Preservatives

101 Toxic Food Ingredients

101 Toxic Food Ingredients

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Preservatives are added to decrease the degradation rate of foods during processing and storage. They include antioxidants, antimicrobials and antibrowning agents.

5.2.3.1 Antioxidants Antioxidants primarily prevent or inhibit autoxidation of fatty acids (see also Part I, Chapter 6) in food products and, consequently, the development of rancidity and off-flavor. They are especially useful in preserving dry and frozen foods for long periods of time. The major antioxidants for the protection of dietary fats and oils are phenols. They are either synthetic or natural substances. The synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), n-propyl gallate and tertiary-butyl hydroquinone (TBHQ). Important natural food antioxidants are the tocopherols. They occur naturally in the majority of fats and oils. The mechanism of the antioxidant action of phenols is shown in Figure 5.1.

Figure 5.1 Diagrammatic representation of the mechanism underlying the antioxidant action of phenolic antioxidants.

Figure 5.1 Diagrammatic representation of the mechanism underlying the antioxidant action of phenolic antioxidants.

Generally, BHA was not believed to be a hazardous substance. However, the results of recent studies in experimental animals suggest that the intake of BHA involves a cancer risk.

The case of BHT is more complex. There is evidence that BHT promotes several types of chemical carcinogenesis in a number of experimental animals. Further, liver damage and cytotoxic effects have been found.

Accurate data on the daily intake of BHA and/or BHT by man are not available. Generally, estimates are in the range of 1 to 5 mg/day. Estimated total use in the US of BHA is 143,000 lb/year and of BHT 670,000 lb/year. In the case of propyl gallate, no evidence of carcinogenicity, mutagenicity or teratogenicity has been provided. BHA, BHT and propyl gallate are almost universally accepted for use in food since the 1950s.

TBHQ is the most recently developed synthetic food antioxidant. It has been designed especially to protect polyunsaturated oils. In long-term animal feeding tests, no indications of carcinogenicity were obtained. As yet, TBHQ is allowed for use in food in the US and a few other countries, but not in the EU.

C(CH3)3

C(CH3)3

BHA OCH3

(CH3)3C

C(CH3)3

(CH3)3C

BHT CH3

C(CH3)3

cooc3h

BHT CH3

C(CH3)3

cooc3h

TBHQ

Propyl gallate

(3, 4, 5 - trihydroxybenzoic acid propyl ester)

Propyl gallate

(3, 4, 5 - trihydroxybenzoic acid propyl ester)

a - Tocopherol (vitamin E)

a - Tocopherol (vitamin E)

There are many phenols of natural origin which are strong antioxidants. Sometimes, they are more effective than the major synthetic phenolic antioxidants. At present, tocopherols and rosemary extract are commercially available.

HO HOOC

HO HOOC

Antioxidants in rosemary extract

Antioxidants in rosemary extract

5.2.3.2 Antimicrobials Antimicrobials are used to prevent or inhibit the growth of microorganisms. They play a major role in prolonging the shelf life of foods. Nowadays, consumers expect all foods to be available all year round and to have a fairly long shelf life. In dietary behavior, however, risks from microbial contamination are often overlooked (see Part 1, Chapter 2, Section 2.3.3).

As far as food safety from a microbiological viewpoint is concerned, some advances have been made without calling in the help of additives. These involve the application of certain packaging and processing methods. Nevertheless, the use of chemical antimicrobials is indispensable for safe food handling. Common antimicrobial food additives are benzoic acid and benzoates, sorbic acid and sorbates, short-chain organic acids (acetic acid, lactic acid, propionic acid, citric acid), parabens (alkyl esters of p-hydroxybenzoic acid), sulfite, and nitrite. Most of these substances are believed to be safe for application in food. They are easily excreted and metabolized by both animal and man. An exception should be made for one of them, namely nitrite. The intake of nitrite can lead to the formation of nitrosamines, which are well-known carcinogens.

Nitrite has been used as meat preservative for many centuries. It contributes to the development of the characteristic color and flavor, to the improvement of the texture of meat products, such as bacon, ham, frankfurters, fermented sausages, and canned meats, and also of fish and poultry products. Its antimicrobial effect was not recognized until the late 1920s. The primary aim of using nitrite as an antimicrobial is to prevent germination of the spores of Clostridium botulinum and hence the production of the botulinum toxin (see also Chapter 2).

Prolonged ingestion of sodium nitrite has been shown to cause methemoglobinemia, especially in infants. The major adverse effect of nitrite intake is the induction of cancer. In many animal species, this is attributed to the formation of nitrosamines in the reaction of nitrite with secondary amines. Nitrosamine formation can take place in the food itself as well as in the body. The normal acidity of the stomach is ideal for nitrosamine formation.

From a food toxicological point of view, three types of nitrosamines are of importance: dialkyl nitrosamines, acylalkylnitrosamines, and nitrosoguanidines. Cyclic nitrosamines are similar to the dialkyl type. The nitrogen atom becomes part of the heterocyclic ring. Nitrosoguanidines are a special class of highly reactive nitrosamides.

general structure of nitrosamines

general structure of alkylacylnitrosamines

general structure of N - alkyl (R) - N' - alkyl (R') -N - nitrosoguanidines

The hazards due to nitrosamines in food depend strongly on the types and levels of precursors present. Precursors can be endogenous substances, products of food components, and endogenous substances, and also contaminants. Tables 5.1 and 5.2 list nitro-samine precursors and the corresponding nitrosamines that can be formed.

Since many nitrosable substances are formed on degradation of proteins and amino acids, nitrosamine formation cannot always entirely be prevented in food and in the body. One of the most effective inhibitors of nitrosation is ascorbic acid. This vitamin reacts rapidly with nitrite to form nitric oxide and dehydroascorbic acid. In that way, it can inhibit the formation of dimethylnitrosamine by more than 90%. Other inhibitors of nitrosation are gallic acid, sodium sulfite, cysteine, and tannins. Nitrosamine levels in food also depend on the temperature at which food is prepared. Table 5.3 gives some examples of nitrosamine levels in foodstuffs. Cooking can increase the nitrosamine level in food. As can be seen from Table 5.3, frying can increase the nitrosamine level in bacon quite considerably. Up to 135°C, cooking or frying does not result in detectable nitrosamine formation. Above 175°C, however, the nitrosamine levels increase rapidly.

Nitrite addition to fresh meat and food products is still under discussion because of the earlier-mentioned toxicological hazards. Up to now, banning of this additive has been blocked by the food industry. It is stressed that so far no other antimicrobial agent has been

Table 5.1 Nitrosamine precursors, endogenous or formed in food

Compound Food Nitrosamine formed

Table 5.1 Nitrosamine precursors, endogenous or formed in food

Compound Food Nitrosamine formed

Creatine, creatinine

Meats, meat products, milk, vegetables

Nitrososarcosine (NSA)

Trimethylamine oxide

Fish

Dimethylnitrosamine

(DMN)

Trimethylamine

Fish

DMN

Dimethylamine

Fish, meat, and meat products, cheese

DMN

Diethylamine

Cheese

Diethylnitrosamine (DEN)

Sarcosine

Meat and meat products, fish

NSA

Choline, lecithin

Eggs, meat and meat products,

DMN

soybeans, corn

Proline, hydroxyproline

Meat and meat products,

Nitrosoproline and

other foodstuffs

nitrosopyrrolidine (NPyr)

Pyrrolidine

Meat and meat products, paprika

NPyr

Piperidine

Meat and meat products, cheese,

Nitrosopiperidine (NPip)

black pepper

Methylguanidine

Beef, fish

Methylnitrosourea

Carnitine

Meat and meat products

DMN

Dipropylamine

Cheese

Di-n-propylnitrosamine

Dibutylamine

Cheese

Di-n-butylnitrosamine

Table 5.2 Nitrosamine Precursors which Contaminate Foodstuffs

Compound Chemical class Nitrosamine derivative

Table 5.2 Nitrosamine Precursors which Contaminate Foodstuffs

Compound Chemical class Nitrosamine derivative

Atrazine

Secondary Amine

N-Nitrosoatrazine

Benzthiazuram

Carbamate

N-Nitrosobenzthiazuram

Carbaryl

Carbamate

Nitrosocarbaryl

Fenuron

Carbamate

DMN

Ferbam

Amide

DMN

Morpholine

Secondary Amine

Nitrosomorpholine

Propoxur

Carbamate

Nitrosopropoxur

Simazine

Secondary Amine

Nitrososimazine

Succinic acid

Amide

DMN

2,2-dimethyl

hydrazide

Thiram

Amide

DMN

Ziram

Amide

DMN

Note: See Table 5.1 for abbreviations.

Note: See Table 5.1 for abbreviations.

found that can provide protection against Clostridium botulinum as effectively as nitrite. In some EU countries (but not in Germany and the UK) and the US, nitrite addition to fresh meat is allowed up to a maximum of 200 ppm.

5.2.3.3 Antibrowning agents Antibrowning agents are chemicals used to prevent browning of food, especially dried fruits and vegetables. Browning of food can occur enzymatically as well as non-enzymati-cally. The latter is dealt with extensively in Chapter 6. Enzymatic browning is mediated by polyphenol oxidase (PPO). This enzyme becomes available for catalysis upon cell disrupture. PPO contains copper and catalyzes two types of reactions (Figure 5.2):

Table 5.3 Nitrosamine levels in food

Food

Nitrosamine

Level (ppb)

Bacon, raw

0

fried

DMN, DEN, NPyr

1-40

NPip

10-108

NPyr

11-38

DMN, NPyr

2-30

Bacon, frying fat

NPyr

10-108

drippings

NPyr

16-39

Luncheon meat

DMN, DEN

1-4

Salami

DMN, DEN

1-4

Danish pork chop

DMN, DEN

1-4

Sausage

DMN

1-3

Sausage, metwurst

NPyr, NPip

13-105

chinese

DMN

0-15

Fish

raw: sable

DMN

4

salmon

DMN

0

shad

DMN

0

smoked: sable

DMN

4-9

salmon

DMN

0-5

smoked and nitrate- or nitrite-treated:

salmon, sable, shad

DMN

4-17

salted marine fish

DMN

50-300

smoked and nitrate- or nitrite-treated:

DMN

20-26

Other fish products

DMN

1-9

Fish sauce

DMN

0-2

NPyr

0-2

Cheese

DMN

1-4

Baby foods

DMN

1-3

Shrimp, dried

DMN

2-10

NPyr

0-37

Shrimp sauce

DMN

0-10

Squid

NPyr

0-10

DMN

2-8

NPyr

0-7

Canned meats (uncooked)

DMN

1-3

Ham and other pork products (uncooked)

DMN

0-5

Beef products (uncooked) (4 days after

DMN

1-2

slaughtering)

Wheat flour

DEN

0-10

Note: See Table 5.1 for abbreviations.

Note: See Table 5.1 for abbreviations.

- hydroxylation of monophenols to catechols, i.e., o-diphenols;

- oxidation of catechols to ortho-quinones.

The ortho-quinones subsequently undergo a sequence of non-enzymatic reactions to yield brown-black melanin pigments.

Generally used antibrowning agents are vitamin C, citric acid, and sodium sulfite; the latter is also a well-known antimicrobial agent. Usually, antibrowning agents are not hazardous. Sulfite, however, can cause allergic reactions. It is one of the most widely used food additives. It is cheap and can be used efficiently in a variety of applications. Recently,

OH

Catechol

Catechol

o - Benzoquinone

Figure 5.2 Reactions catalyzed by PPO: (a) hydroxylation; (b) oxidation.

attention has been drawn to the reactions between sulfite and nutrients and other food components. Although sulfite itself is considered to be safe for the majority of consumers, there is hardly any information on the nature and toxic effects of its reaction products.

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