The United States of America is the largest user of grape juice and grape juice concentrate. About 25 percent of the 1999 crush was channelled into juice concentrate. From 1998 to 1999 the quantity of imported grape juice rose a whopping 50 percent and greatly exceeded exported juice. Imported grape juice and concentrate is on the rise. Thirteen nations combined to import grape juice and concentrate into the United States of America, Argentina having 70.3 percent share in 1999. Grape juice and concentrate accounted for 9.5 percent of the value of all 1999 imports of fruit juices and concentrates and is increasing. For comparison, orange juice market makes up 37 percent, apple 28 percent followed by pineapple and grape (Larsen, 2000).
The volume of grapes processed for beverage use worldwide (including wine and beverage alcohol) exceeds any other individual fruit. Indeed, many of the grape selection and juice preparation steps are common to both even to the extent that fruit not meeting juice standards can be used for wine and ultimately beverage alcohol. In contrast, grapes not meeting wine quality standards would rarely end up as juice. (Curiously, premium "world class" wine grapes make mediocre juice.) An appreciable amount of the fresh market seedless grapes, especially `Thompson seedless', end up as concentrate juice, primarily for blending purposes. However, for juice and juice beverages where grape character is important, other species and cultivars are most prominent.
The first grape juice processed in the United States of America was used as the sacrament on the communion table of the Vineland, New Jersey, Methodist Church. Dr. Thomas B. Welch, a dentist, processed the juice, derived from Concord grapes (Vitis labruscana L.). Concord juice and Concord blends have become the standard for quality red grape juice around the world. For white juice, Niagara along with Delaware and Catawba and various labrusca blends are gaining in popularity.
Nevertheless, any grape cultivar with acceptable fresh eating quality can be used for juice. With creative blending (Chapter 9) even those of marginal juice quality can be utilized, providing the grapes are not spoiled or otherwise contaminated.
The composition of grape juice is similar to that of whole grapes except that crude fibre and oils, which are primarily present in the seed, are removed. Sugars, acids, methyl anthranilate (in Vitis labruscana), volatile esters, alcohols and aldehydes are major flavour constituents. Glucose and fructose are the major sugars present in grape juice. The quality of grape juice largely depends upon sugar level, acid content and flavour constituents such as methyl anthranilate and other volatiles, tannins and colour substances. Changes that occur in grapes during growth and maturation determine quality of the juice.
The principal acids of grape juice are tartaric, malic and citric, but small quantities of other acids are present. Flavour and aroma develop during the ripening process. Colour in grape juice is largely the result of anthocyanin pigments located in and near the skin. Moreover, the types and quantities of anthocyanin pigments are different among grape species. The differences in the types of anthocyanin help to explain why some grapes have better colour stability and are more suitable for juice processing than others.
The specific composition of juice from any grape species can never be assumed since composition varies from year to year and changes continually during ripening (De Golier, 1978). Likewise, the composition of a given species and cultivar will vary from area to area depending upon soil, location and climatic conditions. In general, as fruit matures, the sugar and colour increase and the pH and titratable acidity decrease.
The Concord grape juice industry has determined that the best objective index to determine optimum maturity is percent soluble solids. It has been reported, for instance, that around Lake Erie ideal flavour, acid and colour levels occur in grapes when the soluble solids value is between 16 and 17 percent (Morris and Striegler, Somogyi, et al., 1996b). The juice industry determined that as the percent soluble solids of Concord grapes increased above 18 percent, flavour and acid decreased; consequently, quality decreased. Concord grapes that are harvested in the range of 14 to 15 percent soluble solids have excess acidity and inadequate flavour-aroma components and may have insufficient colour. The Concord juice industry usually uses 15 percent soluble solids as the lower level of acceptable quality and pays a premium for grapes based on each increase in percentage soluble solids up to 18 percent. However, most cultivars of Vitis vinifera, the major wine grape throughout the world, produce grapes that are much higher in percent soluble solids, but lower in acid at harvest. It is not uncommon for these grapes to produce juice that is 22 to 25 percent soluble solids.
Because of the industry emphasis on the importance of the value of percentage soluble solids to quality, most of the literature dealing with the effects of pre-harvest variables on fruit and juice quality has used percentage soluble solids as the major index for quality. However, this is not the best method of predicting quality. To properly evaluate juice quality, it is important to consider all major quality attributes such as flavour, pH, acidity, colour along with percentage of soluble solids.
Among major pre-harvest conditions that influence quality of grape juice are climate, soil, cultivar, vineyard management and maturity. Each of these factors exerts its own influence, but complex interactions among these factors must be kept in mind.
The maximum, minimum and average temperatures as well as the daily pattern of heat accumulation and solar energy level have to be considered in looking at the overall site (Somogyi, et al., 1996b). Rainfall, clouds and fog and their distribution through the season are important along with other water and solar factors.
Loose soils with moderate fertility and excellent drainage characteristics are best. This ideal situation and all conditions that vary from the ideal require different vineyard management systems to obtain maximum juice quality.
Concord is the grape cultivar most widely used for juice production and the United States accounts for the vast majority of the world's Concord production. It is a rare grape cultivar that can produce juice with a balance of sugars, acids, flavouring substances, astringent characteristics and aroma as palatable and as well recognized by the consumer as Concord juice (Morris, 1985). Also, the highly flavoured Concord grape juice imparts a rich flavour after dilution and sweetening.
Other cultivars for dark juice are Fredonia, Van Buren, Sheridan, Ives and Clintonnes. Sunbelt is a new cultivar released from the Arkansas Agricultural Experiment Station. It has proven to be an outstanding juice grape cultivar in southern or warm production regions. Among white grapes, Niagara has become the standard for juice because of its unique aroma and flavour. Commercially, Niagara is usually blended with the less expensive and neutral Thompson Seedless juice from California. Cold-pressed Catawba, Isabella, Ontario and Seneca have been used for white juices, usually blended. California has greatly increased their production of grape juice concentrate, a great deal of it being the Vitis vinifera type. Vitis vinifera grapes are the most widely planted grape cultivars in the world.
The juice of muscadine grapes (Muscadinia rotundifolia) has a unique bouquet. It is appreciated by people in the Southern part of the United States of America, where it is native and its flavour is well known by the consumer. Cultivars vary in colour from almost white to pink, red, blue, purple and nearly black. Blends have a beautiful colour and a refreshing taste (Bates and Sims, 2001; Morris and Blevins, 2001).
Pruning and training systems, fertilization, irrigation, application of growth regulators and pest control measures are vineyard management operations that can influence juice quality. Maintaining an adequate and balanced mineral nutrition program is a major factor in producing high fruit yields and quality grapes. It is not uncommon to create fruit quality problems with excessive nitrogen fertilization that results in excessive vigour and subsequent fruit shading. Also, excessive potassium (K) can result in quality problems. Excessive K levels in the juice were detrimental to fresh juice colour quality and stored juice colour stability, making a balanced K fertilization program highly important in vineyard management of grapes.
Morris (1985) found that in harvest maturity, the flavour and sugar/acid ratio of Concord juice was directly related to maturity, making harvest dates crucial determiners of juice quality. Most grapes used for juice are mechanically harvested. It was shown that mechanically harvested grapes are of better quality than hand-harvested grapes. Effects on the quality of machine-harvested grapes can be altered or influenced by six major factors:
Muscadine grapes present a major problem for once-over machine harvesting, since, unlike other commercial Vitis species, many cultivars of muscadine do not ripen uniformly. The presence of immature fruit in an once-over harvest is undesirable, since it lowers the quality of the processed product. Lanier and Morris (1979) developed a system for sorting machine-harvested muscadine grapes into maturity classes using a density sorting system. It provided a rapid and inexpensive way of removing fruit of undesirable maturity. The ease of berry detachment from clusters, spherical shape of the muscadine berry and relatively small variation in fruit size characterizes it as ideal for mass density sorting.
Figure 12.1 illustrates a generalized flowchart for grape juice production. There are several options for juice extraction and subsequent treatment. Methods for commercial preparation of grape juice have undergone continuous change. In most commercial operations, the continuous pressing method is used. Hot pressing is appropriate for deeply pigmented grapes where maximum colour extraction is desired. Whereas, the immediate or cold press procedure is necessary to maintain the initial colour of light coloured grapes.
Figure 12.1: Grape juice manufacture flowchart.
Figure 12.2:
Hot press enzyme treatment.
Prior to rice hull addition (bucket on
left)
Hot-press juice production involves the addition of a pectolytic enzyme to break down naturally occurring pectins and it uses paper pulp or rice hulls as press aids to facilitate extraction of juice (Figure 12.2). A hot-press method yields more juice that contains higher total solids, more non-sugar solids, tannins, pigments and other substances than a cold-press juice operation. When hot pressing, the temperature and time in processing can be varied within a range to produce juice with uniform colour from grapes harvested throughout the season. Excessive extraction temperatures (exceeding 65°C or 150°F) must be avoided to preserve juice quality.
Figure 12.3:.
Grape stemmer/crusher.
Rice hulls added, auger feeds press
Following the schematic in Figure 12.3, harvested grapes are dumped into a hopper and transported by augers or pumps to a rotary stemmer-crusher that separates the fruit from the stem. The crushed berries are pumped through a steam-jacketed, vacuum preheater in which the pulp is heated to 60 to 63°C and passed into holding tanks. At this point, slow-moving agitators mix pectolytic enzyme and ~7 Kg of purified paper pulp (as a press aid) into each 1 000 Kg (metric tonnes, MT) of grapes. It takes between 30 and 60 minutes for the enzyme to break down the pectin to make the grape pulp ready for pressing. This part of the process helps to extract colour from the skins into the juice (Tressler and Joslyn, 1971).
Next, a dejuicer removes 30 to 35 percent of the free-run juice through a 40-mesh screen. The remaining pulp empties into a continuous screw press. The free-run juice may have as much as 20 to 40 percent suspended solids and is combined with the pressed juice that may have only 5 to 6 percent. The combined juices have most of the soluble solids removed by rotary vacuum filtration, pressure leaf filtration or centrifugation (Figures 6.14, 6.15 and 6.16). This process yields approximately 820 L of juice per MT of grapes. An additional 40 L of juice (after the juice and water have been concentrated) may be obtained by breaking up the press cake, spraying it with hot water and re-pressing. (This operation involving the addition of water to extract additional soluble solids is not permitted in table wine manufacture).
Grapes are unique from other fruits in that after juice extraction, the argols (potassium bitartrate, tartar in crude form) and tartrates must be precipitated. Otherwise, the argols will settle out upon cooling or even when filtered juice is refrigerated. These crystals, although harmless, are aesthetically unpleasant and can be mistaken for glass fragments. Thus to accomplish detartration (cold stabilization), the filtered juice is flash-heated at 80 to 85°C in a tubular or plate-type heat exchanger, rapidly cooled in another heat exchanger to -2.2°C and placed in tanks for rapid settling of argols. Seeding with bitartrate crystals and ion exchange methods exist to accelerate the cold stabilization step. The final processing into a single-strength juice or concentrate can occur once the argols have settled and the juice is racked off. The sediment can be filtered, resterilized and stored to allow the argols to settle again for optimal recovery of juice. The juice is now passed through a heat exchanger (heating it to 77°C) into an automatic filler and then into preheated bottles. The bottles are capped, pasteurized at 85°C for 3 minutes, cooled and labelled. In newer operations hot fill into plastic or aseptic packing are increasingly the methods of choice in grape juice processing (Chapter 8), although glass bottles still present a quality image.
The major difference between this method of juice production and the hot-press methods are the steps that allow for heating of the crushed berries to 60 to 63°C and holding in tanks with pectolytic enzymes. Without these steps, the dark colour from the dark-skinned grapes is not adequately extracted and the juice is a lighter colour. However, light coloured grape cultivars, lacking skin pigment and yielding a light green to yellow juice, cannot be hot pressed. Enzymes may be added to the cold-press juice to facilitate the clarification and filtration process following cold stabilization. However, extended contact time or high temperatures must be avoided to minimize enzymatic browning and undesirable colour extraction. Also, about 100 ppm of SO2 should be added to minimize browning. Juice yields from this method of processing may be only 710 L/MT, depending on the cultivar and pressing efficiency. In view of the tough skin and pulp, bronze muscadine grapes may only yield about 560 L/MT when processed using the cold press method.
Colour is one of the most important qualities of grape products. A typical purple-red is associated with high quality `Concord' grape juice or other red grape juice, but changes in colour from purple-red to brown during processing and storage cause a drastic decline in quality. This is true of all cultivars and species of grapes. The red muscadine grape anthocyanin pigments are extremely unstable under conventional warehouse storage temperatures (Bates and Sims, 2001).
The increase in soluble solids of `Concord' grapes from 14 to 18ºBrix during maturity usually corresponds to an increase in colour. After grapes reach 18ºBrix, colour quality may decrease. With Vitis vinifera cultivars, the colour will continue to increase up to 22 to 26 ºBrix. This condition is cultivar-dependent. The development of the typical purple-red colour in `Concord' grapes begins at veraison (time at which berries commence to ripen) and (Morris and Striegler, Somogyi, et al., 1996b). However, as the pH of `Concord' grapes gets to 3.7 to 3.8 or higher, a change in the pigment occurs which results in a colour shift from purple-red to blue. Therefore, it is important to harvest at a low pH (3.3 to 3.4) to maintain stable colour in processed juice
Extraction temperature influences juice colour by affecting the activity of polyphenoloxidase (PPO), which accelerates the rate of degradation of anthocyanins (colour ingredient) in crushed grapes. Inactivation of PPO by heat prior to depectination prevents loss of anthocyanins during extraction and subsequent storage. Storage temperature and time are primary factors for stability of colour in long-term storage. Research studies have shown that maturity, total acidity and juice storage time affect the amount of tartrates or argols in grape juice. The percentage of total phenols was increased in less-mature grapes and at high extraction temperatures (King, et al., 1988).
Increased storage time is detrimental to juice quality. Studies showed similar results for `Concord' and muscadine grape juice (Morris, 1985). Juice from mature grapes had better quality initially than juice from less mature grapes but declined in quality more rapidly during storage. Storage at 35°C resulted in a more rapid loss of quality than storage at 24°C.
Many studies and trials have been conducted to develop and determine the acceptance of grape juices and grape juice blends from new cultivars. King, et al., (1988) studied the effect of maturity and carbonation on muscadine grape juice. A sensory panel preferred the late maturing juices with high muscadine character and low phenolic and acid levels. Carbonated juices were lighter in colour but preferred equally to non-carbonated juices. Muscadine juices have been mixed with other popular grape juices, cranberry juice and apple juice for unique blends (Sistrunk and Morris, 1985). The dark juices were highly acceptable and retained their colour and flavour quality during a 12-month storage period. The lightest combinations (lighter muscadine with apple and `Niagara' grape juice) were rated highest and remained stable during storage.
One study investigated the effects of amelioration and carbonation on five wine grape cultivars processed for juice (Rathburn and Morris, 1989). The juices with adjusted sugar and/or acid rated higher in flavour than those without the adjustment. Carbonation improved the ratings of the unadjusted juices but generally had no effect on adjusted ones. Two wine grape cultivars, Aurore and Verdelet, produced juices that rated comparable in flavour to `Niagara', the white juice industry standard.
Later studies evaluated consumer preference tests on blueberry juice blended with water and with three different grape juices. On the hedonic scale used, a majority of the panel members ranked the flavour and colour of all four blends in one of the "like" rankings. The Blueberry-Concord blend had the highest ranking for flavour. Blueberry juice blends have been formulated and marketed as a result of this study. There is an excellent market for juice blends that use grape juice as a major ingredient.
Grape juice concentrated to 55, 65 or 68°Brix minimizes transportation and storage costs. This concentrate is diluted for use in single strength grape juice or multi-fruit and sparkling juice. Fruit concentrate is used full strength to sweeten jams, jellies, yoghurt, frozen fruit deserts, cereals, cookies and other bakery products. Many consumers perceive fruit concentrates to be a healthy replacement to table sugar and corn sweetener.
Concentration of juice is a vital operation of the juice processing industry. Juice may be concentrated by evaporation or freeze concentration. Historically, evaporation has been the most widely used concentration process for grape juice (Figure 12.4). Although many types of evaporators are available, all have essentially the same components (Hartel, 1992). Evaporators generally include a heat transfer surface, a feed distribution device, a liquid-vapour separator and a condenser. It is best to heat grape juice for as short a time as possible and to rapidly cool the product. Reduced exposure to heat minimizes the effect on flavour, aroma and sugar components. The following sections describe juice processing systems that are often coupled with essence recovery systems. The recovery systems are generally activated carbon columns that adsorb flavour and aroma compounds. Steam stripping can then be used to selectively remove these compounds for later addition to the concentrate or for other uses.
Figure 12.4: Grape juice concentrate plant, California.
Rising film or long-tube vertical evaporators are sometimes used for juice processing. These evaporators have the advantage of short evaporation times due to high heat transfer rates through thin films at high temperature differentials. The evaporator consists of bundled tubes inside a steam chest. The feed stream is heated and introduced into the bottom of the tubes where some of the product is vaporized. The concentrated fluid rises under vacuum in a thin film along the tubes. The tubes empty into a vapour/liquid separator. The vapour is diverted into a condenser to be liquified or is passed through a carbon column
A falling film is almost identical to a rising film evaporator except that fluid is pumped over the top of the tube bundle. This evaporator is the most popular type because it can handle more viscous fluids than the rising film evaporator is and can be operated at lower temperature differentials.
Plate evaporators operate similarly to plate heat exchangers. The fluid to be condensed passes on one side of a plate and steam flows on the other side. The superheated fluid then passes into a flash chamber. The vapour flashes off and the product and vapour are separated. High viscosity fluids can be efficiently concentrated in these evaporators possibly to concentrations above 60°Brix.
These relatively new evaporators produce a thin film using centrifugal force in single or nested cones. The cones have steam on the alternate side to provide a heat transfer surface. The systems operate under vacuum and allow the total time on the juice transfer surface to be as little as 0.5 seconds with only a small increase in product temperature. They are good for use with extremely heat sensitive and/or high viscosity products. Two major drawbacks are low capacity and high capital cost. However, these evaporators can also be used to distill, degas and deodorize liquids that have high heat sensitivity.
This process is based on the physical phenomenon of freezing point depression. Pure water freezes at a temperature of 0°C, but if a solid is dissolved in the water the freezing point temperature is lower. At a specific critical temperature, pure ice water crystals will form leaving a more concentrated liquid in solution. In freeze concentration, three fundamental elements are employed: 1) a freezer or crystallizer produces a slurry of ice crystals, 2) a centrifuge, wash column, or filter press separates the ice crystals from the slurry and 3) a refrigeration unit reduces the heat from fusion and the heat generated by friction from hydraulic flow, wall scraping and agitation of the slurry. Freeze concentration avoids the difficulties associated with heat-based evaporation methods. It is capable of concentrating most fruit juices to 50°Brix without appreciable loss of taste, aroma, colour, or nutritive value. Even so, freeze concentration has not achieved widespread commercial acceptance due to relatively high capital costs and low throughput.
The process of making grape jelly, jam, preserves, butter or marmalade consists mainly of cooking the grapes and/or their juice in combination with sweeteners and pectins. United States federal standards dictate the ingredients, their proportions and the final concentration of soluble solids level. The ratio of minimum total soluble solids to fruit sweetener as required by the FDA is shown in Table 12.1.
Table 12.1: FDA minima for grape jelly, jam preserves and fruit butter.
Finished
|
Soluble
|
Parts by |
Parts by |
Grape Butter |
43% minimum |
5 |
2 |
Grape Jelly |
65% minimum |
45 |
55 |
Grape preserves/Jam |
68% minimum |
45 |
55 |
Jam, preserves and grape butter are made from whole or crushed grapes. The fruit pieces in preserves are usually larger than in jams. Grape butter is made from screened grapes and differs from jam in the final solids concentration and in the ratio of fruit to sweetener.
The recognition by the jam and jelly industry that liquid sweeteners or syrups offer ease of handling and blending has greatly increased the popularity of corn sweeteners. Syrup from cornstarch may be produced in virtually any combination of viscosity and sweetness with other functional specifications.
Corn syrups are widely used by manufacturers of quality jellies, jams, preserves and butters. United States federal standards have authorized the replacement of up to 25 percent of total sweeteners with corn syrups for these products and up to 50 percent in marmalades. The use of corn syrups is economical and offers these quality improvements:
It is not difficult to substitute corn syrup in any preserve recipe or formula: 0.57 kg of corn syrup is used for every 0.45 kg of sugar replaced. Therefore, 0.57 kg of corn syrup provides 0.454 kg) of solids and replaces the sugar on a solids basis.
A satisfactory gel must be formed to produce a spreadable product. Gel formation requires that the concentration of the water-sweetener-acid-pectin mixture be in the proper proportions. If the grape juice or fruit does not provide sufficient quantities of acid and/or pectin to form a good gel, then it is permissible under Federal standards to add pectin and/or acid in a quantity that "reasonably compensates for any deficiency." Since Federal regulations fix the proportions of the grape juice or fruit, the relative amounts of sweetener, acids and pectins are the only variables.
A specific acidity (pH) is necessary for pectin to form a perfect gel. The optimum pH range for forming a pectin gel is 3.0 to 3.35. Within this pH range the consistency of the product will be primarily determined by the amount of pectin present. When whole grapes are present in the product, the fruit itself provides some spreadability and decreases the need for pectin.
US Federal Regulations allow the addition of vinegar, lemon juice, lime juice, citric acid, lactic acid, malic acid, tartaric acid, or any combination of two or more of these. The quantity of added acid must reasonably compensate for any deficiency in the natural acidity of the fruit ingredient without requiring a label declaration of added acid. The following standard acid solutions will produce the same general gel firmness under comparable conditions. Each acid may impart a slightly different tartness to the final product.
If grape juice has a reduced pH in the natural state, the end product will have a pH lower than the optimum 3.0 to 3.35 and will cause premature setting of the pectin. Buffer salts (sodium citrate, sodium potassium tartrate or any combination of these) may be added to adjust the pH in this situation. The buffer salts may be added in solution or dry when mixed with the pectin. No label declaration is required.
Pectin is a carbohydrate present in all plants, which, along with cellulose, is responsible for structural properties of the plant. Commercial pectins are normally produced from either citrus fruits or apples in accordance with internationally accepted specifications for identity and purity (Chapter 11).
The following discussion on pectins modified and adapted from the Handbook for the Fruit Processing Industry by Hercules Incorporated, Food Gums Group, 1313 N. Market, Wilmington, DE 19894-0001, now out of print.
High methoxyl pectins (HM-pectins) have a degree of methylation above 50 percent. HM-pectins require soluble solids above approximately 55 percent and a pH around 3.0 to form gels. Once formed, additional heating cannot melt the HM-pectin gel. The degree of esterification of HM-pectin determines the gelling rate and gelling temperature of the pectin, as reflected by the designations "rapid set" and "slow set" HM-pectins. At an elevated pH of 4.5, HM-pectin is stable only at room temperature or below. Elevated temperatures cause the pectin molecule to rapidly depolymerize and the gelling properties are completely lost.
Low methoxyl pectins (LM-pectins) are pectins with a degree of methylation below 50 percent. The gelling mechanism of LM-pectin differs substantially from that of HM-pectin. To obtain gel formation in a system containing LM-pectin, the presence of calcium ions is crucial. On the other hand, LM-pectins form gels at much lower percent soluble solids than HM-pectins and greater variations in pH are tolerated without a major effect on the gel formation. LM-pectin gels may melt when heated but show excellent stability at all temperatures in the pH-range 2.5 to 4.5. The right combination of two varieties of LM-pectins can very closely duplicate the texture and taste of a HM-gel at any Brix level from 20° to 70°.
Jellies are normally produced with slow set pectins because there is ample time for any air bubbles to escape from the product before gel formation starts. In jams and preserves, a uniform distribution of fruit particles throughout the container is desired. To avoid fruit flotation, gel formation must begin immediately prior to filling the container.
Choice of the type of HM-pectin for jams and preserves consequently depends on the filling temperature. Higher filling temperatures, used with small containers, normally require rapid set pectin. Medium-sized and large containers, where lower filling temperatures are necessary, require use of medium set and slow set pectins, respectively. In markets where jam standards state a minimum soluble solids of 68 percent, slow set pectin is normally preferred. At these high percent soluble solids, rapid set pectin gels at too high a temperature when pH is in the usual range. In jams with a relatively mild acidulous taste and high pH, rapid set pectin must be used, especially if soluble solids are at the lower end of a 60 to 68 percent range.
Pectin must be completely dissolved to ensure full utilization and avoid unhomogeneous gel formation. Complete dissolution requires dispersion without lumping and can be achieved by means of a high-speed mixer. The pectin is completely dissolved in a few minutes thus pre-blending with sucrose is not necessary.
The use of pectin in jams, jellies and preserves has two major purposes: creation of a desired texture and binding of water. If the water binding effect is not achieved completely, the final gel will show a tendency to contract and exude juice, known as syneresis.
Products based on HM-pectin must have greater than 60 percent soluble solids. High solids counteract the contraction of the gel structure and correctly produced HM-pectin-based products seldom show any syneresis. HM-pectin jellies do not reform their gel texture when mechanically ruptured and once initiated, the syneresis will remain constant or even increase over time. A small amount of syneresis will occur during normal consumption (when the gel is broken), especially if the product is stirred or pumped.
LM-pectins are usually used in jam, jellies and preserves with soluble solids below 60 percent for reduced calorie applications. The tendency for syneresis to occur increases with lower solids, but this phenomenon is partially counteracted by the ability of LM-pectin to reform the gel texture after mechanical rupture, especially if the calcium content of the system is relatively low.
Only a small amount of syneresis should occur after breaking the gel structure in the soluble solids range of 40 to 60 percent. At soluble solids lower than this range, syneresis becomes pronounced and it may be necessary to combine LM-pectin with other water binding hydrocolloids, such as locust bean gum, if a completely syneresis-free product is required.
The basic formula for making grape jelly is given in Table 12.2 and conforms to the Federal standards of identity. This formula has been tested on a commercial scale still, manufacturing processes and conditions may vary. Several test batches should be prepared and evaluated before a formula is used in commercial production.
Table 12.2: Basic jelly formula.
Ingredients |
Pounds |
Percent |
Standard grape juice1 |
82 |
45 |
Sucrose |
75 |
55 |
Corn syrup (43°Baume) acid |
31 |
2 |
Pectin, slow set |
1/2 to 1% |
3 |
1 any juice containing the specified amount of soluble
fruit solids (86.5 percent)
2Quantity may be varied to obtain a pH of 3.0 to
3.35 in the finished product
3Quantity will be varied, depending upon type of
juice and the pectin manufacturer's recommendation.
Source: Corn Syrup in Jams, Jellies and Preserves, Technical
Service Bulletin No. ID1a, Clintonnes Corn Processing Company, Clintonnes, IA, undated.
The United States Federal Regulations stipulate that the juice portion of the basic jelly formula contains a minimum percentage of soluble fruit solids. In mixed juice products, the percentage is the average percent soluble solids of each respective juice as determined by the FDA. The average required soluble solids content of grape juice used to prepare jelly and the corresponding factor (reciprocal of each percentage times 100) are 14.1 and 7.0, respectively.
This factor is used in a short-cut formula to calculate the amount of juice that must be used to equal the soluble fruit solids of a standard juice in the basic formula. For example, the average soluble solids of grape juice is 14.1 percent. Therefore, the 37.3 kg (82 lbs) of standard grape juice in the formula must contain 5.3 kg (11.66 lbs) of soluble fruit solids. An adjustment is necessary in most cases, since the grape juice used by the jelly maker will not contain exactly 14.1 percent soluble solids. If the grape juice on hand contains 10 percent soluble solids, for example:
37.3 kg (82 lbs) of standard grape juice
0.10
(soluble fruit solids) x 7 (factor) = 53 kg (117 lbs) of juice would be required
for each 45.4 kg (100 lbs) of sweetener solids (Clintonnes Corn Processing Company).
