Introduction

The history of broths can be traced back to the Paleolithic period [1], and they continued to be of significance during the Middle Ages, where they were not only used in culinary practices but also played a role in traditional medicine [2]. There are studies that have provided support for the use of chicken broth in the treatment of upper respiratory tract inflammation [3, 4].

Thanks to their sensory characteristics, broths have been an integral part of global cuisine. Indeed, broths exhibit significant variation in their recipes across different geographical regions. The different ingredients, cooking techniques, and seasonings used in broths reflect the distinct culinary traditions and cultural preferences of each location. In Japan, broth is renowned as a noodle dish called Ramen, while in Vietnam, it takes the form of the popular Pho soup. In Italy, broths serve as the basis of most soups, including Minestrone vegetable soup, and of most soups, and in the Czech Republic, they are commonly used as a basis of various soups and sauces, and are even enjoyed as standalone dishes.

In Czech gastronomy, broth is defined as a liquid made by cooking bones, meat, vegetables, and spices [5]. Depending on the primary ingredient utilized, a distinction is made between vegetable broth, bone broth, and meat-based broth. In English cookbooks, one may encounter the terms "broth" and "stock." Broth typically refers to a preparation made from meat, while stock is derived from bones and cartilage. Stock is also cooked for a longer time, resulting in a denser consistency attributed to its higher gelatin content [6]. Finally, a main distinction is made based on the type of animal from which the meat or bones are derived–poultry, pork, beef or fish broth/stock.

Among the general public, there is a common belief that broths are a valuable source of micronutrients. This perception likely stems from the fact that many of the ingredients used in broth preparation are inherently rich in minerals. For instance, meat is known for its iron (Fe) content, while bones are recognized as a source of calcium (Ca). While certain minerals may be present in broths, comprehensive scientific research supporting their significant contribution to overall micronutrient intake is currently missing.

Available research articles indicate that the nutritional composition of broths can vary substantially. Numerous factors, such as pH, temperature, cooking duration, and the specific raw materials used, can influence the final composition of broths. Additionally, the preparation methods of broths are often culturally influenced, leading to a lack of standardized procedures across different regions [7, 8]. Consequently, the existing information on micronutrient content in broths from food databases is limited [9], and only few studies have explored their comprehensive nutritional composition [7, 10].

On the other hand, many of the ingredients from which the broths are prepared are also considered as sources of certain toxic elements, which have negative effects on human health. Bones are known to accumulate lead (Pb) [11], and root vegetables like carrots are considered sources of cadmium (Cd) [12]. In 2015, the State Institute of Health (SZU) in Prague detected Cd and Pb levels exceeding the reference value for chronic exposure in many cases of czech school lunches [13].

The main goal of this study was to assess the content of Ca, Cd, copper (Cu), Fe, K, Mg, sodium (Na), and Pb in different types of broths (beef, fish, poultry, pork, and vegetable) typical for Czech cuisine. A part of the samples was prepared by authors of this study according to Czech recipes, and the second one was obtained from the Czech school canteens where the professional cooks prepared the broths.

The study compared the element contents among different types of broths, as well as the element contents in the broths with the contents in the water used to prepare them. The results obtained were utilized to assess the contribution of broths to the dietary intake of Ca, Cu, Fe, K, and Mg, and the degree of risk resulting from the presence of Cd and Pb in them were discussed.

This work with its findings contributes to the limited number of scientific works dealing with the content of elements in broths so far. As broths play a significant role in various culinary traditions, gaining a deeper understanding of their nutritional content can contribute to more informed dietary choices and recommendations.

Material and methods

The methodological part of the work involved several steps: a laboratory preparation of broths, collection of samples of broths from school canteens, processing and mineralization of samples and determination of the selected elements by means of atomic absorption spectrometry (Ca, Cd, Cu, Fe, Mg, and Pb) and atomic emission spectrometry (K and Na).

The laboratory broths were prepared using recipes commonly used in school canteens. To validate and broaden the interpretation of the results obtained from the laboratory-prepared broths (n = 12), the study involved collecting samples of broths directly from school canteens (n = 18) from different districts of Brno (Czechia). These broths were prepared by professional cooks.

Preparation and collection of samples

Laboratory prepared broths

Four types of broth, including poultry, beef, vegetable and fish, were prepared in a professionally equipped kitchen according to recipes used in Czech cuisine. Broths were prepared in triplicates (3 × 2 l broth of one type) which is consistent with similar study designs found in previous research [7, 14,15,16]. To ensure consistency and control over the experimental conditions, each type of broth was prepared under identical conditions. Broths were not salted. Supplementary Table S1 provides an overview of the raw materials with their origin and amount that were used to prepare individual types of broth, and also shows the process of preparation of all types of broths. After cooking, all broths were strained through a sieve with a cheesecloth. Samples were taken from the finished broths and prepared for laboratory analysis. As broths were prepared using tap water to meet real conditions, a sample of the water was taken as well.

Broths from school canteens

Total 18 samples of broth were obtained from various school canteens in Brno (Czechia) where they were prepared by professional cooks. In school canteens, broths are an essential part of soups and often other dishes consumed daily by most children. The samples included pork (n = 2); vegetable (n = 2), poultry (n = 6), and beef (n = 8) broths. These were basic broths without additional ingredients that are needed to prepare soups or other dishes.

Cooking time of beef broths had a wide range of cooking values (0.8–13 h) with a median cooking time of 3 h and an average of 4.14 h. Poultry broths were cooked for 2–3 h, with the median 2.75 h and the average 2.58 h. Pork broths were cooked for 3–4 h, and vegetable broths were cooked for the shortest time, 0.5–0.75 h.

At the same time, samples of tap water (n = 18), which was used to prepare broths, were taken from school canteens. The samples were transported to the laboratory in the shortest possible time, where a similar pretreatment of the samples took place as in the case of laboratory-prepared broths.

Sample preparation and analysis

In the laboratory, broth samples were subjected to acid digestion using a microwave digestion system. Five ml of broth was mixed with 5 ml of nitric acid (Analpure grade, Analytika, Czech Republic) and 1 ml of hydrogen peroxide in teflon vessels. The digestion process was carried out using a Multiwave Go Plus microwave digestion system (Anton Paar, Austria) at a temperature of 180 °C for 10 min. After digestion, clear digests were diluted to a volume of 25 ml with water. Samples of tap water (20 ml) were only acidified with 0.2 ml of nitric acid.

In addition to the samples, two certified reference materials, NIST 1570a Spinach leaves and BCR-191 Brown bread, were digested and together with recovery tests employed to ensure the accuracy of the method.

The determination of the elements of interest was performed using flame emission (K, Na), absorption (Ca, Mg), and graphite furnace (Cd, Cu, Fe, Pb) atomic absorption spectrometry. The measurements were conducted using Unicam Solaar 939 (Cambridge, UK) and ContrAA 800G (Analytik Jena, Germany) spectrometers. Detailed measurement conditions are listed in Supplementary Table S2.

Evaluation of benefit of broths consumption in relation to the elements content

To assess the significance of broths as sources of selected elements (Ca, Cu, Fe, K, and Mg), their contents in a serving size of 250 ml are compared to the Nutrient Reference Values (NRV). These NRVs are established by Regulation of the European Parliament (EU) No. 1169/2011, which also establishes the conditions for the use of nutrition claims on food packaging [17]. Nutrition claims state or imply that a particular food has beneficial nutritional properties due to its energy value, nutrient content, or other substances present in increased or decreased amounts, or the absence thereof [18, 19].

According to the regulations, if a broth contains at least 15% of the NRV for a specific element per 100 ml or regular portion size (for broth is 250 ml), it may be labeled with the nutrition claim “source of the given element”. To use the nutritional claim “with a high content of given element”, it is necessary to meet double the stated value [18].

Statistical evaluation of data

Statistical analysis of the results was conducted using STATA 13.5 software. For evaluation and comparison of laboratory prepared broths, parametric statistical tests were used. For the samples of broth from school canteens, nonparamateric statistics was used as the results were distributed non-normally. Due to the small sample size, vegetable (n = 2) and pork broths (n = 2) obtained from the canteens were not included in the statistical evaluation. All tests were evaluated at a significance level of α = 0.05.

Results

Laboratory-prepared broths: the elemental composition

Concentrations of the selected elements differed significantly among individual types of broths (Tukey test, p < 0.05). Vegetable broth had the highest mean concentrations of Ca (9.75 mg/100 ml) and Fe (44.9 μg/100 ml), and the lowest concentration of Na (5.63 mg/100 ml). The Ca concentration was 10 times higher and the Fe concentration was almost 6 times higher in vegetable broth than in beef broth. Fish broth contained significantly more K (161 mg/100 ml) and Mg (5.35 mg/100 ml) compared to other broths. Poultry and vegetable broths contained significantly more Cu than beef and fish broths. Beef broth contained the lowest concentrations of Ca (0.96 mg/100 ml), Mg (1.55 mg/100 ml), Fe (7.8 μg/100 ml), and Cu (4.9 μg/100 ml) of all types of broth.

Based on the results of a one-sample t-test, the concentration of Ca was significantly lower in beef, poultry, and fish broth compared to the tap water used for their preparation. We observed the same Ca concentration in vegetable broth as in the tap water which from the broth was prepared. The reason may be that vegetable broth had the lowest pH = 5.6 (compared to pH = 8.6 in beef, 7.1 in poultry, and 6.3 in fish broth).

School canteens broths: elemental composition

Table 1 shows the results detected in the samples of broths obtained from school canteens. As expected, a significant variability in elements’ concentrations was observed among the samples. The contents of Fe and Cu were found significantly higher in beef broth compared to the poultry broth. The interquartile range of Na concentrations in broth samples was 0.6–3.2 g/l. The large range of the values was related to the fact that some broths were salted or were commercial semi-finished products, while others were prepared without salt. On average, broths prepared without salt contained 0.3 g Na/l, while the broths with salt 2.6 g Na/l.

Table 1 Elementary composition of samples
Full size table

The same pattern as observed in chapter 3.1, i.e. the lower Ca content in the broths compared to the water used for their preparation, was also observed in the real samples. According to the paired t-test, broths contained significantly less Ca than tap water from corresponding school canteens (on average, 8.1 mg/100 ml vs. 13.5 mg/100 ml, respectively).

Content of elements in all samples of broth in terms of the NRV

The standard serving (250 ml) of all samples of broth covers less than 15% of the NRV for Ca, Cu, Fe, Mg, and also for K. However, laboratory-prepared fish broth can be considered as “a source of K” because it contains more than 15% of the NRV per serving [18].

NRV for Na is not established. There is only a value of 2000 mg/day set by the EFSA, which indicates a safe and adequate value of Na intake. Compared with this value, only fish broth covers more than 5% of it [19].

Content of toxic elements

In all involved school canteens, concentrations of both Pb and Cd in tap water were below the regulatory limits (10 μg/l for Pb and 5 μg/l for Cd) [20]. Pb concentrations above the detection limit (0.5 μg/l) were determined in seven samples, with the highest observed concentration of 2.5 μg/l. Cd concentrations were below the detection limit (0.05 μg/l), indicating minimal presence of Cd in the tested tap water samples.

In the broth samples, the Pb content was generally below the limit of detection (2.5 μg/l) in the majority of samples. However, one sample of beef broth exhibited a Pb content of 12 μg/l, surpassing the limit of detection. The elevated lead content suggests the presence of some contaminated ingredient as the tap water used for the preparation of this particular broth contained 1.9 μg Pb/l. Cd concentration below the detection limit (0.25 μg/l) was observed in eight out of eighteen broth samples with maximum value of 2.4 μg/l, indicating potential contamination from certain ingredients or cooking process, although to a limited extent.

Discussion

This work addresses the commonly accepted assumption that broths are good sources of trace elements and minerals, as well as concerns about the presence of toxic elements.

Some of the interesting findings observed for laboratory-prepared were also valid for the real samples obtained from school canteens, namely: (a) above-mentioned Ca trend; (b) the counter-intuitive highest concentration of Fe in vegetable broths, (c) the fact that amount of elements in broth portion did not cover NRV.

Laboratory-prepared broths

Most remarkably, we observed Ca concentrations for most broths to be lower than that in tap water used (see Table 1). The only exception to this trend was vegetable broth, whose Ca concentration was about the same as in the original tap water. The reason may be the lowest pH = 5.6 of the vegetable broths compared to the other types. According to Rosen et al. (1994) the addition of vegetables to the broth causes the mild acidic environment which can also increase the solubility of Ca [21]. Most of the available studies have primarily focused on investigating the element concentrations in broths prepared from distilled water [7, 14, 15, 21, 22]. In contrast to broths prepared from distilled water, our study aimed to simulate real conditions by using tap water, making direct comparisons with such studies unfeasible. Although Rosen et al. (1994) observed increasing Ca content with cooking time (Ca concentration at the beginning of cooking 2.7 ± 0.3 μg/ml and at the end of 24 h of cooking: 5.8 ± 0.8 μg/ml) the resulting amount was still much lower than the common Ca content of water [21].

Broths from school-canteens

Among the samples of broths obtained from school canteens, a statistically significant higher concentration of Cu and Fe was observed in beef broth compared to poultry broth. Among the laboratory-prepared samples, the vegetable broth exhibited statistically higher amounts of Ca and Fe, and the least amount of Na compared to the other types of broths. The higher concentration of Fe in vegetable broth contradicts the intuitive assumption that meat broths would have the highest content of Fe. Vegetable broth contained approx. 6 × more Fe than beef and 2 × more than poultry and fish broths.

Higher amount of Ca in vegetable broth in comparison with meat-and-bone broth also was not expected due to the fact that bones are a reservoir of Ca. Bone broths are widely promoted by the general public as a Ca-rich food for individuals who do not consume dairy products. Based on previous studies, the Ca content in meat-and-bone broth ranges from 0.58 to 7.99 mg/100 ml, regardless of the origin of the animal from which it was prepared [7, 14, 16, 21].

In this work, the content of Ca in meat-and-bone broth was found lower than in tap water from which the broths were prepared (see Table 1). Zou et al. (2021) state that this change in the concentration of Ca can be explained on the basis that Ca may form oil-in-water emulsions together with fatty acids and fatty acid salts, and at the same time, some macromolecules and colloidal particles may bind to polyvalent metal elements, which may decrease the concentration of Ca in the final broth [16]. In this work, the decrease in Ca was deeper in broths of animal origin than in vegetable broths. It could be caused by a collection of a protein precipitate, which formed when bones are boiled in water, from animal broths. Further losses of Ca may have occurred during the filtering of the broth samples, when excess fat was removed from the broths [21].

In this work, the mean concentration of Ca (8.1 mg/100 ml) in samples from school canteens was lower than that in tap water (13.5 mg/100 ml).

Nutritional significance of broths

It has been shown that some types of broth contain more of some elements than other types but even so, they do not contain a sufficient amount of them. All samples with their content of elements in this work did not cover even 5% of the NRV (per a serving) exception for K. Potassium is present in all types of broths at levels exceeding 5% of the NRV, and fish broth contains even more than 15% of NRV per serving [18].

Due to the fact that the addition of salt into the broth is common and necessary for taste, the broths cannot be considered low-sodium. This could be observed in samples from school canteens (some samples were salted) in comparison with the laboratory-prepared broths which were not salted at all.

Toxic elements

Besides the essential elements, concentrations of toxic Cd and Pb were also determined in both broths and tap water obtained from school canteens. Pb is a neurotoxin that negatively impacts the develo** brain. Pb contamination in tap water, both in homes and schools, has been identified as a significant source of Pb exposure in the United States [23], and elevated levels have also been detected in certain schools in the Czech Republic [13]. In this work, the concentrations of Pb and Cd in all samples of tap water did not exceed the respective drinking water regulations' parametric value given in Directive (EU) 2020/2184 (10 μg/l for Pb and 5 μg/l for Cd in drinking water [20].

Bones are known to accumulate Pb, and the release of Pb from animal bones into the broth has already been observed [10]. In the broth samples, the Pb content was generally below the limit of detection (2.5 μg/l) in the majority of samples as well as Cd concentration except contaminated samples. Generally, the observed concentrations of Cd and Pb in the broths were found to be comparable to those reported for animal broth-based foods in Taiwan, resulting in a similar conclusion of minimal risk in this regard [7].

Conclusion

The general assumption that broths are a good source of essential elements cannot be supported by sufficient scientific evidence including the results of this work.

It has been shown that some types of broths contain more of some elements than other types in dependance on factors including recipe, preparation process, ingredients, and pH. However, a regular portion size of broth, regardless of type, does not contain significant amounts of Ca, Cu, Fe, nor Mg in relation to their values of NRV. The potential risk of toxicity from the presence of Cd and Pb in broths was assessed to be minimal.

As broths play a significant role in various culinary traditions, gaining a deeper understanding of their nutritional content can contribute to more informed dietary choices and recommendations. Future work should focus on the establishment of a technological procedure that could be able to maximize the content of beneficial nutrients including essential elements.