Food Chemistry as a Vital Science: Past, Present, Future

Jun 11, 2019 - As food consumers become increasingly selective, food chemistry is becoming a vital science. Food items that are most in demand are nat...
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Chapter 16

Food Chemistry as a Vital Science: Past, Present, Future

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Lili He* Department of Food Science, University of Massachusetts, 102 Holdsworth Way, Amherst, Massachusetts 01007, United States *E-mail: [email protected].

As food consumers become increasingly selective, food chemistry is becoming a vital science. Food items that are most in demand are natural, organic, not genetically modified, low in sugar, low in salt, and low in fat while simultaneously being affordable. Natural additives, such as colorants, sweeteners, and preservatives, are increasingly popular. However, scientists’ knowledge about the chemistry of these ingredients is limited, so the food industry is hesitant to regularly use them. As more food products are branded as “natural,” “100% natural,” or “all natural,” scientists also have greater concerns about the authenticity of so-called “natural” ingredients. These types of dilemmas have encouraged scientists to develop better analytical methods for the study of food chemistry. Their ultimate goal is to facilitate efforts to ensure the food supply will be high-quality while also safe for human consumption.

Understanding the Chemistry of “Natural” Food Ingredients In recent years, food consumers have demanded products that are more “natural,” with simple and recognizable ingredients. As a result, there is an increasing trend toward replacing artificial food additives with natural alternatives. Food colorants, for example, have been used extensively as additives to enhance product appeal and quality. A recent report estimates that the natural and artificial food color market in the United States will be worth $3.75 billion by 2022 (1). Currently, there are seven main artificial colorants approved for human consumption in the United States, all of which are required to undergo FDA certification with the manufacture of each new batch. A 2014 survey of about 810 products in one North Carolina grocery store found that 43% contained artificial colorants. The most common artificial colorants were Red 40, Blue 1, Yellow 5, and Yellow 6 (for more information, please see Chapter 7 in this book, “Carotenoids, Cochineal, and Copper: Food Coloring Through the Ages” (2)). The structural formulas for all of the permitted colorants can be found in the previous chapter “Carotenoids, Cochineal and Copper: Food Coloring Through the Ages” by Orna, M. V. The highest percentage of items with artificial colorants was found in products marketed to children, including candies, fruit-flavored snacks, and drink mixes/powders (3). However, the possible health issues connected to some artificial dyes—such as potential © 2019 American Chemical Society

Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

carcinogenicity of azo-dyes, association with attention deficit hyperactivity disorder in children, and possible correlation with allergies and asthma—have raised public concern (4, 5). Currently, the FDA has approved approximately 25 colorants derived from certain natural sources to be used as additives. These items are exempt from FDA certification. Still, natural colorants are generally more expensive and less stable than artificial colorants (6). Some of the key challenges associated with applying natural colorants in food matrices include light exposure, heat sensitivity, and interactions with other ingredients. Due to the complexity of food matrices, it can be especially difficult for scientists to understand how different ingredients interact with each other. For example, in the same food matrix, vitamin C enhances the stability of carotenoids but promotes degradation of anthocyanins (7, 8). A synergetic effect between gum arabic and Fe2+ has been reported to improve the heat and acid stability of norbixin (9). However, controversial findings regarding the effect of metal ions on the stability of anthocyanins have also been reported (10, 11). Clearly, there is a lack of systematic and comprehensive chemical studies on the interactions of natural colorants with other food ingredients. This is often keeping natural colorants from being included in complex food matrices. Similar challenges exist for other natural additives such as preservatives and antioxidants. Scientists must understand and utilize more chemistry before they can effectively improve the stability and efficacy of these natural additives in food products. Still, some progress is being made; strategies such as encapsulation have been developed to improve the stability of these natural additives. Emulsion-based delivery systems, meanwhile, have been demonstrated to enhance the efficacy of nutrient absorption (12–14).

Ensuring the Authenticity of “Natural” Ingredients Aside from the demand for more “natural” products, consumers are also reporting concerns about the authenticity of product ingredients. Food adulteration is both a historical and current issue. In 1995, the U.S. Government Accountability Office estimated as much as 20% of the orange juice sold to school meal programs could be adulterated (15). Economic juice adulteration can fall into three categories: (1) over-dilution of juices with water, (2) use of cheaper solid ingredients such as sugars and sweeteners, artificial colors, and acids, or (3) blending of more expensive juices with cheaper juices. Another example, the use of Sudan dyes, which were used in the past but are now prohibited because of their harmful effects, was also confirmed. In a number of cases, banned colorants have still been found in food and present a threat to public safety (16). More recently, in the growing market for natural food products, officials have confirmed numerous cases of misleading product labels; these labels claim 100% natural colorants, but the items actually use artificial ones. According to Food Litigation News, several large food companies have been involved in class action lawsuits due to mislabeling products as “natural,” “100% natural,” or “all natural” when those products actually use artificial additives (17–19). However, the exact frequency of food fraud and product mislabeling in the current market is unknown. This discrepancy urges scientists to develop effective chemical methods ensure the authenticity of the “natural” ingredients.

Developing More Advanced Analytical Techniques for Food Food matrices typically include different complex chemical components such as water, carbohydrates, lipids, proteins, vitamins, minerals, colors, flavors, and a variety of food additives. Food can take one of three physical forms: liquid, solid, and semi-solid. Food products are typically 232 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

produced in bulk quantities, which requires officials to develop methods for rapidly screening them in large numbers. Numerous regulations have been set up to control food additives and contaminations, necessitating appropriate sensitivity and accuracy of these methods. Techniques currently used to analyze food colorants in complicated food matrices usually require separation and extraction steps, such as liquid–liquid or solid phase extraction, during sample preparation (e.g., thin layer chromatography (20, 21), liquid chromatography (LC) (22), and capillary electrophoresis) (23, 24). These techniques are often coupled with ultraviolet–visible, fluorescent, or mass spectroscopy (MS) as detection methods. Natural colorants may also be altered or degraded during sample preparation, which makes detection more difficult. As different types of colorants usually require different extraction methods, the identification of unknown colorants is challenging when handling foods containing mixtures of colorants. Other methods used to detect colorants include micellar electrokinetic capillary chromatography (25), adsorptive voltammetry (26), capillary isotachophoresis (27), and spectroscopic methods such as Fourier transform infrared (28). These techniques provide alternative approaches to the standard detection processes; however, they do not offer substantial improvement regarding sample preparation, sensitivity, and detection time. Detecting food fraud is more challenging. According to the U.S. Government Accountability Office’s report in 1995 (15), it is both difficult and expensive to detect juice adulteration. The costs of analyzing juice samples for adulteration range from $15 for a basic test to identify dilution with water to $700 for a test to identify the presence of pure beet sugar. As juice can be adulterated in a variety of ways, a battery of tests is needed to confirm whether anything has been added to, or substituted for, pure juice. Although this can be costly, it is necessary to ensure the authenticity of a purchased product, to protect consumers, and to help companies be certain that juices from a variety of global sources are high-quality. Detecting dilution with water is the simplest test, which can be performed by measuring solid content and certain elements (e.g., potassium and nitrogen) (15). The process of detecting added sugar and acid can be complex, as all juices contain a variety of different sugars and acids. It requires a technique that identifies a specific type of sugar and acid or a profile of overall sugars and acids. For example, a quantitative isotope NMR method (i.e., 13C-SNIF-NMR) can separate pineapple sugars from cane or corn sugars (29). Another isotope method detected synthetic L-malic acid in apple juice (30). Sorbitol/sucrose ratios and sorbitol/total sugar ratios are used to determine whether apple juice has been adulterated with sugar solutions or pear juice (31). An anion-exchange chromatography with a pulsed amperometric detector was used to discover several oligosaccharides in beet medium invert sugar that were either in low concentrations or not present in the orange juice samples. Analysis of these intentionally adulterated samples using LC detected 5% beet sugar in orange juice (32). Infrared spectroscopy was also used to detect adulteration of apple juice (33) and orange juice with added sugars (34). Identifying the presence of a low-cost juice in an expensive juice can be the most challenging aspect of the food fraud detection process. Many researchers focus on biomarker identification for targeted adulteration. For example, dihydrochalcones in apple products are characteristic of apples, because they have not been detected in any other fruits. By detecting them, scientists have been able to create useful tools for assessing food authenticity through a reversed-phase high-performance liquid chromatography (HPLC) method (35). Another HPLC-based method employed neoeriocitrin and naringin as markers to detect the addition of bergamot juice to lemon juice at the 1% level. When using neohesperidin as a marker, the minimum amount of detectable bergamot juice was about 2% (36). The amino acid profile of pomegranate juices was compared to apple amino 233 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

acids, and l-Asn was proposed as a marker for the adulteration of pomegranate juices with apple juices using a chiral micellar electrokinetic capillary chromatography–laser induced fluorescence method (37). In order to identify adulteration, scientists have also measured the changes of certain profiles, other than just one or two biomarkers. For example, reversed-phase HPLC has been applied to quantify levels of polymethoxylated flavones and carotenoids in orange and tangelo juices. Lower levels of sinensetin and tetramethyl-o-scutellarein and higher levels of heptamethoxyflavone and tangeretin relative to nobiletin indicated the addition of tangelo to orange juice (38). With the volatile composition analyzed by a gas chromatography (GC)–based method, the adulteration of pomegranate juice with grape juice created significant levels of acetic acid, isoamyl butyrate, and especially 1-hexanol and linalool. Meanwhile, the adulteration with peach juice created significant increases in butyl acetate, isobutyl butyrate, benzyl acetate, and especially isoamyl butyrate (39). A metabolomics approach has been applied more recently to discover biomarkers that might be used to trace food adulteration with pineapple, orange, grapefruit, apple, clementine, and pomelo juices. Through an ultra-performance liquid chromatography quadrupole time of flight mass spectrometry with multivariate data analysis, scientists were able to determine how untargeted metabolite fingerprinting contributed to the separation between pineapple, orange, and grapefruit juices (40). Adulteration through addition of grapefruit juice was determined by 3D-front-face fluorescence spectroscopy, followed by independent components analysis and by more classic methods such as free radical scavenging activity and total flavonoid content (41). As chemical-based methods of food analysis continue to develop, the focus will likely remain on modifications of the current gold standard methods based on LC or GC-MS/MS. Sample preparation is also key for these techniques. Experts are also exploring how to integrate big science data and deep machine learning algorithms in order to facilitate the analysis and interpret the results. Other techniques may find niches for specific applications. For example, surface-enhanced Raman scattering (SERS) is an innovative analytical technique that combines Raman spectroscopy with nanotechnology (42–44). Raman spectroscopy measures the molecular vibrations of analytes resulting from inelastic scattering processes that could be considered a chemical signature. However, conventional Raman spectroscopy signals are extremely weak and suffer from fluorescence interference. This limits how often the SERS method can be applied to analyze bio-materials containing low analyte levels. To overcome this problem, analysts might use certain metallic nanostructures that can greatly enhance the Raman signal; they might also suppress fluorescence emission signals known to interfere with the analysis. In some cases, the limit of SERS detection can be as low as a single molecule. As a result of electromagnetic and chemical enhancement mechanisms, Raman signal increases significantly in the presence of noble metal nanostructures. In food science, SERS has been utilized to detect multiple classes of food contaminants, including chemical, microbial, and engineered nanomaterials (45, 46). Specifically, analysts have applied SERS to extensively study pesticides, colorants, and some chemical adulterants. These applications of the SERS method have helped demonstrate its capability, but more research is still needed to further advance the technique for the purposes of food analysis.

Conclusion Understating the chemistry of food, particularly “natural” ingredients, is vital to maintain the sustainability of the food system. The development of better analytical methods will facilitate a more effective study of food chemistry and will enhance the quality, nutrition, and safety of food items.

234 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

Biographical Information Lili He is associate professor of food science at the University of Massachusetts Amherst. She received her Ph.D. from the University of Missouri in 2009 and did her postdoc training at the University of Minnesota between 2009 and 2012. She then joined the faculty in the food science department at the University of Massachusetts Amherst as an assistant professor in 2012. Dr. He’s major research focus is to develop and apply the most advanced and innovative analytical techniques to help solve critical and emerging issues in food science. Her group has developed various techniques for food safety and food chemistry applications based in SERS. She was the recipient of the 2012 Young Scientist Award from the International Union of Food Science and Technology, the 2015 Young Scientist Award from the Agricultural and Food Chemistry Division of the American Chemical Society, the 2016 Young Investigator Award from the Eastern Analytical Symposium, and the 2016 Samuel Cate Prescott Award for Research from the Institute of Food Technologists. She was also selected as one of the Talented 12 by C&EN, the official magazine of the American Chemical Society, in 2016.

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