Pretreatment grape before drying

Fresh grapes are very sensitive to microbial spoilage during storage, even under refrigerated conditions, having relatively high moisture contents (80% - 85% wet basis). Therefore, within a few weeks following harvest, they must either be consumed or processed into various products. Drying is the most common method for grape processing.

The reduction of water activity in dried grapes is of greatest importance, because the products become more stable and with storage under appropriate conditions the optimum microbiological, nutritional, and sensory properties of the finished product are achieved.

Pre-treatment is a necessary step in raisin production in order to ensure the increased rate of water removal during the drying process. A faster water removal rate decreases the rate of browning and helps to produce more desirable raisins.

The peel of the grape plays a critical role in controlling the drying process. Grape peel consists of a wax layer as a protective barrier against fungal pathogens. However, this coating reduces permeability and therefore hinders the process of water transfer.

Pre-treating helps keep light-colored fruits from darkening during drying and storage and it speeds the drying of fruits with tough and waxy skins.

One way of removing this waxy layer is to apply pretreatments before drying grapes so that water gets migrated easily, thus improving the process. This not only decreases the energy consumption but also promotes quality preservation of the final product, such as retention of antioxidant during drying.

Due to this, prior to drying, several chemical and physical pretreatments are used to enhance permeability and increase the drying rate, while preserving the physical, chemical, nutritional and sensory qualities of the final product.

Therefore, chemical dip treatments (both hot and cold) have been used to increase the drying rate of grapes which not only reduce the drying time but also improve quality of the raisins. The hot dip treatment with dipping solution causes cracking and perforation in the waxy cuticle and thus increases the drying rate over the cold dip treatment.

The effects of pretreatment solutions on seedless grapes during drying were reported in literature.

Study shows dipping grapes in to 0.5% NaOH solution at 90 °C for 2-3 sec repeated for two to three times and then dipped in to Lye solution 0.5% + Ethyl oleate 1.0% + Potassium metabisulfite 1.0% will improve the physical and chemical properties of raisins and also reduce the time required for drying.
Pretreatment grape before drying

Manufacturing process of raisins

Raisins are the second most important product of the grapevine wine, being the first. The quality of raisins depends on the size of the raisins berries, the uniformity and brilliance of the berry color, the condition of the berry surface, the texture of the skin and pulp in the berry, moisture content, chemical position and presence of decay, mold, yeast and foreign matter.

Raisins are produced from grapes by sun during and artificial drying. In the traditional drying method, grape bunches are spread over either the ground or on a platform or on wooden trays in a thin layer between the rows of grapes vines.

The trays are tilted to face the sun. After they are partially dried, the grape bunches are turned and allowed to dry to the point where no juice can be pressed out. The trays are then stacked, and the air drying is continued in the shade until moisture content of about 17% reached. After drying, the raisins are placed in sweat boxes to equilibrate, or to even out, the moisture that is present, and then they are packed in retail size containers or in larger containers to be sold to the bakery trade.

This method of drying is most cost-effective and takes approximately 8-10 days to produce dried products.

In artificial drying, grapes are first dipped in 0.25 – 1 % lye (sodium hydroxide) solution at 200 – 212 degree F for 2 – 5 sec to remove a natural wax that impedes drying and to check or crack the skin of the grape to facilitate drying

They are then washed, placed on trays, and treated with sulfur dioxide to prevent enzymatic and nonenzymatic browning during drying. Hence instead of having the dark brown color of raisins that normally be expected, the raisins will be of a light yellow color when dried.

Raisins are dried at temperatures not exceeding 165 degree F and at a low relative humidity. After the moisture content has been lowered to about 16 – 18%, the raisins are packaged in containers of various sizes to be sold at retail or for use by the food industry.

In California, light-colored raisins are produced mostly by the golden bleach process. First, Thompson Seedless grapes are sorted, then they are dipped to produce slight checking cracking of the skins, and they are cooled and washed in a spray of cold water. They are exposed to the fumes of burning sulfur in a sulfur house. After being sulfured, the grapes are dehydrated at a temperature of 140 to 160 degree F.
Manufacturing process of raisins

Packing process of grape fruits

In packing grapes, different types of containers are used: Lug boxes are most generally used, and the depth of the lug is varied according to a variety and consumer preference. Sawdust chest packs are used for grapes to be exported.

Grapes to be shipped as fresh are packed in wooden crates, then pre-cooled to about 40 ° F (4.4 °C) in railroad cars or refrigerated rooms.  Grapes destined for immediate delivery are sometimes shipped without pre-cooling though they should be shipped in a refrigerated container.

Grape bunches can be arranged in different ways in the containers. In the stem-up pack arrangement clusters stems are in full view but packs are attractive. Fresh grapes can be packed in this manner with very little handling injury.

Generally, the grapes will be fumigated with sulfur dioxide prior to or during cooling to prevent mold growth.

Grapes that are to be stored for future shipment should be packed in crates, pre-cooled to 36-40 °F (2.2 - 4.4 °C), place in refrigerated storage 29- 32 ° F (-1.67 – 0 °F), and fumigated with sulfur dioxide and should be held ih this manner until shipped.

Periodic re-fumigation with sulfur dioxide may be required to prevent spoilage by molds.

Most table grapes are packed as they are picked to avoid extra handling. A large part of the California grape crop is packed in the vineyard. This method has some advantages over the house-pack method:
• Less capital investment in packing facilities
• Only a vineyard crew to mange rather than a picking and lacking crew whose production must be coordinated
• No large inventory of filed lugs
Packing process of grape fruits

Grape juice processing

In general grape juice production can be divided into four basic principal stages.
*Front end operation
*Juice extraction
*Juice clarification and refining
*Juice pasteurization and concentration

Grapes at the plant are first conveyed to a steamer which remove residual stems, leaves and petiole from fruit. After cleaning, grapes are crushed to facilitate juice extraction during the pressing step.

The crusher consists of several ruffled rotary rollers that break the berries while they are passing through. In most commercial operations, the continuous pressing method is used. Hot pressing is appropriate for deeply pigmented grape where maximum color extraction is desired.

Whereas, the immediate or cold press procedure is necessary to maintain the initial color of light colored grapes.

Additional enzyme treatment is required before the juiced is further processed For concentration it is necessary to complete the de-pectinization process to preclude gelation of the concentrate once produced.

For bottled juice, de-pectinization speeds and simplifies polish filtration and clarification prior to bottling.

Juice may be concentrated by evaporation or freeze concentration. It is desirable to heat the juice for as short a time as possible and to rapidly cook the product in order to minimize the effect on flavor, aroma and sugar concentration.
Grape juice processing

Cold pressing of grapes

The cold-press method is the simplest and quickest and yields brilliant, transparent juices with are not only generally superior in flavor and aroma but also much more pleasing in appearance. Cold ropes minas the pressing of grapes at the same temperature as received from the vineyard.

Cold pressing of grapes has an advantage over hot pressing in that a fresh grape flavor is more readily obtained from many varieties of grape that is obtained by hot pressing.

The only difference between this method of juice production and the hot-press methods are steps that allow for heating of the crushed berries to 60-63 C and holding in tanks with pectolytic enzymes.

Cold- pressed, light colored juices are invariably muddy and may have sediment. This necessitates clarification to produce an appealing product. Enzymes may be added to cold press juice to facilitate the clarification and filtration process following cold stabilizers.
Cold pressing of grapes