Introduction

Food is one of the most important basic needs for human sustenance (Iwuozor 2019). Its adulteration in all its forms has been an existent practice for as long as the manufacturing and processing of food has been. Food adulteration has been on the increase in recent years, and there are various reasons why this has become rampant in our world today. Food adulteration is an act/process whereby food is intentionally and/or unintentionally debased through the removal of vital nutrients (wholly or partly) from the food and the substitution or addition of extraneous substances (adulterants) to the food, rendering it unsafe for consumption (Ayza and Belete 2015; Tibola et al. 2018). According to the FAO and WTO (2017), food fraud and adulteration are the deliberate substitution, addition, adulteration, or misrepresentation of food or food ingredients for economic gain.

Adulteration of food has been reported to be economically as well as criminally motivated (Manning and Soon 2014). As the global population increases, the demand for food and food commodities rises alongside the race to meet this demand and increase profit margins. This places the major reason for food adulteration as purely for financial gain and, from it, other ‘sub-categories’ of reasons may be deduced, namely to increase quantity, increase shelf life, improve appearance, and reduce production costs.

Sugar is a generic name for saccharides, classified under sweet-tasting carbohydrates used as food. They occur in simple forms called simple sugar (monosaccharides, e.g. glucose, fructose, galactose) and in compound forms as disaccharides, e.g. sucrose, lactose, and maltose (Afiomah and Iwuozor 2020; Zaitoun et al. 2018). This review focuses on sucrose, extracted and concentrated from sugarcane and sugar beet plants. It is processed as shown in Fig. 1 into its concentrated form as "table sugar" for consumption and for industrial applications (baking and confectionery industry, fruit drinks and beverages industry, ethanol production, etc.) (Iwuozor et al. 2021a, 2021b). Sugar is also susceptible to adulteration and increasingly so. Sugar was reported as the third most adulterated product in India with 37% adulteration using chalk (Choudhary et al. 2020). Sugar adulteration has been reported in countries like India and Indonesia, and this practice may lead to a range of health problems. This formed the need for this study. To the best of the author’s knowledge, there is no study in published literature that describes the adulterants, effects, and detection techniques for sugar. This study was aimed at studying some of the possible adulterants in sugar and possible ways they can be detected. The study also discusses some futuristic measures that can be taken to curtail this menace.

Fig. 1
figure 1

Summary of the white sugar production process

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Properties of Some Adulterants

There are various reasons why people would want to engage in the adulteration of sugar, and they are shown in Fig. 2. Common adulterants as shown in Fig. 3 include other sugar products, plastics, chalk, urea, washing soda, and sugar substitutes. The essence of this section is to study the properties of these adulterants to understand the purpose of their usage as adulterants as well as the human defects they can cause, as shown in Fig. 4.

Fig. 2
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Some reasons for the adulteration of sugar

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Fig. 3
figure 3

Some common sugar adulterants

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Fig. 4
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Some health defects of adulterated sugar

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Chalk, mainly composed of calcium carbonate (where it derives its white colour from), is one of the sugar adulterants. Chalk is made of fine, soft grains that can be pulverized. It may be used due to its water absorption ability, which makes it remove moisture from sugar to increase its shelf-life, its ability to add to reduce the colour of sugar, and finally, its ability to increase the weight of the sugar. Ingesting chalk in large amounts can cause appetite loss, lead poisoning, constipation, kidney stones, and tooth damage (Lin et al. 2015).

Urea is a white crystalline or granular solid that can be odourless or sometimes have a slight smell of ammonia. It is utilized in the production of adhesives, plastics, cosmetics, and resins. Urea can be synthesized in the body through the breakdown of amino acids in the liver. Urea may be used to reduce the colour of the sugar. Excess urea results in skin infection, skin blisters, rashes, and bodily irritation (Dickerson et al. 2018).

Plastics are synthetic organic polymeric compounds that are light in weight, have low thermal conductivity, and are chemically stable. Its mouldability property could make it look very much like sugar crystals. Plastics are currently a global problem due to their tendency to create a wide range of health hazards. Ingestion of small quantities of plastics may not be harmful, but in considerable quantities, it may lead to necrosis, oxidative stress, apoptosis, cancer, asthma, and infertility (Alabi et al. 2019).

Washing soda simply refers to hydrated sodium carbonate. It is an odourless, white, crystalline solid. Like its name, it is used like a detergent to remove dirt and stains from surfaces and materials. It can be used to reduce the colour of sugar. Just like the other adulterants, it is not harmful in minute quantities when ingested, but in large doses it could cause vomiting, shock, bodily pains, low blood pressure, diarrhoea, skin/eye irritation, and asthma (Mukherjee et al. 2015).

Sugar substitutes are substances that are not sugar and provide some amount of sweetness. Common examples of sugar substitutes are artificial sweeteners and sugar alcohols. Artificial sweeteners contain more sweetness than sugar, which limits the quantity one would need with respect to sugar to get the same level of the sweetness. Usually, they have low calorific content, which makes them a better alternative for people who want to lose weight. Examples of artificial sweeteners are aspartame, advantame, stevia, sucralose, neotame, acesulfame potassium and saccharin (Chattopadhyay et al. 2014). Sugar alcohols are compounds that are not alcohols and are not sugars. Rather, they have some properties of both alcohols and sugars. They are generally utilized as sweeteners and thickeners in the food industry. Some common sugar alcohols include erythritol, maltitol, sorbitol, xylitol, and lactitol. Unlike artificial sweeteners, sugar alcohols are less sweet than sugar but have lower calorific content than sugar. In large amounts, sugar alcohols may cause flatulence, bloating, and diarrhoea (Mäkinen 2011).

Techniques for Detecting Sugar Adulterants

Dissolution in Water

The test by dissolution in water can be performed based on the solubility strength of the adulterants. Sugar, which is majorly composed of sucrose, is soluble and therefore dissolves in cold water. Adulterants such as plastics and chalk (which is composed majorly of calcium carbonate) have low solubility in water and therefore would not dissolve in cold water. Therefore, adulterants like plastics and/or chalk would be insoluble in a sugar solution. On the other hand, adulterants like urea and washing soda are soluble in water.

In cold water, urea gives off ammonia and carbon dioxide, as shown in Eq. (1). The solution gives off CO2 and NH3. The latter gives off a strong urine-like or sweaty-like odour.

$${\text{C}}_{12} {\text{H}}_{22} {\text{O}}_{{11\left( {\text{s}} \right)}} + {\text{NH}}_{2} {\text{CONH}}_{{2\left( {\text{s}} \right)}} + {\text{H}}_{2} {\text{O}}_{{\left( {\text{l}} \right)}} \to {\text{C}}_{12} {\text{H}}_{22} {\text{O}}_{{11 \left( {{\text{aq}}} \right)}} + 2{\text{NH}}_{{3\left( {\text{g}} \right)}} + {\text{CO}}_{{2\left( {\text{g}} \right)}}$$
(1)

In cold water, washing soda decomposes to give sodium hydroxide, which is a strong base, as seen in Eq. (2). This raises the pH of the solution to a high alkaline level, and it can be checked with the aid of a pH meter or litmus paper. It should be noted that the pH of sugar in demineralized water is about 7.

$${\text{C}}_{12} {\text{H}}_{22} {\text{O}}_{{11\left( {\text{s}} \right)}} + {\text{Na}}_{2} {\text{CO}}_{3} \cdot 10{\text{H}}_{2} {\text{O}}_{{\left( {\text{s}} \right)}} + 2{\text{H}}_{2} {\text{O}}_{{\left( {\text{l}} \right)}} \to {\text{C}}_{12} {\text{H}}_{22} {\text{O}}_{{11 \left( {{\text{aq}}} \right)}} + {\text{H}}_{2} {\text{CO}}_{{3\left( {{\text{aq}}} \right)}} + 2{\text{NaOH}}_{{\left( {{\text{aq}}} \right)}} + 10{\text{H}}_{2} {\text{O}}_{{\left( {\text{l}} \right)}}$$
(2)

The strength of this technique lies in the fact that it is cheap and easy to perform but it is not reliable.

Polarimetry

A polarimeter/saccharimeter is an instrument that is popular in sugar and allied industries. It is a device that monitors the direction and extent that an optically active compound rotates plane polarized light. A polarimeter is made up of the following components: a detector, a polarimeter tube, a polariser, a light source, and an analyser. The analyser is spun manually or mechanically, depending on the kind of device, until the maximum intensity of light falls on the detector (Anyika et al. 2012). The rotation angle in polarimetry provides information about the molecule’s structure and purity. The unit of measurement is known as polarization (usually referred to as Pol (°Z)). Refined sugar and plantation white sugar have pol values of ≥ 99.7°Z and ≥ 99.5°Z, respectively. All optically active substances such as sugar, calcite, and lactic acid change the direction of plane polarized light either to the right or left. This is the major drawback to detecting sugar adulteration using this method. While this method is critical for determining the quantity of sugar present in a sugar sample, it wrongly interprets the presence of other optically active adulterants such as sugar alcohols too, but all non-optically active adulterants such as urea and chalk can be detected by this technique.

Spectroscopic and Chromatographic Detection

Infrared spectroscopy (IR) is one of the most frequently utilized spectroscopic methods in the quality control of manufactured products or in food product adulteration. For qualitative and quantitative food analysis, IR obtained from the mid-infrared (MIR) and near-infrared (NIR) regions has become popular. MIR spectroscopy, which exists at a wavenumber of 4000–400 cm−1, gives more precise and intense chemical information than NIR because it depicts basic vibrations rather than combination bands and overtones recorded in the NIR area. The advantages of IR that make them better suited for adulteration analysis are its non-destructive nature and the issue of cost and long-time analysis for other techniques, which makes them unsuitable for large samples and regular analysis. FTIR was used to detect adulteration in brown sugar that was intentionally doped with various concentrations of coconut sugar (Roosmayanti et al. 2021). The study confirmed the potential of using FTIR for the adulteration of palm sugar.

The study of the absorption of radio frequency radiation by a nucleus is known as Nuclear Magnetic Resonance (NMR) spectroscopy. It can assess the content and structure of diverse organic and inorganic substances qualitatively and quantitatively. 1H-NMR, 2H-NMR, SNIF-NMR, DOSY 1H-NMR, and LF-NMR are examples of NMR methods. Furthermore, the magnetic resonance imaging approach allows for visual investigation of the inside of meals. It may collect information about the chemical composition and internal structure of food samples and so improve the capability of online food quality assessment (He et al. 2020b). It has become a responsible and promising method for detecting food adulteration since it can measure distinct chemicals and offer structural information about compounds in a combination using a single NMR experiment with great repeatability and reproducibility. NMR has been successful in the detection of adulteration in honey samples (He et al. 2020a).

Gas chromatography is commonly used to authenticate and identify chemical compounds, differentiate between different types of the same product, and detect food fraud. It is most often used in addition to hyphenated methods and detectors such as mass spectrometry (GC–MS), Fourier Transform Infrared Spectrometry (GC–FTIR), Nitrogen Phosphorous Detector (GC-NPD), Electron Capture Detector (GC-ECD), Atomic Emission Detector (GC-AED), and Flame Ionization Detection (GC-FID) to identify distinct chemical components. Pascual-Maté et al. (2018) studied the sugar composition of 54 honey samples. The results obtained showed the presence of a total of 14 carbohydrates including sucrose, present in the honey samples.

One of the most extensively utilized techniques in analytical chemistry is High-Performance Liquid Chromatography. In an industrial environment, HPLC is one of a number of control and research technologies that are commonly used. HPLC has also been utilized in the detection of various adulterants in foods like honey, milk, and aloe vera gel (Cordella et al. 2002). HPLC is employed as a quality control technique in the sugar industry because it can separate diverse chemical components of mixtures, but it is also often used to identify adulteration as well as characterize sugar products. HPLC is sometimes attached to various detection systems such as florescence, refractive index, evaporative light scattering, and ultraviolet–visible detection systems in the analysis of sugar to improve its efficiency. Tihomirova et al. (2016) successfully utilized HPLC with a refractive index (RI) detector for the qualitative analysis of sugar in hydrolysed hay. (Montesano et al. (2016)) attached the evaporative liquid scattering detector with HPLC to identify and quantify sucrose, glucose, and fructose in Goji berries samples.

A summary of techniques that can be utilized for the detection of sugar adulteration is given in Fig. 5.

Fig. 5
figure 5

Some of the sugar adulteration detection techniques

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Recommendations and the Way Forward

Though there is almost no scientific study in published literature on the adulteration of sugar, it is imperative that certain measures are put in place to reduce this menace to the barest minimum. Some of these measures are discussed below.

  1. 1.

    Intentional research should be encouraged and sponsored to study the properties and preparation of the mixture of the adulterants with sugar at various ratios. This would lead to the creation or generation of a big database that would be able to provide qualitative and quantitative information on the threat, and scale of adulteration in an adulterated sample.

  2. 2.

    Most of the detection techniques that currently exist are very expensive and the equipment cannot be acquired by the average person. Therefore, the development of cheap and easy-to-use sugar adulteration detectors is encouraged so that they can be utilized by the average person. In the same vein, regulating bodies and institutions should be provided with different types of detection equipment to ease the testing of sugar samples.

  3. 3.

    It is pertinent that citizens are educated by the government and responsible bodies on the effects of consuming adulterated sugar and adulterated sugar products. The awareness of the adulteration of sugar would help curb and reduce this menace as it would encourage consumers to buy sugar and sugar products from trusted dealers who are less likely to adulterate the products in their possession. This would also discourage retailers and wholesalers from buying from unauthorized dealers.

  4. 4.

    Periodic checks of the quality of sugar available to consumers in the market should be performed on a regular basis by regulating bodies in each state and country. Research institutions in various countries, such as the Nigeria Sugar Institute, Ilorin, and the National Sugar Institute, Kanpur, should also assist the government in detecting adulterated sugar in the market, and culprits should be handed over to security agents.

  5. 5.

    Government regulation is paramount in the fight against adulterated sugar. The quality of products from the industries should be regulated. The supply chain from the industry to the consumer should also be regulated. There should be laws against sugar adulteration in various countries and national sugar laws in respective countries, which could include the banning of imported refined sugar, should be implemented.

  6. 6.

    Product development should be encouraged and sponsored for the production of sugar blended with safe alternatives such as sugar alcohol to obtain the required property as requested by the consumer. The producers should also be mandated to ensure that their label claim is in conformity with the content of their products.

Conclusion

In this study, possible sugar adulterants and their effects, and detection were reviewed. It was observed that the reasons for sugar adulteration could be an increase in weight an improvement in colour and taste, a lower cost of production and an increase in profit. The properties and health effects of adulterants such as chalk, plastics, urea, washing soda and sugar alcohols were also discussed. Detection techniques such as dissolution in water, polarimetry, gas chromatography, FTIR, NMR, and HPLC were also discussed. Some of the useful recommendations which could minimize the growing menace of sugar adulteration are the development of a sugar adulteration database, increased government regulations, periodic checks of sugar in the market, education of citizens and the fabrication of cheap and better detectors.