FOOD AND INDUSTRIAL MICROBIOLOGY

Module 4. Microbiology of food preservation

Lesson 14
CHEMICAL METHODS

14.1 Introduction

Preservatives are substances that limit, slow down or stop the growth of microorganisms (bacteria, yeasts, molds) or present within the food and thus prevent the deterioration of the goods or food poisoning. They are used in cooked food, wine, cheese, fruit juices and margarines. Chemical preservatives prevent microbial growth without loss of nutrient but some people are sensitive to some chemical preservatives. Chemical food preservatives are the best and the most effective for a longer shelf life and are generally fool proof for the preservation purpose. Chemical substances are the artificial preservatives that stop or delay the growth of bacteria, spoilage and its discoloration. These artificial preservatives can be added to the food or sprayed on the food. Several kinds of chemicals can be used for food preservation including propionic acid, sorbic acid, benzoic acid , and sulfur dioxide. These acids are acceptable because they can be metabolized by the human body. Acids such as citric acid, acetic acid (vinegar) and ascorbic acid are also known to confer protection against product deterioration. In these cases, the pH of the product is shifted to being low, that is, more acidic, where very few moulds, yeast and bacteria are able to grow and multiply. Food additives such as benzoate and sorbate are quite commonly used in the fruit drink industry to protect against microbial spoilage, while nitrites are used in meat processing. The most commonly used preservatives are the acids such as sorbic acid, benzoic acid and propionic acid. These check mainly the growth of yeasts and molds. Sorbic acid is used for preservation of syrups, salads jellies and some cakes. Benzoic acid is used for beverages, margarine, apple cider etc. Propionic acid is an ingredient of bread and bakery products. Sulphur dioxide, as gas or liquid is also used for dried fruits, molasses and juice concentrates. Ethylene oxide is used for spices, nuts and dried fruits. Some antibiotics can also be used, depending upon local laws and ordinances. Tetracycline, for example, is often used to preserve meats. Storage and cooking normally eliminates the last remnants of antibiotic. Antioxidants are also the chemical food preservatives that act as free radical scavengers. In this category of preservatives in food comes the vitamin C, BHA (butylated hydroxyanisole). Unlike natural food preservatives some of the chemical food preservatives are harmful. Sulfur dioxide and nitrites are the examples. Sulfur dioxide causes irritation in bronchial tubes and nitrites are carcinogenic. All of these chemicals act as either antimicrobials or antioxidants or both. They either inhibit the activity of or kill the bacteria, molds, insects and other microorganisms. Antimicrobials prevent the growth of molds, yeasts and bacteria and antioxidants keep foods from becoming rancid or developing black spots. They suppress the reaction when foods come in contact with oxygen, heat and some metals. They also prevent the loss of some essential amino acids and some vitamins. These substance works on the principle of controlling water activity, pH and osmotic pressure of food.

14.2 Water Activity

Water is the most important factor in controlling the rate of deterioration of a food. It is the availability of water for microbial, enzymatic, or chemical activity that determines the shelf life of foods. This water availability is measured as water activity (a_w). a_w of milk is 0.97, honey is 0.5-0.7 and dried fruits is 0.5-0.6. Food spoilage micro-organisms, in general, are inhibited in food where the water activity is below 0.6. However, if the pH of the food is less than 13.6, micro-organisms are inhibited when the water activity is below 0.85. The water activity (a_w) values for inhibition of some of the microorganism are Cl.botulinum 0.97 , Ps. fluorescence 0.97, E.coli 0.95, B.cereus 0.93 and S.aureus 0.86. Generally, Intermediate moisture food (IMFs) possesses water activities that range from 0.6 to 0.85. This enables the food to be stable at room temperature, because the growth of most micro-organisms is inhibited at these levels. Binding the water that's present preserves intermediate moisture foods, for example, cookies, cake and bread. This reduces the availability of the water for deteriorative reactions. Water is immobilized by adding permissible humectant additives such as glycerol, glycols, sorbitol, sugars and salts. The addition of some chemicals inhibits microbial growth in foods. These chemicals include not only those classified as preservatives . Salt, sugars, wood smoke and some spices also inhibit the growth of micro-organisms.

14.3 Types of Artificial/ Chemical Food Preservatives

These are generally grouped as:

1. Antimicrobial agents
2. Antioxidants
3. Chelating agent

In antimicrobial the benzoates, sodium benzoate, sorbates and nitrites are generally used. Antioxidants include the Sulfites, Vitamin E, Vitamin C and Butylated hydroxytoluene (BHT) and the c helating agent like the disodium ethylenediaminetetraacetic acid (EDTA), polyphosphates and citric acid.

14.3.1 Some common chemical preservatives

a) Nitrates and nitrites: Nitrates are converted to nitric acids which form stable red colour in meat. Nitrites have inhibitory action against Clostridium botulinum in meat products but forms carcinogenic nitrosamine. It is also used to preserve meats such as sausage, ham, bacon, beef, etc. The side effects are: allergy, asthma, nausea, vomiting and headaches and sodium nitrite can be converted to nitrous acid in the body and cause cancer

b) Sulfites (sulfur dioxide and metabisulfite): Sulfites form sulphurous acid which is active antimicrobial compound and can kill the microbial cell by reduction of sulphide linkage, formation of carbonyl compounds and inhibiting the respiratory mechanism. These are used to prevent fungal spoilage and browning of peeled fruits and vegetables.The commonly used level of sulphites is 0.005 -0.2% . The side effects are allergy , asthma, nausea, vomiting, joint pain, palpitation, and headaches.

c) Sodium benzoate or benzoic acid: They are more effective against yeast and molds and are used at a concentration of upto 0.2%. The action of benzoates against microorganisms are by inhibiting enzymes necessary for oxidative phosphorylation, inhibiting membrane protein function and also by destroying membrane potential. They are added to carbonated drinks, margarine, flour, pickles, fruit purees, and fruit juices. The side effects are severe allergic reaction and cancer.

d) Propionates: Calcium and sodium propionate are effective against mold and bacteria at 0.1-0.2% but not effective against yeast at this concentration. The inhibitory action is due to cytoplasm acidification and destabilization of membrane proton gradient.

e) Sorbate: Sodium, calcium or potassium salt is used at the concentration of 0.05-0.2%. It is more effective against yeast and mold than bacteria. The activity of sorbic acid increases as the pH decreases. Sorbic acid and its salts are tasteless and odourless when used at levels below 0.3%. It is used in non-alcoholic drinks, alcoholic drinks, processed vegetables and fruits and dairy desserts.

f) Butylated Hydroxytoluene (BHT) and Butylated Hydroxyanisole (BHA): They serve as antioxidants that prevent the oxidation of fats (rancidity). They are used in preserving fresh meat, pork, sausages, potato chips &crackers, beer, baked goods, drink powder, dry cereals, and frozen pizza. These compounds have several side effects and they can cause cancer and liver disease.

g) Mono-glycerides and Diglycerides: They are used as preservatives for cookies, cakes, pies, bread, peanut butter, roasted nuts, shortening, and margarine. They may cause cancer and birth defects.

Salt and sugar: Salt and sugar have long been used as effective means of extending shelf life of various products as these solutes bind water, leaving less water available for the growth of microorganisms. Essentially the water activity (a_w) of the product is reduced. Since most microorganisms require a high water activity, they are unable to survive.

Table 14.1 Examples of commonly used preservatives in different foods