Food Chemistry

Module 12. Jams, jellies and pickles

Lesson 27
CLASSIFICATION, COMPOSITION AND PRESERVATION

27.1 Introduction

Jams and jellies are products made principally from fruits, but they can also be made from some vegetable materials, such as sweet potatoes, tomatoes, carrots, and some legumes. Generally a preserve or jam is a product manufactured with one or a permitted combination of fruit ingredients, and one or any combination of some optional ingredients. Fruit ingredients should be mature and properly prepared, including fresh, concentrated, frozen, or canned. Pickling is an ancient art of food preservation. It is the process of preserving food articles by anaerobic fermentation in brine (salt solution) to produce acid, or storing it in acid solution (usually vinegar). The resulting food preparation is called a pickle, i.e edible product preserved and flavoured in a solution of common salt and vinegar along with spices and oils.

27.2 Jams and Jellies

Jams are produced usually from one kind of fruit. They are thickened by boiling and constant stirring of the whole or sliced fresh or fresh stored raw material, or of fruit pulp. Ordinary jams are also made from fruit slurry. Boiling under a vacuum at 65–80 °C offers the advantage of preserving the aroma and color. The disadvantages are the absence of sucrose inversion and the low caramalization. These reactions produce the characteristic taste of jams boiled in an open kettle. The optimal pH of 3.0 required for gelling is adjusted by the addition of lactic, citric or tartaric acid, if necessary.

Jellies are gelled food made of one or a permitted combination of fruit juice ingredients and one or any combination of the optional ingredients. Such a mixture is concentrated with or without heat. Fruit juice used in jelly manufacture is the filtered or strained liquid extracted with or without application of heat and with or without addition of water from mature, properly prepared fruits that are fresh, frozen, or canned. Some of the fruits used for making fruit jellies include apple, apricot, blackberry (other than dewberry), black raspberry, cherry, fig, gooseberry, grape, grapefruit, guava, orange, peach, pineapple, pomegranate, pear, quince, raspberry, red raspberry, strawberry.

27.2.1 Essential Ingredients in Jams and Jellies

Jams and jellies are products based in texture formation. They are characterized by the formation of a special viscous structure in jams and gel formation in jellies, and both properties are developed by the interaction of sugar, pectic substances, and acidity (pH). In jams, the viscosity is the result of an interaction between sugar and pectin in the presence of high fiber content. All cell wall materials are present in the product and the effects of cellulose and hemicellulose molecules do not permit the formation of a continuous gel. In jellies, clarified or strained juices with very low fiber content are used; hence the relationship between pectin and sugar permits the formation of a continuous gel structure.

a) Sugar: Most jams and jellies are added with 65% of sugar. The types and concentrations of sugar are responsible for some of the taste in jams and jellies; added sugar, normally sucrose, does not have the same effect on this important quality factor. Jam and jellies fall under the category of so-called intermediate products, having a_w of 0.80 to 0.85. These products are not self-preserved because the water activity values are not low enough to control microbial growth or chemical reactions. The principal microbiological problems are molds and yeast, not bacteria.

b) Pectin: Pectin is a very complex molecule formed by a polymer of D-galacturonic acid. The degree of esterification indicates the capacity of the pectin to form a gel. Gel formation is produced by the relationship between pectin, water in the fruit, and sugar, under a controlled pH. High methoxyl pectins gel at acid pH (less than 3.5) in the presence of sugar. Low methoxyl pectins, on the other hand, gel at higher pH in the presence of some divalent cations, of which the most relevant is Ca²⁺.

Fig. 27.1 Monomeric unit of high-methoxyl pectin

c) Acid: Acid is also an essential component in jams and jellies. Normally fruit used for making jams and jellies has a low pH. Acid stabilizes the relation between pectin and sugar. Berries have low pH due to their content of some common organic acids, such as ascorbic, citric, tartaric, and malic acid. All these acids can be used to increase the acidity in jams and jellies. Acids also help to produce the inversion of sugar at the beginning of the process. Sucrose is converted into glucose and fructose, which may improve the quality of products by increasing the brightness, reducing crystallization, and reducing the sugar flavor in products.

Table 27.1 Composition of various jams (average values in %)

27.2.2 Mechanism of formation of pectin gels

The most commonly accepted theory of gel formation is that the mechanism of jellying involves stacking of polysaccharide chains to form junction zones. For gel formation sugar, acid, water, pectin must be present. Protons of the acid shift the equilibrium between ionized and unionized groups. Added sugar further decreases the hydration of pectin by competing for water, there by lowering a_w. Hence water is less free to solvate the polysaccharide and so there is increased hydrophobic interaction between methyl esters group. Thus, due to loss of some of their charges and hydration, the polymer molecules can now associate over a portion of their length forming junctions and thus a network of polymer chain is formed that entraps aqueous solution of solute molecules. Finally when cooled the unstable dispersion of hydrated pectin forms gels.

27.2.3 Factors affecting gel formation

A firm gel is that which is firm enough to stand without appreciable deformation and yet tender enough to spread readily on bread. A firm gel depends on following factors:

a) Pectin: As % pectin increase in the mixture, the firmness of jellies produced on cooling increases. A satisfactory jelly is obtained with around 1% pectin, but quantity will vary with quality of pectin preparation, average molecular weight of pectin molecules and degree of methylation.

b) Degree of methylation in pectin: Excellent jellies can be prepared from pectin with wide range of methoxy content but max jellying appears at about 8%. This represents esterification of half of the carboxyl group. By definition, preparations in which more than ½ of the carboxyl groups are in methyl ester forms are classified as high methoxy pectins. The high methoxy pectines gel when sufficient acid and sugar is present. Preparations in which less than 1/2 of –COOH groups are in methyl ester form (< 7 %) are called low methoxy pectins. Low methoxyl pectins gels only in presence of divalent cations and can form a gel even if percentage of solids are very low. Low methoxy pectins do not require the presence of sugar for the formation of gel. The divalent ions (Ca²⁺) react with carboxyl group on the molecules of pectic acid and form a bridge between them.

c) pH: Most pectic products do not form jellies until pH is lowered to 3.5. The firmness of jellies increases as pH decreases. With very low pH, the amount of pectin required is very less and satisfactory gel still formed.

d) Sugar: Sugar is necessary for formation of pectin gels and must be present in minimum concentration. Most jellies are made with 65% of sugar. If amount is increased above 65%, crystallization tends to occur on jelly’s surface and occasionally even within the jelly.

27.3 Pickles

Pickles are easy to prepare with right ingredients and can be preserved for months. Pickles serve as a flavor enhancer and consumed typically in small quantities along with usual meal. They add to palatability of a meal, aid in digestion and are good appetizers. There are a wide variety of different pickles made and each is usually made with a mixture of fruits or vegetables which are chopped and immersed in a liquid (often oil or lemon juice) and a variety of different spices and salt. Varieties of pickles include, pickles from lemon, mango, amla, ginger, green chilly, mix vegetables, cucumber, cabbage, garlic, carrot and sometimes fish, prawns, eggs or meat etc.

Addition of salt and acid to pickle gives the food a salty or sour taste. Most distinguishing characteristic of pickle is a low pH i.e. pH 4.6. This prevents the bacterial spoilage of foods and preserves perishable foods for months. Antimicrobial herbs and spices, such as mustard seed, garlic, cinnamon or cloves, are also added to pickles. Edible oils can also play a part as an oxygen-excluding covering for pre-pickled matter.

27.3.1 Types of pickling

a) Long, fermentation-based pickling: Requires a 'curing' period (up to several weeks) at room temperature.
b) Quick, unfermented pickling: It is made by adding acid (e.g. vinegar) to prevent bacterial growth.

27.3.2 Classification of pickles

a) Acid-based Pickles: The most common liquid for acid pickling is vinegar. This is an impure, dilute solution of acetic acid, obtained by the fermentation. Examples of vinegar-based pickles are pickled ginger, pickled vegetables (a mixture of onions, carrots, cauliflower, etc.), pickled sausages, etc.

b) Dry-salted Pickles: Salt has two effects when added to fruit or vegetables. It draws water from them by osmosis and triggers the fermentation process of the lactic bacteria. The resultant fermentation produces a particularly rich range of complex flavours. The most common dry-cured pickle is sauerkraut, dry salted pickled limes and lemons, plums, etc.

c) Brine-based Pickles: Brine-pickling works by a combination of osmosis and lactic fermentation. Cucumbers are traditionally pickled in brine along with other flavourings. Other examples are brine-pickled vegetables, garlic, chillies, etc.

d) Lye Pickling: Olives cannot be eaten in their raw state and require pickling to render them digestible. Before pickling, they require treatment with lye to remove substances which would be toxic to the bacteria causing fermentation. Once pickled, olives are packaged in various forms, with the addition of various herbs and spices, in brine, vinegar, and oil or dried and salted.

e) Pickles in Sugar: Fruits are sometimes first pickled (using vinegar) before being stored in a syrup or honey. Alternately, sweet-sour syrup is often made by adding sugar to vinegar. Such pickles are normally served along with meats or cheeses. Examples are watermelon rinds, walnuts, etc.

f) Oily Pickles: Oil finds its way into pickles. Various species of mushroom which are brine-pickled before storage in olive oil. Mustard oil and other vegetable oils are added to dry salted fruit and vegetables (lemon, mangoes, chillies, etc) along with spices.

27.3.3 Preservation principles

Commercial preservation of many pickles relied upon conversion of fermentable carbohydrates to organic acids during bulk storage and/or the addition of sufficient amount of sugar, vinegar and other ingredients to the fully cured and packed products to preclude any microbial growth. Organic acids, oils, salt and spices all have antimicrobial properties at suitable concentration especially in combinations which preserve the pickles. Pickles in brine as such or after fermentation however, need some amount of preservative or pasteurization to prevent the spoilage.

27.3.4 Pickling process

Pickling is done in two stages

1. By curing or fermentation with dry salting or fermentation in brine or salting without fermentation.
2. By finishing and packing.