measurements in bakery products were found to be in accordance with the regulations on salt contents in food products, which is1.5% NaCl for breads in Turkey. The detection limits of LIBS for Na and NaCl are adequate for detection of even dietary products.
The reliability of the LIBS technique was confirmed through comparison with the reference methods. Eight commercial samples of biscuits, crackers and different types of bread with various NaCl concentrations were obtained from a market to evaluate the reliability of LIBS technique and to compare with the results of the reference methods. The measured Na intensities for the commercial samples were calculated as NaCl and Na concentrations according to calibration graphs in Fig. 3. The comparison results are presented in Table1. The standard deviation was based on two or more replicate analyses of each sample. The calibration curves for NaCl and Na concentrations can be used for accuracy assessment. However, the calibration curve method depends on the known accuracies of the NaCl concentrations in the standard bread samples and on how close the matrices of standard bread samples are to those of the commercial samples. For minimizing the errors of the matrix effect, a standard addition method was performed. As it can be inferred from Table1, the LIBS technique is reliable for determination of NaCl and Na concentrations in the commercial samples. The accuracy of LIBS compared to AAS and titration for bakery products was evaluated with relative accuracy (RA) value. RA is calculated as fallows (Gondal et al., 2010):
,- =
|/|0*.+.× 1.234 /√7 8
(3) 10
d: Difference between LIBS measurement and reference method S.D. : Standard deviation of LIBS measurement n: The number of measurements t0.975: t value at 2.5% error confidence
The atomic absorption results of the commercial samples as a function of Na level showed low RA values at about 0.19, which means that LIBS is a quite acceptable technique for Na measurements in bakery products (Table1) (Gondal et al., 2009).
The calculated NaCl concentrations of the commercial samples showed low correlation with the titration results due to some coloured products. Coloured products, such as biscuits, caused erroneous results in the titration method because the colour of the K2CrO4 indicator resembles that of the biscuits, and it wasn’t possible to observe the real turning point. To overcome this drawback of Mohr’s titration method, de-coloration pretreatment is required for deep coloured foods (Skoog et al., 2000). Therefore, these products were de-colored as pretreatment of titration method. 10 g of sample was treated by stirring with 1 g of activated carbon and distilled water under boiling for 10 min. After that, samples were filtered to remove activated carbon, diluted and titrated. The titration results of the commercial samples as a function of NaCl level showed low RA values at about 0.21, which means that LIBS is an applicabletechnique for NaCl measurements in bakery products (Table1).
The ratio of NaCl to Na concentrations in the commercial samples showed that the Na concentration originated mainly from NaCl in the ingredients. Therefore, following the Na emission intensities by LIBS and correlating the results as a function of NaCl concentration is convenient for analyzing Na content in bakery products. Because the difference between the
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reference methods and LIBS is not significant, it can be concluded that LIBS provides accurate results and is an adequate alternative for determination of Na and NaCl in the analyzed samples. Working with LIBS has great advantage over the atomic absorption spectroscopy and titration methods; and thus, analysis can be performed much easier, faster, cheaper and in situ.
4. Conclusion
In many countries salt (NaCl) has been considered as the main cause of several health damages such as kidney and heart diseases, and hypertension. Thus, the World Health Organization (WHO) has set a maximum salt intake regulation to reduce daily salt intake of adults. In accordance with these recommendations, many countries have developed their own guidelines and controlled salt content in different kinds of food. Bakery products, especially bread, have been identified as the main contributor to the daily salt intake. Despite the limitations of salt content, some products contain exceeding amounts of NaCl. Salt measurements in bakery products are important not only for health, but also for quality control in food industry. Using LIBS is an alternative approach for faster control of bakery products. It could become a valuable tool to quantify NaCl by following Na emission at 589 nm. The results of the present study demonstrated that LIBS can be used to analyze Na and NaCl contents in bakery products such as some kinds of breads, biscuits and crackers. Reliability of LIBS was confirmed by reference methods, namely the titration and atomic emission spectroscopy methods for commercial samples. Unlike the reference methods, LIBS does not require sample preparation and can analyse samples in less than 1 min. Furthermore, the coloured products such as biscuits which cause false results in titration method can be analyzed by LIBS accurately. Detection limits of LIBS is convenient even for low salty
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bakery products, therefore, it can be used safely as a rapid and real-time monitoring, and an in situ technique for Na and NaCl measurements.
Acknowledgements
This research is partly supported by TUBITAK 2232 research grant Project No. 113C034. In addition, authors would like to thank Assoc. Prof. Dr. Aysel Berkkan for performing atomic absorption spectroscopy analysis.
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Figure captions Figure 1. Experimental setup of laser-induced breakdown spectroscopy (LIBS). Figure 2. LIBS spectra of standard bread samples. Figure 3. Calibration curves of standard bread samples for the atomic Na signal as (a) a function of NaCl concentration, which was determined by titration, and (b) as a function of Na concentration, which was determined by atomic absorption method.
Table Captions Table1. Comparative results between reference methods (AAS, titration) and LIBS for commercial products.
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19
20
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Highlights • • • •
We detected Na and NaCl in bakery products using LIBS LIBS is a rapid and in situ method for Na and NaCl measurements of bakery products LIBS’ results were confirmed with reference methods LIBS owns low detection limits in bakery products
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