Food Science: Courses: FS 532

The Commercial Canning of Green Beans

Introduction

There are several types of beans used for canning. Some are round, others are flat, and still others in-between. Commercial varieties now are of the "bush" type and are developed for mechanical harvesting.

Cleaning and Snipping

Beans for canning are usually bought on a graded basis with higher prices for the smaller sizes.

The harvested beans usually contain some dirt, leaves, pieces of vines, etc. These items are removed mechanically by shakers and air cleaners. The beans are then washed by various means to remove all other foreign matter. This can be accomplished by immersion in water followed by a spray washer.

After washing, the beans are pre-graded into small, medium, and large sizes and then put through snippers where both ends of each bean are removed. The snipper consists of a metal cylinder having narrow slots. As the cylinder revolves, the ends of the bean are caught in the slots, and fixed knives, lying snugly against the lower surface of the cylinder cut off the tips and stems that have been caught in the slots. The snipped beans then pass over inspection belts where defective beans are removed. Any beans that escaped snipping are returned to the snipping line again.

Figure 1: Mechanical Green Bean Snipper
Grading

The beans are also put through specially constructed size graders for separation into the desired number of sizes. The size is based on thickness, or diameter and is classified as follows:

No. 0 size 12/64 inch or smaller (4.8 mm)
No. 1 size from 12/64 to 14.5/4 (4.8-5.8 mm)
No. 2 size from 14.5/64 to 18.5/64 (5.8-7.3 mm)
No. 3 size from 18.5/64 to 21/64 (7.3-8.3 mm)
No. 4 size from 21/64 to 24/64 (8.3-9.5 mm)
No. 5 size from 24/64 to 27/64 (9.5-10.7 mm)
No. 6 size over 27/64 (10.7 mm)
Cutting

The small sizes (No. 3 and smaller) are usually packed whole. The larger sizes are cut crosswise by machine into lengths of 1-1½ inch (2.5-3.8 cm) for regular cut, or lengthwise for "French style." Lengthwise cutting is usually done after blanching to avoid excessive loss of flavor and nutrients, and to reduce the amount of small cut pieces and seeds that would otherwise accumulate in the blancher.

Figure 2: Front view of Bean cutterFigure 3: Rear view of Bean cutter
Blanching

All beans are blanched to remove gasses from within the tissue and to wilt the tissue, which helps in filling the cans. The blanching should be as short as possible to minimize sloughing of the bean epidermis, splitting and lack of firmness. A typical blanching process would consist of holding the beans in water at 170-180°F (77-82°C) for anywhere from 1.5 to 5 minutes. Longer blanches may be desirable for certain situations.

Filling

Cut and blanched beans are filled by machine on the basis of volume rather than weight. Fill-in weights may vary considerably depending on the style of pack, length of blanch, maturity of the beans, processing time, desired drain weight, and other factors. At the time of filling, a brine at 200°F (93°C) is added to the can. The strength of the brine is usually 1.5-2.5% salt.

Exhausting and Closing

If brine is added at a temperature near boiling and the beans have been properly handled, it should not be necessary to exhaust the filled can before closing. However, when the contents of the can has an average temperature below 165°F (74°C) as sometimes occurs in larger cans, a steam flow enclosure should be used or a steam or hot water exhaust sufficiently long to raise the temperature of the contents to this temperature. This ensures the removal of permanent gases. The filled hot cans are immediately closed by a mechanical can seamer.

Thermal Processing

The beans are heat processed to both sterilize and cook them. The objective of the sterilization process is to produce a condition called "commercial sterility." "Commercial sterility" of foods means the condition achieved by application of heat which renders the food free of viable forms of microorganisms having public health significance, as well as any microorganisms of non-health significance capable of reproducing in the food under normal non-refrigerated conditions of storage and distribution. The recommended sterilization processes are not designed to kill all microorganisms in canned foods. In other words, canned foods are "commercially sterile," but not bacteriologically sterile. The necessary heat treatment depends on the type of pack, the closing temperature, and the size and type of container. Bulletin 26-L from the National Food Processors Association in Washington, D.C. list minimum processing conditions for processing green beans without agitation. An example for Whole or Cut Green Beans follows:

Figure 4: Outside view of Steritort and Controls Figure 5: Close up of Steritort
Max. Fill Weight Min. Initial Temp. Minutes at Retort Temp.
240°F245°F250°F
Can Sizeoz.g °F°C (116°C)(118°C)(121°C)
211x3045.21477021171210
1204916129
303x406102847021221613
12049201511
401x411215957021231714
12049211613
603x7007220417021292218
12049272016
Source: A Complete Course in Canning and Related Processes, Book III, 13th Edition

These processes consider only the matter of sterilization in a still retort and actual processes used in commercial plants will exceed these times and temperatures to optimize product shelf life and quality. Also, the times and temperatures will vary for the type of retort being used. Due to the additional variables involved in the use of an agitating continuous retort, standard recommendations such as those shown above are not available and each processor must establish their own individual parameters.

Cooling

At the conclusion of the thermal processing, the cans are immediately cooled in water until the average temperature of the contents are at or below 105°F (41°C).

References

A Complete Course In Canning And Related Processes, Book I, Basic Information on Canning, 12th Edition. 1987. CTI Publications, Baltimore, MD.

A Complete Course In Canning And Related Processes, Book III, Processing Procedures For Canned Food Products, 13th Edition. 1996. CTI Publications, Baltimore, MD.

Luh, B.S. and Woodruff, J.R. 1975. Commercial Vegetable Processing. AVI Publishing, Westport, CT

Green Bean Canning Laboratory Exercise
Purpose

To demonstrate (1) the procedure for canning a low-acid food product, green snap beans, and (2) to examine the effect of using zinc salt during the blanching process on maintaining the bright green color that is typically lost during canning.

Materials
Procedure
  1. Make sure the beans are clean. Wash in cold water if necessary.
  2. Mechanically cut the green beans and collect in stainless bowls.
  3. Weigh out equal portions of the cut beans in each of two blanching sieves.
  4. Prepare a 200 ppm ZnCl2 in one of the blanching tanks (16 g ZnCl2 in 40 kg soft water).
  5. Add 40 kg soft water to the other blanching tank. This will be the control.
  6. Heat each of the blanching solutions to 170-180°F (77-82°C).
  7. Blanch the beans for 25 minutes. One batch of beans should be blanched in the ZnCl2 solution and the other batch should be blanched in the control blanching tank.
  8. During the blanching process, prepare about 10 kg of a 2% NaCl solution and add to the Groen mixing tank. Start stirring and heat to 200°F (93°C).
  9. When blanching is completed, drain the sieves and weigh 250 g of beans into appropriately labeled cans. There should be enough beans to fill 3-6 cans from each batch. The cans can be filled by hand using vinyl or latex disposable exam gloves. The beans will be hot so handle with care.
  10. After each can is filled with beans, add hot NaCl solution until the can is full. Do not leave any headspace.
  11. Immediately close each can with the mechanical can sealer.
  12. Using heavy plastic gloves to handle the hot cans, put the cans in the retort. The retort is an FMC Steritort, which is a pilot version of an agitating continuous retort commonly used in commercial plants.
  13. Seal the retort and cook at 252°F (122°C) for 25 minutes.
  14. Immediately cool the cans to 105°F (41°C) minimum by turning off the steam and adding cold water to the retort. This will take an additional 10 minutes before the retort can drained and opened.
  15. Remove the cans from the retort and open one can of each batch to observe for differences in color.
  16. Clean up any items used during this lab.
Color measurement

Determine color of beans by using the Hunter Colorquest colorimeter. Report values using the Hunter Lab L, a, b values.

Questions
  1. The Hunter Lab color scale is one of a few color scales in existence. Name one other and discuss why color measurement by an instrument is necessary and why this is difficult?
  2. What might be ways to further enhance the green color in the beans?
  3. The beans that underwent the Veri-green process are somewhat greener than the others. Discuss what is happening to the vitamin content of the beans. What is a way that the vitamin content can be maximized?
  4. Why is it important to cool the beans to ~105°F immediately after retorting?
Discussion

The laboratory exercise described above is a demonstration of a commercial process, known as Veri-Green. This process has been developed to improve the color of canned green vegetables by adding metal ions to the blanch solutions. The improved color that results is due to the presence of zinc complexes of pheophytin and pyropheophytin. The FDA has allowed for the use of ZnCl2 as an optional ingredient provided that the concentration of Zn++ in the product be no more than 75 ppm. Further information on this process can be found in the following references:

LaBorde, L.F. and von Elbe, J.H. Effect of Solutes on Zinc Complex Formation in Heated Green Vegetables. Journal of Agricultural and Food Chemistry 1994 42(5):1096-1099.

Segner, W.P., et al. Process for the Preservation of Green Color in Canned Vegetables. U.S. Patent 4,473,591. 1984.

Figure 6: Veri Green vs. Regular processes Figure 7: Hunter Color Scale
University of Wisconsin College of Agricultural & Life Sciences
Copyright © 2003, Department of Food Science,
University of Wisconsin-Madison. 		Updated February 7, 2003.
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