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CHAPTER 13

TREE FRUIT : APPLE, PEAR, PEACH, PLUM, APRICOT AND PLUMS

13.1 Raw material

Apples are more widely grown than any other fruit; apple trees of one kind or another are grown all around the world. (Root, Somogyi, et al., 1996b). Apple production can vary from one year to the next by as much as 20 percent, depending on the climate of any given year. There are hundreds of apple cultivars, but only about 20 cultivars are commercially important. More than 90 percent of this production is represented by 14 cultivars and only five of these account for most of the world's apple production: Delicious, Golden Delicious, McIntosh, Rome Beauty and Granny Smith.

Newer cultivars are becoming increasingly common in the marketplace. Some newly popular cultivars are Gala, Fuji, Jonagold, Braeburn and Lady Williams. Many new commercial cultivars are red strains of the primary cultivars. There is a wide variety in their characteristics. For instance, Gala matures in 100 days or less while the Western Australian cultivar Lady Williams needs more than 200 frost-free days to mature. Some need long cold winters to break dormancy while others can be grown in very mild climates such as Israel.

While some cultivars are grown exclusively for use in processing, at least some of the harvest of all commercial apple cultivars is used in processed products. Only sound, ripe fruit should be used for further processing because decay, damage, maturity, firmness, colour, soluble solids, acids and tannins of the fruit impact the quality of the product. Perfectly good fruit from the commercial fresh market cultivars (an average of 20 percent) are used for processing. Some fresh market cultivars produce excellent juice and still others produce superior sauce. Some apples are grown specifically for processing, but most of the apples that are sold to the processor are salvaged fruit grown for the fresh market. Premium price is paid for large, bruise-, disease- and insect-free apples delivered to the processor. This requires apple producers to pay full attention to their cultural details whether growing for fresh or the processing markets. Production practices for apples will vary not only with the apples' destination, but also with the climate and soils in which they are grown.

13.2 Harvest

The majority of the apple crop is hand harvested because only a very small percentage of that crop is intentionally harvested for further processing. To use mechanical harvesting, a grower or cooperative of growers must be producing at least 40 000 bushels (~ 700 MT), in order to justify the cost. Since apple processing is mainly thought of as a salvage operation, the amount of apples available to process is largely dependent on the size of the fresh market harvest and its quality. Consequently, processing apples are harvested and stored in the same manner as premium, fresh market apples.

Salvage operation or not, it is absolutely essential that fruit for juice (or any other application, for that matter) not be "drops", i.e. apples that have fallen from the tree and are collected off the ground. There have been numerous incidences of food poisoning associated with the use of drops for fresh, unpasteurized apple juice (Table 4.1). Even with improved sanitation and a microbial kill step between processor and consumer, drops will invariably contain many damaged or partially rotten fruit, impossible to grade out. In addition, it doesn't take many mouldy juice apples to exceed the 50 parts per billion limit in juice of the fungal metabolite, patulin, a human carcinogen (Ashurst, 1995). While most aflatoxin analyses have been done on major crops such as apple, grape and orange, it is likely that tropical fruits contain levels worth establishing (and probably reducing).

One advantage that apples have over other more perishable fruit crops is that the fruit may be successfully kept in storage for a few weeks to several months. However, to maintain their high quality for processing over storage time periods, it is extremely important that they are picked at the proper stage of maturity and storage conditions are optimized for specific apple cultivars. The processor must determine when the apples for processing are to be harvested.

Figure 13.1: Apple maturity, Northern Hemisphere.
(Childers, et al., 1995)

Generally speaking, there is a window of about five to 20 days, depending upon cultivar and climatic and cultural conditions, during which the fruit can be picked with reasonable assurance that the apples can be stored until they can be processed. Figure 13.1 illustrates these conditions (Childers, et al., 1995). Apples continue to increase in size to maturity, when the apple drops from the tree. Harvest must begin as late as possible and yet while the fruit still adheres to the tree. Fruit mature enough to drop will not store well or make the best product. Of course, sound Good Agricultural Practices (GAPs) logically prohibit the use of drops for juice manufacture (FDA, 1998a).

While no one method is entirely dependable under all circumstances, there are several methods available to determine the proper time to pick. These indices may serve one cultivar differently from another. The intended use of the cultivar is also a consideration. It may be best to combine several of these indices with plain good judgement and experience to determine the proper harvest date. Some cultivars have such a body of research attached to them that these indices can be more specific to these cultivars.

13.2.1 Time elapsed from full bloom to maturity

It is important for the processor to predict when the apples will be arriving at the processing plant. Full bloom is considered to be that time when 80 percent of the blossoms on the north side of the tree are open. The time from bloom to maturity can be predicted for each cultivar for each growing region. Seasonal mean temperature seems to have little effect on this time interval, but other conditions may affect its accuracy. High temperatures just before harvest may cause excessive fruit drop before harvest time. Trees heavily fertilized with nitrogen may delay skin colour in the apples, or fruit drop may be more pronounced. A light crop tends to mature sooner than a heavy crop. These variables may be very disconcerting.

13.2.2 Starch/iodine test

This quick, widely used method uses a standardized iodine solution applied to a cut apple cross-section. A darkened pattern develops which indicates the level of starch remaining in the apple. The pattern is compared to established charts that are available commercially. If the starch level indicates that the starch is about half cleared from the fruit, then the apple is ready for harvest and storage. When it is almost all cleared, it is ready for consumption. Samples should be taken twice a week near to harvest of 10 apples/block from tagged, typical trees. Select two typical apples/tree.

13.2.3 Pressure test

One of the best methods of determining the proper harvest date is a firmness test. Firmness of flesh can be determined by a pressure gauge (Figure 13.4) designed for this purpose and available commercially. The determination is based on the fact that firmness gradually decreases as fruit matures. Several readings are averaged to get the result. Season, cultivar, sampling and testing techniques can influence firmness. Instructions on the proper use of the instrument are available from the manufacturer.

Figure 13.2: Penetrometer (Puncture pressure gauges).
Penetration pressure is one indication of ripeness

13.2.4 Soluble solids

Since starch in apples gradually changes to sugar as the fruit matures, a few drops of apple juice may be tested for sugar development right in the field with a hand-held refractometer (Figure 13.5). However, soluble solids values can be more variable than firmness readings. It is wise to blend the juice of several typical apples before testing for sugar. Cultivar, crop size, cultural practices and growing season can influence the readings.

   

Figure 13.3: Hand and bench refractometers.
For soluble solids (Brix) and specific gravity determinations.

13.2.5 Ground colour change

The green "ground" colour of the unexposed side of the apple changing to yellow can be an index of maturity. However, even this can be influenced by the nutritional condition of the tree and vary from cultivar to cultivar. Too much nitrogen may delay the change. Growers must know their cultivar. The colour change of Golden Delicious at 135 to 150 days from full bloom can give a good index of maturity. However, other cultivars may drop from the tree before colour change.

13.2.6 Flesh colour change

Flesh colour change from light green to white in Delicious, Golden Delicious and some other cultivars is useful. Harvest before the change may result in storage scald for susceptible cultivars. In contrast, seed colour change from white to brown is a poor index of maturity.

13.2.7 Water core

Water soaked areas in flesh for McIntosh and Jonathon can be an indication they are past good storage life. Water core early in Red Delicious can indicate prime condition for storage, but if water core areas are coalescing, storage will be shortened and apples should be moved early.

13.2.8 Easy removal of fruit from spur

When some cultivars are ready to pick, they can be separated from the spur without breaking the stem by lifting, with or without slight twisting. Some cultivars (McIntosh and Delicious) may drop before maturity because of early frost or other factors, while there are cultivars like Jonathan and Stayman that may retain fruit until over mature. Thus this index may only indicate when picking is necessary to save the crop. With the use of "Stop-Drop" chemical sprays to "stick" the apples on, this index is of little value.

13.3 Storage

Since most processors cannot use the whole harvest they receive as they receive it, some fruit is stored, short term, as they come in, not refrigerated. Other fruit is stored refrigerated in a temperature range of 1 to 4C, depending on the cultivar. The next level of storage is controlled atmosphere (CA). CA storage usually consists of a modified atmosphere, 2 to 3 percent oxygen and 1 to 4 percent carbon dioxide, at a reduced temperature. The exact specifications are adjusted to the cultivar being stored. Apples can maintain quality under these conditions for 4 to 6 months. Only the highest quality apples destined for the fresh market are placed in CA storage. However, many times the fresh market price will drop to the point that CA apples will be dumped to a processing market. Apples from CA storage should be allowed to "normalize" for a few days before processing. These apples and apples from refrigerated storage are capable of producing good quality processed product. Processors take into consideration that different qualities of juice or applesauce can be manufactured from the same cultivar, depending on the type of storage, time of storage and stage of maturity when processed.

Advances in controlled atmosphere technology have had a dramatic effect on apple storage logistics and opened up markets hitherto unavailable for fresh and processed apple products. This is an advantage not fully shared by other fruit crops whose shelf life extension by CA is much less.

13.4 Storage facilities

There are essentially three types of storage buildings for apples: air cooled storage, mechanically refrigerated storage and refrigerated and controlled-atmosphere (CA) storage.

13.4.1 Air-cooled storage

These storage houses cool by admitting cold night air (applicable climates) at inlets near the floor of an insulated building and forcing upper accumulated warm air out at outlets near the ceiling. Both openings are closed during the day. These storages are economical and effective in areas where the night air becomes cooler than the accumulated air in the storage house.

13.4.2 Mechanically refrigerated storage

For longer periods of storage than is afforded by the air-cooled storages, mechanical refrigeration is needed. This would become necessary to extend the season on fresh-market apples or to extend the cooling of processing apples because the volume is so large they can not be completed in the period of time afforded by the air-cooled storage situation. Specifications of room size, room construction, capacity, compressors, condensers, expansion coils, etc. for both mechanically refrigerated storage and CA storage can be found in Childers, et al., 1995.

13.5 Processing

Apples are processed into a variety of products, but by far the largest volume of processed apple products is in the form of juice. Apple juice is processed from apples that are unsuitable for peeling, such as "eliminator" apples, smaller than ~57mm diameter, too small to peel, etc.

Apple juice can be produced and sold in several forms. Fresh apple juice or sweet cider is juice of ripe apples, bottled or packaged with no form of preservation. This form needs to be sold at the orchard or at outlets close by. Even under these conditions it is important to pasteurize the juice to eliminate E. coli or other dangerous organisms. This recommendation has been established after several incidents of serious E. coli problems associated with unpasteurized apple juice in the United States of America.

Apple cider is considered around the world as the fermented juice of the apple, but in the United States apple cider refers to sweet cider, the simple juice of early season, tart apples. Shelf-stable apple juice is sweet cider that has been treated for preservation. This could include clarified juice (depectinized, filtered, pasteurized and bottled), crushed apple juice (pasteurized and with a high pulp content), natural unfiltered juice or juice concentrate; frozen (natural or clarified and concentrated to 42Brix) or high Brix (clarified and concentrated to 70Brix)

Table 13.1: Nutrients in the edible portion of 454 g of apples.

 

Energy

Protein

Fat

Carbohydrate

Calcium

Phosphorus

Products

(Calories)

(g)

(g)

(g)

(mg)

(mg)

Raw fresh

242

0.8

2.5

60.5

29

42

Applesauce

413

0.9

0.5

108.0

18

23

Unsweetened

186

0.9

0.9

49.0

18

23

Apple juice

213

0.5

0.1

54.0

27

41

Frozen slices

422

0.9

0.5

110.2

23

27

Apple butter

844

1.8

3.6

212.3

64

163

Dried, 24%

1 247

4.5

7.3

325.7

141

236

Dried, 2%

1 601

6.4

9.1

417.8

181

299

13.6 Apple juice manufacture

There are a number of procedures employed in apple juice production, depending upon the end product desired. Figure 13.6 is a generalized flow scheme for producing some of these products.

Figure 13.4: Apple juice flowchart.

Figure 13.5: Apple receiving at juice plant for immediate processing or storage.

13.6.1 Prejuicing

Apples are brought to the processing building and dumped by the truckload or out of pallet bins, into a water-filled tank (Figure 13.7). (Such practices should be discouraged for fresh juice manufacture, due to the potential for contamination, particularly when dump water temperature is lower than the fruit temperature. In this case water-borne pathogens may be sucked into the fruit and hence protected from subsequent fruit surface sanitary measures). Fruit are then spray washed and sorted (removing damaged and diseased fruit). Depending on process logistics, clean, sorted fruit may be stored as described in Sections 13.3 and 13.4 (and inspected again before juicing) or juiced immediately. To prepare them for juicing, a disintegrator, hammer mill or grating mill may be used to grind the apples. The mashed apples need to be free of large pieces yet not so fine that pressing becomes difficult. The type of extraction equipment may dictate the chopping method to achieve highest efficiency. The hammer mill adjusts more easily to different pulp consistencies.

Although apples contain potent browning enzymes, pectin enzymes are in low concentrations. Thus, commercial macerating enzymes are usually added. There are a number of enzyme products prepared just for apple mash pretreatment that break down cell walls to free the juice, lower the viscosity and reduce pulp slipperiness.

13.6.2 Extraction

Extraction may be accomplished through pressing chopped apple continuously or in batches. There are a number of pressing systems:

Apple juice from any of the presses described is invariably cloudy and contains particles (bits of apple and press aid particles) that can be removed by screening. A cylindrical "cider" screen, which is made of stainless steel screening of approximately 100 to 150 mesh, revolves on a system of rollers. The revolving action keeps the screen clean by causing the pomace to gather into small balls and finally into a continuous roll which falls off the end of the slightly sloping screen. A stainless, dewatering shaker screen can also be used (Figure 6.14). Screened juice reduces the load on the filter.

13.6.3 Unclarified juice

For the "natural" look associated with fresh apple cider, the ground apple pulp is treated with ascorbic acid before pressing to minimize browning. The juice is screened or settled, but not otherwise filtered. The ascorbic acid is best added directly to the mill, to be mixed with the pulp as soon as possible after the apples are crushed and pulp exposed to air.

About 30 grams of ascorbic acid added to 100 kg of apples seems to be effective, if all processing is done without delay. (This amount may be cultivar dependent since a cultivar may have a very active oxidative enzyme system requiring an increase in ascorbic acid.) In view of recent fresh cider food poisoning outbreaks, the majority of unclarified juice is flash pasteurized.

13.6.4 Enzyme treatment

An enzyme step is not employed if the end product desired is a cloudy or "natural" looking apple juice. Otherwise, after juice extraction, the raw apple juice must be treated with enzymes to remove suspended solid materials. If not removed, this colloidal material can clog filters, slowing production and can cause the juice to form a haze later on. Enzymes work by hydrolyzing soluble pectinaceous materials, hemicellulose and other polymers and colloids that increase juice viscosity, thereby leaving the juice more easily filtered. Many enzyme preparations are available both in liquid and powder forms. They are all subject to conditions that can influence enzyme performance such as pH, temperature, enzyme concentration and length of reaction time. Considering these variables, it is recommended that test trials be conducted with specific enzymes under typical operating circumstances to determine the proper concentrations and conditions.

There is a hot and a cold method for enzyme treatment. In the hot method, the enzyme is mixed into juice at 54C and held for 1 to 2 hours. In the cold treatment, the enzyme is mixed into the juice at room temperature, 20C and held 6 to 8 hours. The enzyme activity can be monitored by adding five millilitres of juice to 15 ml of HCL-acidified ethyl alcohol, observing the mixture for 5 minutes for gel formation. No gel formation means that the depectinization has been completed.

13.6.5 Tannin and gelatin treatment

For highly astringent apples, tannin removal is beneficial. Many of these tannins can be precipitated with addition of gelatin. However, in order not to remove all tannins and therefore some of the flavour and colour of the juice, it is often the practice to first add more tannins and then precipitate a certain amount with gelatin. A classic, older procedure of possible value with overly astringent juices is described (Walsh, 1934):

Solution 1. Dissolve 9.45 g of tannin (tannin acid) in 176 ml of 95 percent ethyl alcohol. Then add 704 ml of water and mix thoroughly.

Solution 2. Dissolve 21.2 g of gelatin in 704 ml of water and add 176 ml of 95 percent ethyl alcohol.

Heat a portion of the water and add the gelatin slowly, stirring continuously. Then add the rest of the water and dissolve the gelatin by heating in a pan of hot water or double boiler and stirring. Add the alcohol and mix well.

These solutions should be kept in stoppered bottles and may be used as needed, the alcohol acting as a preservative in both cases. In some cases the gelatin will gel when cold, but can be liquefied when needed by putting the container in hot water.

Four clear glass quart bottles should then be filled to the neck with apple juice and numbered 1, 2, 3 and 4. Then add to each bottle the following amounts of Solution 1 (tannin) and Solution 2 (gelatin).

 

 

Bottle No. 1

Bottle No. 2

Bottle No. 3

Bottle No. 4

Solution 1. (ml)

10

10

10

10

Solution 2. (ml)

5

10

15

20

Measure and add the amounts of solutions shown to each bottle, adding the tannin first  in all cases and shaking well after the addition of each solution. Let the bottles stand for 10 minutes. The bottle showing the clearest juice is the one to which the proper proportions of tannin and gelatin were added.

The quantities of tannin and gelatin to use for 380L-batches of apple juice are then found by referring to the table below. For smaller amounts of cider, proportionate amounts of tannin and gelatin are used. For example, if bottle 3 showed the clearest juice at the end of 10 minutes, 35 gm of tannin and 126 gm of gelatin should be added to each 380L of juice; for 190L, one-half these amounts should be added.

AMOUNTS OF GELATIN AND TANNIN TO BE USED FOR 100 GALLONS APPLE JUICE

 

Bottle No. 1

Bottle No. 2

Bottle No. 3

Bottle No. 4

Tannin (grams)

35

35

35

35

Gelatin (grams)

42

84

126

252

The actual clarification of apple juice according to this procedure is carried out by first stirring into the apple juice a solution containing the proper amount of tannin. A few minutes later the correct quantity of gelatin, dissolved in hot water, is added, stirring constantly. It is most essential that the juice be very thoroughly stirred after the addition of the treating chemicals. After standing overnight, the clear supernatant liquids are drawn off and filtered. In some plants, the liquid is not separated from the sludge since the filter retains the sludge. This speeds up the operation and eliminates the waste due to discarding juice with the sludge."

The success of the tannin-gelatin method of clarification is due to some extent on the experience of the operator. Too much gelatin in the juice can slow filtering and cause the finished juice to cause a cloud or precipitate upon storage.

13.6.6 Heat clarification

Flash heating the apple juice between 82 and 85C will coagulate the particles that interfere with juice filtration. The juice is then rapidly cooled and filtered or centrifuged. There are some difficulties with this method.

13.6.7 Centrifuged apple juice

Centrifuged juice is slightly clearer than unclarified juice but quite cloudy and viscous compared to the filtered juice although free of visible suspended particles. The pressed, screened juice is fed to either a tubular bowl or continuous centrifuge (Figure 6.15). Both types work well, but the continuous type is self-cleaning and therefore does not have its operation interrupted for cleaning.

13.6.8 Bentonnesite fining

A means of reducing both haze and juice astringency uses the adsorbtive properties of bentonnesite. The finely divided bentonnesite clay particles have an enormous surface area to which tannins and protein-tannin complexes adsorb. Since this may be at the expense of colour and flavour, treatment amounts must be carefully chosen. Fining is also effective in reducing harshness and astringency in some wines (Amerine, et al., 1980; Vine, et al., 1997). As such, it has an unexploited juice flavour cleanup potential.

13.6.9 Filtration

To obtain a brilliantly clear apple juice polish filtration is necessary. Filtering freshly pressed juice is a difficult operation due to the pectinaceous nature of apple juice and the potential for post filtration haze formation. Untreated juice can be rough filtered in large capacity filters with large filter areas that can be easily cleaned. The juice from this method has superior flavour and excellent body. It may have a slight haze that increases with time as proteins and tannins react. Filtering juice that has not been depectinized reduces the filtration rate to about 1/3 of enzyme treated juices.

There are many types, styles and capacities of filters available. Plate and frame filters with disposable pads or sheets are easy to clean, but readily clog after extended use (Figure 6.16). Continuous filters with back flush capability are preferred. The juice contact surface should be stainless steel or food grade plastic. In a pre-coated filter, the liquid, containing suspended filter aid, is forced by pressure of vacuum through a coated membrane (Figure 6.16). In the case of apple juice, the medium is usually diatomaceous earth. This medium is available in many grades. It takes skill and experience to operate a filter in an apple juice plant. There is a great deal to know about pre-coating, filter aids, special pumps, etc. Either a new filter must arrive with complete directions, or an experienced operator needs to be on hand for consultation (Tressler and Joslyn, 1971).

13.7 Pasteurization

The most important method of preserving apple juice is pasteurization, which involves heating the juice to a given temperature for a length of time that will destroy all organisms that can develop, if juice is put hot into containers that are filled and hermetically sealed. Flash pasteurization is, true to its name, the rapid heating of juice to near the boiling point (greater than 88C) for 25 to 30 seconds. Steam or hot water passes the juice between plates (Figure 8.5) or through narrow tubes that are heated. Design of the heat exchanger provides juice flow turbulence and even heating to prevent scorching and burn-on in the unit. There are numerous flash pasteurization heat exchangers available. They are all adaptable to a continuous operation set-up.

Juice can be canned or bottled in cans, glass or plastic. Cans used are enamel or lacquer lined to resist corrosion from the juice. As the cans travel the canning line, they must pass through a can washer, be filled from filling machines and immediately sealed on a can-closing machine. After closure cans should be positioned or inverted so that the hot fill will be in contact with the lid and thus, pasteurize it. From here, the cans must be removed to a cooling room where they will be cooled to near 38C to stop the effect of high heat on the contents. If cooled to a temperature lower than 35C, the labels will tend to detach, the can will not dry and will be susceptible to surface rusting. This necessitates that the cans travel continuously from washing, to filling to cooling to labelling and packing.

Bottling juice requires specialized equipment. Cans can be roughly handled, but bottles are fragile ("bruising" not visible to the eye can cause breakage later) and susceptible to thermal shock. Temperature changes of greater than 7C should be avoided. Bottles must be cleaned before filling then heated (steam jets) within 7C of the fill temperature. The best filler draws the liquid into the bottle by evacuating the bottle, thus reducing oxidation. Bottle closures can be screw caps, crown caps or vacuum caps. The vacuum caps have the advantage of allowing less headspace; if the contents ferment only the cap will blow off rather than the bottle exploding. Bottles also must be cooled. This can be accomplished in a special cooler that sprays hot water on them and decreases the temperature as the bottles move along. At the end, they emerge close to 38C, still warm enough to dry.

Newer packaging and processing systems use plastic containers that can be hot filled and rapidly cooled without the danger of thermal shock. In addition, aseptic processing greatly reduces heat-induced flavour changes. Still, glass bottles are the traditional standard and carry a quality image not implicit in cans, plastic bottles or aseptic packs. Recent concerns about contaminated fresh cider have resulted in United States Federal Regulations that strongly discourage unpasteurized juice and advise pasteurization of all apple juice products, even at small roadside stands and country markets that prepare juice on site. The alternative is a not very appealing warning label on fresh apple juice (FDA, 1998b).

13.8 Concentration

Apple juice may also be concentrated as a form of preservation, for use as reconstituted juice and in further processing. Evaporating systems such as rising film evaporators, falling film evaporators and multiple effect tubular and plate evaporators can be used (Chapters 8, 11 and 12). Because apple juice is so sensitive to heat the multiple effect of evaporation with essence recovery is the one most commonly used. This method heats the juice in stages. The juice is evaporated to 20 to 25Brix at 90C and the aroma captured by fractional distillation. This concentrate is brought to about 40 to 45Brix at about 100C. In the third stage it is heated to about 45C and concentrated to about 50 to 60Brix. The final heating at 45C will bring it to 71Brix. The concentrate is cooled to 4 to 5C and standardized to 70Brix and then bottled, barrelled or stored.

13.9 Applesauce

Although not strictly a beverage, applesauce merits mention as a co-product of apple processing operations. For making applesauce, clean, sorted, peeled apples, plus all the apple sauce recipe ingredients, are chopped and cooked in a cooker heated by live steam or jacketed steam. Liquid sugar is the preferred sweetener because it imparts a desirable "sheen" to the finished product appearance. The chopped apples are cooked at 93 to 98C for 4 to 5 minutes to soften the apples and inactivate the enzymes responsible for browning. The cooked apples are passed through a pulper with a screen for the purpose of removing undesirables and for sizing the sauce. Baby food goes through a ~0.8 mm screen for the finest of textures. The sauce is next inspected by being poured over a backlit flat plastic sheet. Inspectors remove all dark pieces with a flexible vacuum tube.

The applesauce is now ready to be canned or bottled. It is heated to 90C and piston-filled into cans or bottles. Applesauce must be filled and sealed at 88C in the seamer or capper. To insure a vacuum in the container, a jet of steam may be passed over the top of the container just prior to sealing. As the steam condenses, a vacuum is created in the container. This step is important in cans to prevent headspace detinning. The containers are held for 1 to 2 minutes prior to cooling to insure sterilization of the lids or caps. Water-cooling takes place in a draper belt, walking beam or reel cooler to an average of 35 to 40C to prevent "stack cooking" in the warehouse.

13.10 Pears

Pears are processed very similarly to apples. They should be ripened to a pressure test of 0.9-2.2 kg before processing. Pears may be harvested while they are still firm 7.6 to 9 kg pressure test and safe to handle without damage. If they are stored at 1C for one week they can then be ripened uniformly for four to five days at 10C and 85 percent humidity. To produce nectar, pears are washed and mechanically peeled and cored, after which they are heated to 85C for 3 minutes in a continuous steamer. This hot fruit is passed through a continuous extractor with a 0.084 cm screen and then a brush-type finisher with a 0.08 cm screen. After adjustment with sugar, citric acid and water to a values of about 13 to 15Brix and 3.9 to 4.2 pH, the nectar is flash pasteurized at 96C in a heat exchanger then hot-filled into cans, sealed at (at least) 88C and held inverted for 3 minutes before being cooled. Can linings should be used that have no reaction with the nectar .

Clear pear juice is generally obtained through the help of enzymes. Heated, sliced ripe pears are put through the finisher and cooled to 38C. A macerating enzyme treatment is used for several hours at this temperature to depectinize the puree. At room temperature, the time should be extended to over night. When a clear sparkling juice can be obtained from the puree, then the depectination has taken place (this can be tested in a centrifuge). The amount of press aid will vary for each operation, but about 1 to 2 percent is added to purees put through most rack-and-frame presses. This juice and about 0.25 percent filter aid can then be clarified in a pressure filter with pre-coated plates.

13.11 Peaches

In contrast to the fruits previously mentioned, peaches are not usually extracted as juice, but rather as a pulp or puree. The juice, employing pectic enzyme treatment, is light in colour, flavour and body compared to the pulp. When dealing with peaches, the whole pulp refers to the peach pulp after mashing and before fibre and other coarse material is removed. Pulp refers to the whole pulp after fibre and coarse material is removed. Pulp base is pulp with such additives as sugar, acid, colour and, stabilizer. This product can be stored and used in a variety of ways. Peach juice drink is a product prepared by dilution of peach pulp or peach pulp base. Cloudy concentrate is pulp base with some of the water evaporated. And clear juice concentrate is peach pulp or pulp base that has the insoluble material removed by pectic enzyme treatment and filtration, followed by evaporation.

Peach cultivars show a wide variation for ripening date, Brix, total acidity, flavour, colour and freedom from browning. Most of the cultivars grown for the fresh market are suitable for pulping. The ideal peach for processing will score high on these points: flavour; balanced soluble solids and acids; suitability for mechanical peeling, pitting and pulping without producing seed fragments; maximum of yellow colour and minimum of red colour; difficulty in turning brown; uniform ripening of whole peach; enough firmness and stability to withstand a reasonable amount of handling after ripening. This ideal peach is fully ripe and soft fleshed. The best peach is obtained from peaches graded for ripeness and free from rot, other spoilage and insect damage, rather than from orchard-run (second grade) fruit.

Peaches may be peeled by steam peeling or by lye peeling. In the lye peeling operation, whole peaches are either immersed in or sprayed with a lye solution (up to 5 percent in strength) at a temperature of at least 99C for 15 to 20 seconds. They are held for another 60 seconds and then spray washed with cold water. Some operations disregard the peeling step and simply pulp the whole fruit with the skin intact. This yields 13 to 15 percent more whole pulp. The aroma and the colour of the pulp can be influenced by disregarding the peeling and is also a function of the cultivar and stage of maturity.

If peaches are heated before pulping, the pulping process can be made easier, oxidation reduced and cloudiness stabilized. This is best accomplished by heating in a continuous thermo-screw for 2 minutes at 93C. The jacket around the thermo-screw should be kept at ~125C during this blanching/softening step.

Pulping the fully ripe peaches removes the seed and reduces the peach pulp by passing it through a ~3 mm screen. The speed of the paddle is best at about 1 000 rpm. Faster speeds will whip air into the product. This pulp is then passed through a finisher with screens having 0.061 to 0.084 cm perforations, operating at about the same speed. Pulp is separated from the fibre and unripe portions at this stage and is reduced to a liquid. A 1 MT load of peaches will produce about 494 L of puree. For peach nectar, 241 L of 30Brix sucrose syrup should be added to each 380 L of puree. If the pH needs to be reduced to 3.7 to 3.9, then it may be necessary to add citric acid.

At this point, the puree may have air mixed in it, which can lead to deterioration of the nectar's colour and flavour. It may be advisable at this point to pass the nectar through a deaerator. The nectar is now ready for flash pasteurization. Processors mix in 0.14 percent ascorbic acid at this juncture and then feed the nectar uniformly into the pasteurizer. Puree is pasteurized at 88 to 93C and quickly cooled to 1.7C if it is destined to be aseptically canned and refrigerated. Otherwise the puree may be filled hot directly into cans or bottles. These cans should be closed using vacuum and nitrogen or a vacuum produced with a steam jet. Cans should then be cooled with water spray, dried with warm air, labelled and stored in a cool dry place.

13.12 Apricot

Apricots for juice or concentrate are processed in the same manner as peaches, but are not peeled, although the skin and other fibrous material are eventually removed in the juicing process. Fruit, unsuitable for cutting, but without rot, are heated to 99C in a thermal screw, then run through a pulping unit to remove the stones. A series of finishers removes the skin and fibrous material. The juice can be made into nectar with the addition of sugar, water and acid. The juice made into concentrate is brought to 32Brix. Puree is sweetened with approximately 1.8 times its volume of 15 to 16Brix sugar syrup and acidified with citric acid. Apricot puree can be processed in cans or hot filled directly from the pasteurizer (Somogyi, 1996b; Tressler and Joslyn, 1971).

13.13 Plums and prunes

In areas of heavy plum production, harvest date is determined by skin colour changes that are described for each cultivar. There are colour chip guides available designed to determine maturity for each cultivar. Some cultivars have skin ground colour that is masked by full red or dark colour development during maturation. For these cultivars, flesh firmness is measured for an indicator of maturity. Plums are less susceptible to bruising than most peach and nectarine cultivars at comparable firmness.

Plums do not usually give up a juice upon crushing and pressing and must be treated with a macerating enzyme in order to yield an actual fruit juice. The method for obtaining this juice is similar to the following:

Prunes, which are dried plums, have long been used for juice. Prune juice is reported to have laxative properties. This effect plus the shrivelled appearance of prunes has resulted in a negative image for the product. Hence, the industry is attempting to replace the word prune with "dried plum", hoping for a more positive image.

The plum changes appreciably during dehydration with both enzymatic and Maillard reactions contributing to the dark colour and typical heated fruit flavour of prunes. At ~18 percent moisture, it is impractical to extract juice, so prunes are subjected to an aqueous extraction (Figure 6.7) to produce the sweet brown juice with prune-like characteristics and similar physiological effects.

Two methods are used to produce prune juice from dried plums. Manual diffusion is seldom employed commercially anymore, although it is mentioned here because this method is simpler, less expensive and requires less equipment. This method calls for dried fruit to be steeped in large vats (holding 160 to 180 kg of prunes and 300 to 380 L of water) at 83C for 2 to 4 hours. This extracted juice is combined with the juice of a second extraction of the same fruit, done the same way, except only 63 L of water /100 kg of fruit is used. These combined juices are added to the juice of a third extraction; again performed the same way, only using 40L water/100 kg fruit. This combined extract is either used to extract a fresh vat of prunes (in which case the Brix is generally over the desired level) or is concentrated to the desired Brix of 18.5 to 21. The extracted residue pulp is discarded.

The second method is called the disintegration method. As much as 550 kg of prunes are washed and vigorously boiled and agitated for 60 to 80 minutes, until disintegrated. This mash is then pressed as described for apple pulp or put through a high-speed centrifuge (approximate 4 000 rpm). The resulting juice, about 10 Brix, is allowed to settle, siphoned off and then must be concentrated to the desired Brix of 18.5 to 21.

Curiously, other dried fruit extracts are not as popular as prune juice, although the dramatic effects of dehydration followed by extraction should yield some rather novel, high value beverages worth exploring.

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