Definitions And Standards

The name preserves covers a broad range of products, including jams, butters, marmalades, and conserves, as well as ordinary preserves. Preserves contain the largest fruit pieces, whereas jams contain smaller pieces that are crushed or chopped with added acid. Fruit butters are made of fruit pulp cooked to a smooth consistency. They are pressed through a coarse strainer and are more concentrated than jams. Scorching can be a problem because of their high viscosity. Marmalades have the characteristics of both jellies and preserves. They contain thin citrus peel or fruit pieces and are chiefly made from citrus fruits, alone or in combination with other fruits. Conserves are similar to jams, except that two or more fruits are cooked together and raisins and nuts can be added. Jellies are in a class by themselves. They are clear sparkling spreads in which fruit juices as the source of flavor, and, in some cases, the thickening agent.

The Federal Standards and Definitions do not differentiate between preserves and jams (1,2). A preserve is minimally 45 parts prepared fruit with 55 parts of sugar and is concentrated to 65% or higher solids, resulting in a semisolid product. Jellies are similar to preserves, with 45 parts of clarified fruit juice and 55 parts of sugar, resulting in a minimum of 65% solids. Both categories can utilize a maximum of 25% corn syrup for sweetness as well as pectin and acid to achieve the gelling texture required. Fruit butters are prepared from mixtures containing not less than five parts by weight of fruit to two parts of sugar.

GELATION-PECTIN MECHANISM Typical Gel Formations

Gelation, the formation of the polymer network that gives commercial fruit preserves and jellies their texture, depends on four essential ingredients—pectin, sugar, acid, and water—added in the correct proportions. A pectin gel is a system resembling a sponge filled with water. This polymer is in a partially dissolved, partially precipitated state. The chain molecules are locally joined by limited crystallization, forming a three-dimensional network in which water, sugar, and other solutes are held. Some fruits such as tart apples, red and black raspberries, oranges, and cranberries have enough pectin and acid present. Still others, such as ripe apples and plums, contain sufficient pectin but lack enough acid. Pectin or acid must be added when using most fruits. Sugar is always needed when high-methoxyl pectin is used.

Since fruits vary widely with regard to maturity, climatic conditions, and storage, it can be difficult to ensure the proper composition. Fruit should be picked just before processing to ensure taste and texture. It should be picked as ripe fruit in the early morning to ensure quality. Overripe fruit will have reduced sugar quality, and the pectin will suffer molecular breakdown from enzyme activity. If fresh fruit is not available, frozen, cold-pack, or canned fruit can be used for jams and preserves.

The juice of grapes, currants, lemons, sour oranges, and grapefruits contains sufficient pectin and acid for jelly manufacture. Strawberries, rhubarb, and apricots usually contain sufficient acid but may lack pectin. On the other hand, sweet cherries and quinces may lack acid yet have enough pectin. Commercial pectin, either liquid or powder, can be added as a supplement. The viscosity of a fruit juice is an index of its gelling power.

Pectin is found in the flesh, skins, and seeds of most fruits. It can be extracted when fruit is boiled. Pectin is a complex carbohydrate consisting of polygalacturonic acid chains having a wide variety of molecular weights. The chains contain some carboxyl groups that are partially methylated, forming the ester known as pectin.

Generally, a degree of methylation (DM) of 50% divides commercial pectin into two main groups: high-methoxyl (HM) pectins and low-methoxyl (LM) pectins. The LM-pectin group includes both conventional and amidated versions. The HM pectins are the predominate choice for the standard jellies and preserves. The LM pectins can be utilized in low-sugar fruit spreads.

The degree of methylation of pectin has a critical influence on the solution and gelation characteristics of preserves and jellies. The highest DM that can be achieved by extraction of the natural raw material is about 75%. Pectins of DM ranging from 0 to 70% are produced by de-methoxylation in the manufacturing process.

The DM of HM pectins controls their relative speed of gelation; hence, the terms slow-set and rapid-set HM pectin. If a higher degree of methylation of pectin is used, the higher will be the pH required for a fast set. A fast set is necessary to suspend fruit pieces and prevent fruit flotation or sinking. A slow set is necessary for a clear jelly, so that air bubbles are removed. The pectin's quality is standardized on the basis of its 150° standard, which means that, under controlled conditions, 1 lb of 150-grade pectin will gel 150 lb of sugar. This method is known as SAG.

Another method for testing pectin gels is the Voland-Stevens LFRA texture, analyzer in which the gel's elasticity is exceeded. The Tarr-Baker gelometer was the original jelly strength tester. However, after it was discontinued in 1965, the Voland-Stevens analyzer was declared an acceptable replacement because of its operational similarity, portability, compactness, reproducibility, and ease of use and calibration (3). This flexible instrument can be attached to a printer to measure elasticity and other attributes of a gel. This method is comparable to the SAG method.

Jellies are usually produced at a pH of 3.1 and jams at 3.3. HM pectin can gel sugar solutions with a minimum of about 55% soluble solids within a pH range of approximately 2.0 to 3.4. For any soluble solids with a value above 55%, there is a pH value at which gelation is optimal for a particular HM pectin and a pH range within which gelation can be controlled.

Sugars have a general dehydration effect on HM-pectin solubility. At higher solids values, there is less water available to act as a solvent for the pectin. Hence, there is an increased tendency to gel. Because gelation relies on the proper balance of soluble solids and pH in the medium, it is possible to compensate for a reduction in soluble solids by reducing the pH. Any HM pectin can gel rapidly or slowly and the rate can be controlled by the soluble solids and pH.

An attempt was made to quantify fruit content in jams by combining chemical composition data, particularly the inorganic elements that are stable to processing such as ash, magnesium, and potassium, with the rheological forces such as yield stress and flow index in a regression analysis that could explain 90% of the variability in fruit content (4).

Novel Gel Systems

The typical manufacturing methods for jams and jellies use the four necessary components: fruit, pectin, sugar, and acid. Some combination pectin sources have been devised. In one (5), an emulsifler is added to the sugar surface and blended with the bulk of the sugar. A very fine pectin is then mixed with acid and combined with the emulsifier-sugar complex. The emulsifier acts not only as a glue, causing the fine pectin particles to adhere to the sugar surface, but it also acts as an antifoaming agent and a dispersing agent in the final gel production.

Another convenient product, one-step pectin gelling composition, has been developed (6). Pectin particles are mixed with moistened coarse sugar particles. Acid may be added in a dry form and will adhere to the sugar, or it can be predissolved in water and sprayed onto the mix. Much less pectin can be used in this method as compared with dry blending fine pectin, acid, and sugar. This is due to the use of larger sugar particles and smaller pectin particles, so that the pectin dissolves faster while the concentration of dissolved sugar solids is retarded.

Instant pectins are technologically optimized pectins with better dispersion and solubility properties. They dissolve rapidly even at low temperatures and work into jam mixes without prior mixing with sucrose or corn syrup (7).

The LM pectin offers another gel formation method for preserves and jellies. However, the gel formed does not conform to the federal standard of identity. The use of this pectin does result in reduced-sugar jams, jellies, and preserves, the LM-pectin group includes both conventional (acid demethuylated) and amidated types. These pectins require calcium ions for gelation, not sugar or acid.

The gelation of LM pectin is controlled primarily by the reaction of a divalent cation with the acid groups of the pectin chains. LM pectin can be used at solids levels as low as 10%. The pH range for LM pectin is 3.0 to 6.0 because the role of acid is minimized. For successful gelation to occur, 50 to 100% of the acid groups must be complexed with calcium. The amidated pectin, which has fewer free acid groups, requires less calcium for gelation and relies on hydrogen bonding between the amide and free acid groups. Amidated pectins form gels that are more rigid than those formed with conventional pectins. Conventional pectins produce a thickening effect and are aptly used for jams and preserves; amidated pectins are used for jellies.

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