Introduction to the Chemistry of Foods and Forages János Csapó Introduction to the Chemistry of Foods and Forages

Emulsifiers are surface active agents (tensides) that are located on the interface and decrease the surface tension therefore stabilize the integrity of droplets in emulsion. Ionic tensides are good for the stabilization of o/w emulsions. The polar part of the surface active agent consists of charged particles and counter ions. The resulting electrostatic double layer around the droplets has the same electric charge and the resulting repulsion prevents the aggregation of droplets (Fig 26.). Neutral tensides are to stabilize w/o emulsions. The polar part is nonionic in this case and anchors the tenside molecule on the outer surface of the water droplets. The alkyl residues form a lipophyl bilayer in the apolar dispersed phase (Fig 27.).

The surface tension reducing capacity of tensides is proportional to concentration, until the critical micelle concentration (CMC) is reached. Above CMC the surplus of the tenside molecules do not emulsify the two-phase system, instead of forms spherical micelles. The hydrophilic-lipophilic balance (HLB value) is used in order to characterize the ratio of the strength of the hydrophilic and lipophilic groups. The higher the HLB value, the more emphasized the hydrophilic feature that is the application area shifts from w/o to o/w. Tensides with HLB value between 3–6 are used to stabilize w/o emulsions while additives between 8–18 are o/w-emulsifiers.

Monoglycerides and diacylglycerides are the most important groups of synthetic emulsifiers. They can be modified with estersification. Acetem is acetate ester of mono- and diglycerides while citrem and lactem are esters of citric acid and lactic acid, respectively. Monoacetyl- and diacetyltartaric acid are called datem . It is used as volume increaser in biscuits. The baking result notably depends on the carbon number of acyl residue of monoglyceride.

Stearyl-2-lactylate as a free acid is a tenside for w/o emulsions. The sodium or calcium salt of stearyl lactylates can be applied for the stabilization of o/w emulsions when it is frozen and thawed (Fig 28.).

Sorbitan is a sorbitol derivative. Sorbitan esters of fatty acids are used for the stabilization of w/o emulsions. Sorbitan tristearate is used to avoid the formation of moldy looking white coating on the surface of chocolate (i.e. fat bloom).

Fatty acids with 14-18 numbers of carbons can be esterified with lactose and sucrose and the resulting sugar esters are used as emulsifiers. Their HLB value is high therefore can be applied for the stabilization of o/w systems moreover in powdered and dehydrated instant foods.

Humectants are food additives with hygroscopic properties. They are mostly diols and polyols (e.g. mannitol, sorbitol, glycerol, 1,2-propanediol). In confectionery product they maintain the softness and prevent crystallization. Another field of application is the enhancement of the rehydration properties in dried fruit/vegetable products.

Polysaccharides are often used as food additives. They can facilitate gel formation therefore they can be applied as thickening agents due to viscosity enhancement and also as stabilizers. They have an important role in the aroma encapsulation in dehydrated food items.

Obesity may have an adverse effect on health. The importance of fighting against obesity is increasing in industrial developed countries. Fat replacers are a group of food additive that can be an important tool to decrease the total energy intake with food and decrease the ratio of energy originated from fats. But one can be aware of the fact that fat is not only an energy source: it has several other functions in food and fat replacers cannot fulfill all of these requirements.

Some of the natural fat mimetics are made of polisaccharydes that are nondigestible in the small intestine (i.e. dietary fibers). Some energy is derived from these compounds due to the microbial fermentation of large intestine but it can be negligible related to that of fat (8.4 kJ/g v.s. 38 kJ/g). Some examples belongs to this group are bran (wheat, barley, oat, soybean); fibers (apple, pea); resistant starches (i.e. retrograded starch); cellulose, fructosanes, pectins, and modified starch. Microparticulated proteins in the particle size range of 0.1–3 μm provide a creamy (’melt-in-the-mouth’) feeling like fats.

Fat substitutes can be synthesized with substitution of parts of the glicerides e.g. with the incorporation of other alcohols instead of glycerol, or the implementation of unusual (long-chain or branched) carboxylic acids.

Fatty acid esters of mono-, oligo- and polysaccharides (often sucrose) are also can be applied. The esterification degree of these carbohydrate polyesters should be high otherwise ester bonds are hydrolyzed in the intestine. These substituents are thermally stable and tasteless.

Retrofats consist of polybasic acids (e.g., malonic acid, citric acid) that are esterified with long carbon chain alcohols. The nomination of this group means that instead of the alcohol part (as in the case of glycerides) the acid part has more functional groups.
Chapter 11. Toxic Compounds in Food

The magnitude of the toxicological effect of a given compound is influenced by several factors i.e. age, state of health and consumption habits of the individual, moreover the synergistic effects of the other materials present in the included food. In order to assess toxicity of a given compound different tests can be carried out with animals i.e. the level of acute toxicity (LD₅₀) or the level of subacute toxicity (4 weeks interval). In the interval of chronic toxicity (6 months – 2 years) the attention is paid to the observation of occurring mutagenic, teratogenic and carcinogenic symptoms.

No Observed Effect Level (NOEL) is a dosage of the material under investigation (mg∙kg^-1∙BW^-1∙day^-1) in which there is not any harmful effects during the lifespan of several generations of test animals. The toxicologically acceptable dose per day related to bodyweight unit is expressed with Acceptable Daily Intake (ADI) which is calculated from the NOEL value. The individual deviations could be significant at particularly sensitive organisms therefore the NOEL value is multiplied by an empirical uncertainty factor (usually 10^-2) to obtain ADI. Permissible Concentration (PC) (mg/kg) is stipulated from the permissible intake (mg) for a person divided by the amount of consumed food (kg).

The main groups of food contaminants are pesticides (from agricultural chemicals for crop protections); feed additives and vetenary medicines (from livestock and poultry husbandry); mycotoxins and enterotoxins (that are toxic metabolites of microbes); toxic compounds originated from packaging materials of food (e.g. polymer stabilizers, monomers, PCBs) and pollutants derived from industrial contaminations (e.g. PAH, toxic elements, dioxines).

Pesticides are plant-protective agents in order to moderate losses in crop yield and stocks and control of insect spread diseases. Their toxicological properties are assessed during the registration procedure on different organisms i.e. aquatic, birds and mammals. Moreover the decomposition rate in the plant and in the soil is also determined.

The contamination with a pesticide is direct when the plant food is treated before storage or distribution, e.g. fungicides in fruits and vegetables, insecticides in cereals. Indirect contamination occurs when the pesticide residue in the soil is uptaken by the subsequent crop.

Non-specific herbicides are used before sowing therefore weeds and cultivated plants are also eliminated. If resistance genes (e.g. in soybean) are introduced non-specific herbicides can be applied during the life cycle of plant. Selective herbicides inhibit only the growth of weeds e.g. 2,4-dichlorophenoxyacetic acid hampers the growth of dicotyledonous weeds, but not monocotyledon cereal plants.

The group of fungicides are found most often as an unwanted residue in fruits and vegetables. Systemic fungicides penetrate into the plant and eliminate hidden seats of disease while contact fungicides provide prevention of fungal attack on the surface of plant. Fungicides can be also classified based on the chemical composition e.g. inorganic fungicides, organometallic compounds. Dithiocarbamates of zinc and manganese (Maneb group) often found as residue in foods. The most numerous group is the group of organic fungicides.

The half life of chlorinated hydrocarbon insecticides is long (e.g. 4–30 years for dichlorodiphenyltrichloroethane (DDT)) therefore they are persistent in the environment. They accumulate in the fat tissue and milk of human beings and animals. Their use is banned in the EU and the USA. The half life of organophosphate compounds, carbamates and pyrethroids is in the range of days to a few months (e.g., carbofuran 30–60 days). Pyrethroids are formed via the synthetic modifications of pyrethrin. Pyrethrum is a natural insecticide. It can be isolated from different varieties of chrysanthemums.

Most of the insecticides are nerve poisons. Other possible ways of action are hampering the respiratory chain or hindering the development of pests (e.g. blocked chitin biosynthesis). Some of the insecticides registrated long time ago are very toxic to mammals e.g. chlorpyriphos, methidation, parathion. The development of resistance is a matter of concern and the synthesis of new effective agents is a continuous need. One trend is to improve the activity of insecticides in order to decrease the required dose. Another important consideration is to develop new agents with less toxic effect on mammals and it is also applied for the synthesis of herbicides or fungicides.

Based on the results of food monitoring programs the target compounds for the risk assessment were selected. Compounds that were included being present at least 5% of the samples and at least 3 or more foodstuffs are affected. The assessment of pesticide intake based on the analytical results of food samples and national consumption studies. The estimated daily intake of the selected pesticides was mostly less than 1% of the ADI values with the exception of dithiocarbamates when the intake was 7.7 – 18.3% of the ADI. The upper value was reached by the group of children presumably due to their higher fruit consumption as dithiocarbamates are used against fungal diseases.

The primary application goals of feed additives and vetenary medicines are therapy, prophylaxis and economic aims (e.g. to diminish the risk of losses in intensive animal farming). The low, but continuous burden of these agents could be a health hazard for human beings. Antibiotics are applied in the therapy of infectious diseases and used to be applied also as growth promoters until it was banned in the EU (from January 1, 2006) because some of them used to treat both human and animal infections became resistant to antibiotics. Anthelmintics are applied against worms caused diseases. Coccidiostats are to fighting against coccidiosis diseases (e.g. enteritis) caused by parasite protozoans and they are used mainly for poultry and rabbits. Coccidiostat residues may be quantified in eggs.

The presence of not approved therapeutic agent should be screened e.g. fattening aids with estrogenic activity (17-estradiol), chloramphenicol, nitrofurans and the same is true for the banned feed additive residues. Antibiotics can be detected by the growth inhibition of bacteria and they can be accurately identified with mass spectrometric techniques. In the case of approved medicines their quantification should be achieved because their concentration should be below the permissible upper limit.

Mycotoxines are produced by the fungi from Aspergillus and Penicillium genus (e.g. aflatoxins, ochratoxins and patulin) and fungi from Fusarium genus (e.g. zearalenone (F2), fusariotoxin T2, vomitoxin (deoxynivalenol).

Aflatoxin B1 is the most powerful known carcinogen agent. The most important source is plant material, primarily nuts and fruit. Aflatoxins pass from feed to animal products, primarily milk. They have no threshold below which no harmful effects are observed (NOEL). Ochratoxin A is nephrotoxic. It is uptaken by the fodder and can be found in the blood and kidney tissue moreover to a less extent in muscle, liver and adipose tissue.

In the course of food monitoring the mycotoxin content of several foods were determined and relatively large ratio of the samples (21%) contained some of the above mycotoxins e.g. aflatoxins, ochratoxin A, patulin, zearalenone, deoxynivalenol and fumosins. Pistachio was severely affected. In the case of deoxynivalenol the intake was 34.1–82.5% of the ADI value with upper value of children at the age of 4–6 years. The possible explanation is the higher ratio of cereal product consumption of this age group. The ochratoxin A intake was also high at children may be attributed to the emerged cereal product and fruit juice consumption. However, there is still insufficient data available for the exact evaluation of the effects of mycotoxines for human health.

Bacterial toxins that are synthesized in the digestive tract called enterotoxins . Enterotoxins can be classified as exotoxins (the microorganisms release the toxin into the environment) and endotoxines (the toxin produced remains firmly bound to the bacterial cell wall but after the cell dies and disintegrates the toxin is excreted).

The sources of exotoxins are primarily gram-positive bacteria. They are composed mostly from proteins possessing antigenic properties and being very toxic. Botulin toxin of Clostridium botulinum is a neurotoxin. The symptoms of poisoning are diarrhea, vomiting, paralysis. The cause of the Clostridium infection is inadequate heat penetration in the course of heat treatment of meat products or insufficient use of antimicrobial agents.

The toxin released by Staphylococcus aureus is relative thermostable. The symptoms of poisoning are stomach ache, vomiting, diarrhea. It occurs mainly in food of animal origin (e.g. meat products, poultry, cheese).

Endotoxins are formed primarily by gram-negative bacteria. They can contain both protein, polysaccharide and lipid structure elements and compose relatively heat stable molecules.

Toxins of Salmonella species cause salmonellosis. Typhoid fevers are an enteric illness caused by Salmonella typhi while paratyphoid fevers are caused by Salmonella paratyphi. The sources of infections can be egg, poultry, ground beef or confectionery products.

E. coli species are used in general to indicate the fecal contamination. The toxins of enterotoxic strains of E. coli cause severe diarrhea and can occur in insufficiently heat treated milk, meat, apple juice, moreover in unwashed fruits and vegetables.

Among the industrial contaminants polychlorinated biphenyls (PCBs) had been used worldwide since 1950 and entered into the food chain. They are persistent and soluble in fat therefore they occur more frequently and increasing concentrations in fatty foods. Owing to thermal treatment of food highly toxic dioxins can be formed. The manufacture and application of PCBs are banned since 1989. Polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF) (the latter called informally ’dioxins’) have got large number of congeners (i.e. isomers) (Fig. 29.). The LD₅₀ value of the most toxic congener for rodents is 0.6 μg/kg. They accompanying components (impurities) of chemicals or can be formed during the combustion processes of chemicals containing inorganic or organic halogens.

Health endangering compounds originated from heat treatments are polycyclic aromatic hydrocarbons (PAHs), acrylamide and furan.

PAHs have three or more linearly or angularly fused benzene rings. They possess carcinogenic properties. Among the several congeners benzo[a]pyrene (Bap) is used as indicator component. They are formed owing to the pyrolytic reactions during the combustion of organic materials e.g. wood smoking of fish and meat products. The combustion of fossil fuels (fuel oil, coal) can also result in the formation of PAHs. Contamination falls out from the atmosphere and can concentrate on the surface of leafy vegetables and fruits. PAHs can also contact with food if cereals are dried directly with the combustion gases or during roasting of coffee and barbecuing.

Acrylamide has been used in several industrial processes but owing to its properties it was assigned as carcinogenic working substance. Recently acrylamide was also discovered in thermally treated foods (e.g. chips, cakes). Its concentration greatly varies depending on the process conditions and the properties of the raw material e.g. in potato products the amount of acrylamide formed is proportional to the fructose and glucose concentration of potatoes before frying. The promoters of acrylamide formation have been shown to be asparagine and reductive carbohydrates. The formation of acrylamide can be suppressed with the lowering of the heating temperature, with the reduction of pH and the enzymatic cleavage of the amino group from asparagine prior to treatment.

Thermal decomposition of amino acids may result acetaldehyde and glycolaldehyde that can deteriorate further giving rise the level of furan (Fig 30.). Other possible precursors are PUFA, carbohydrates, carotinoids and ascorbic acid. Furan is carcinogenic. It can be present in roasted coffee, carrot mash and potato/spinach mash (baby food).

The investigation of residues of disinfectants and cleansing agents is especially important in the area of milking in large-scale animal husbandry (udder disinfection agents) and in processed meat production (chemicals remaining back on the surfaces of the equipment) moreover in fruits and vegetables (antimicrobial agent).

Nitrate is uptaken from food mostly from vegetables while nitrite is derived mostly from cured meat. Endogen synthesis is also possible in human from arginine.

Nitrite can convert hemoglobin to methemoglobin (with Fe³⁺) which cannot transport oxygen. Methemoglobin can be converted back to hemoglobin with methemoglobin reductase. In case of infants methemoglobin reductase has low activity therefore an excess intake of nitrite can cause cyanosis.

Nitrate in itself is not toxic to animals. Bacteria reduces nitrate to nitrite in the digestive tract. The most part of the nitrite present in the digestive tract can be originated from nitrate reduction.

Nitrosamines and nitrosamides possess very strong carcinogenic properties. The main promoters of their formation are secondary amines, N-substituted amides (Fig 31.) and nitrous acid. Primary and tertiary amines could also be precursors to a less extent. The most often found compound is dimethylnitrosamine. These compounds can form in cured meat products containing nitrite and nitrate. Nitrosopyrrolidine can derive from proline via nitrosation and subsequent decarboxylation during roasting or frying of meat products. As the bacterial reduction of nitrate to nitrite in the intestine is unequivocal, parallel to this process, the possibility of the endogen formation of nitrosamines in the intestine via nitrosation of the amines present was evaluated. The endogen nitrosamine synthesis has been shown to be insignificant.

The formation of nitrosamines can be diminished with the application of inhibitors of nitrosation reaction (e.g. tocopherols or ascorbic acid) or with the decrease of the incorporated amount of nitrite and nitrate into processed meat. The latter possibility is limited due to the health hazard of enterotoxin-forming bacteria - especially Clostridium botulinum). In the case of vegetables the nitrate content can be diminished with appropriate agriculture (reducing the amount of nitrogen fertilizer).

Mercury, arsenic, cadmium and lead are regarded as toxic trace elements.

Mercury originated from fungicides or seed dressing compose into organomercury compounds (e.g. dimethyl mercury). These compounds are lipid soluble therefore readily adsorbed and accumulated. The industrial waste water containing inorganic mercury salt resettles in lakes or rivers. Microorganisms convert them into methyl mercury compounds.

Organomercury compound can be concentrated in fish that can lead to food poisoning.

Arsenic is very dangerous from the viewpoint of toxical activity, frequency of occurrence and probability of man’s exposure. This element is more toxic than mercury, lead, PCBs and benzo[α]pyrene, vinyl chloride, benzene or cadmium.

Cadmium ions can be absorbed easily by the plant. They evenly distributed throughout the organs therefore decontamination by dehulling is not achievable than in the case of mercury and lead. Foods that are most often affected are peanuts, linseed, some wild mushrooms.

The lead emission from the internal combustion engines of cars has dropped but the contamination from other industrial sources increased. The most contaminated plants are being cultivated close to the emission sources and having larger leaves e.g. cabbage, spinach. The lead uptake of animals from contaminated forage is low because the absorption of lead is minimal. However, the lead burden of man in preindustrialized times was higher than nowadays.

On the whole, the level of the toxic trace elements (lead, mercury and cadmium) in food seems to be shown a decreasing tendency.
Chapter 12. Food Items

1. Milk and Milk Products

Milk is an oil in water emulsion containing the next dispersed particles: fat droplets, casein micelles, globular proteins (whey proteins), lipoprotein particles (microsomes) and somatic cells (leucocytes). Minerals and carbohydrates are solubilized compounds. Its color is derived from the light absorption milk fat globules and protein (skim milk is also white). Its yellowish color is contributed to the carotene originated from the forage. Lactose provides its mildly sweet taste.

The ratio of the individual protein fractions in milk depends on genus. Casein has got several fractions (α, β, γ, κ) and subfractions. Whey proteins consists of α-lactalbumin, β-lactoglobulin, serum albumin, immunoglobulins (G₁, G₂, A, M) and proteose-peptone fraction. The main portion of casein is aggregated to 50–300 nm particles that are called casein micelles . The subunits consist of casein monomers. These subunits are connected through phosphate bridges to aggregate casein complex (Fig 32.). κ-Casein is situated on the surface of submicelles and prevents the coagulation of casein micelles. Rennet (chymosin, rennin) cleaves the peptide chain of κ-casein forming para-κ-casein and glycopeptide. The very hydrophobic para-κ-casein remains on the surface and the resulting hydrophobic interactions are the driving force for gel formation. Casein coagulation by souring is based on a different mechanism: in the case of acidification the micelle structure changes due to swelling and the dissolved casein reassociates giving a gel network.

The main sugar of milk is lactose that is less sweet than fructose, glucose or sucrose. Glucose, amino sugars and oligosaccharides are also present in small amounts. Lactulose is an artifact derived from lactose during thermal processing of milk.

Lactose free milk produced for people having lactose intolerance. Prior to processing lactose is hydrolized to glucose and fructose with β-1,4-galactosidase.