WO1996039875A1

WO1996039875A1 - Method of thermally processing low acid food with added tartaric acid

Info

Publication number: WO1996039875A1
Application number: PCT/US1996/009723
Authority: WO
Inventors: Joseph Robert Powers; Charles William Nagel
Original assignee: Washington State University Research Foundation
Priority date: 1995-06-07
Filing date: 1996-06-06
Publication date: 1996-12-19

Abstract

Low acid vegetables such as asparagus spears, green beans and thelike are canned in a solution acidified with tartaric acid. Sufficient tartaric acid is added to the canned product so that the pHrange of the canned product after thermal processing is within a range of about pH 4.1 to about 4.4. The reduced pH allows less extreme thermal processing than standard canning parameters to achieve microbiological safety, thereby enhancing the organoleptic qualities of the food.

Description

METHODOFTHERMALLYPROCESSINGLOWACIDFOODWITHADDEDTARTARICACID

Technical Field

This invention relates to a process for treating vegetables for canning, and more specifically to a process for acidifying low-acid vegetables prior to thermal processing.

Background information "Canning" is the process by which food is enclosed in hermetically sealed containers and heated so that the shelf life of the food may be increased. The most commonly used commercial method of treating canned foods so as to prevent spoilage or other degradation is to expose the containers to heat for a known period of time in a retort. Generally speaking, the goal of "thermal processing" is to "commercially sterilize" the contents of the container to ensure sufficient shelf life, and more importantly to ensure a safe product. Commercially sterilized foods may not be "sterile" in the microbiological sense because most commercially sterile foods contain microbial cells or spores that survive the thermal treatment. However, any surviving cells or spores are usually unable to initiate growth in the medium of the container.-

Generally, then, the goal of thermal processing is to kill or inactivate substantially all of the vegetative cells of "microbes" (e.g., bacteria, yeasts, and molds) as well as their spores contained in the food product. A secondary goal is to inactivate enzymes that could have a detrimental impact on the storage life of the food. These goals accomplished, the canned food product may have an extensive shelf life as long as the container remains sealed.

Food products have been preserved in thermally processed sealed containers for many, many years, and the process is well known and documented in the art. It is also well known that several factors affect the processing parameters used to thermally treat canned foods. These factors include thermal processing time, te perature and pressure, and the pH of the canned product. In general, the heat resistance of microbial cells and spores is greatest in a food product that is at or near a neutral pH. Stated another way, the thermal processing required to kill microbial cells and spores in a food that is at or near a neutral pH is more severe than the processing required with acid or alkaline foods. Similarly, a decrease in pH (i.e., increased acidity) is more effective than a corresponding increase in pH in terms of reducing the thermal processing required to inactivate microbes. Accordingly, the pH of canned food products is a primary factor used to determine the extent and severity of thermal processing required to stabilize the food and ensure "microbiological safety."

Foods have traditionally been categorized according to their pH values as either "low acid" or "high acid" for the purposes of thermal processing. Thus, in the Code of Federal Regulations, "low-acid foods" are defined as any foods (other than alcoholic beverages) with a finished equilibrium pH greater than 4.6 and a water activity greater than 0.85. 21 C.F.R. § 113.3(n). Under 21 C.F.R. § 114.80(a)(1), "acidified foods" are those foods so manufactured, processed and packaged that an equilibrium pH value of 4.6 or lower is achieved within the time designated for the processing. This then is the basis in common parlance for calling low-acid foods those with an equilibrium pH value greater than 4.6 and a water activity greater than 0.85. High-acid foods, conversely, have an equilibrium pH value of 4.6 or less.

This traditional categorization for the purposes of thermal processing is based at least in part upon the effects of heat on Clostridium botulinum, an anaerobic spore-forming bacterium that will grow in sealed containers and produce a deadly toxin if the container is insufficiently thermally processed. C. botulinum growth is inhibited at pH 4.6 and below. High-acid foods such as fruits generally require less severe thermal processing than low-acid foods such as many vegetables. High-acid foods are often filled into containers hot, at or near 100°C (212°F) , and sealed. Alternatively, the filled containers may be sealed and processed at atmospheric pressure at temperatures approaching 100°c (212°F) . Either of these treatments can result in the killing or inactivation of substantially all vegetative cells of bacteria, yeasts and molds and their spores. Any surviving bacterial spores or cells cannot grow in the acidic environment inside the containers.

Conversely, low-acid foods like most vegetables must be processed at temperatures higher than 100°C (212°F) to ensure the safety of the food. For example, according to the National Food Processors Association Research Laboratories publication, Thermal Processes for Low-Acid Foods in Metal Containers , Bulletin 26-L, Twelfth Edition, 1982, asparagus spears with a typical pH in the range of 5.5 to 5.8 in a 16-ounce can should be thermally processed at 250°F (12l°C) for 19 minutes with an initial minimum temperature of 70°F (21°C) . The same publication recommends a processing time of 14 minutes at 250°F (121°C) with a minimum initial temperature of 70°F (21°C) for green beans, which have a typical pH in the range of 5.3 to 5.5.

Although the canning process is known to extend the shelf life of foods significantly, it also results in undesirable and often excessive softening of the canned food product. The impact of thermal processing on the flavor of foods may also reduce the quality of the food if the food is perceived by consumers as overly "cooked." This is especially true with canned low-acid foods given the excessive thermal processing necessary to ensure microbiological safety. Low-acid foods such as asparagus, green beans, cauliflower, broccoli and carrots are known to suffer considerable degradation in texture and flavor during the canning process. There is, therefore, a trade-off between the quality of the canned product and the requirement for sufficient thermal processing.

While fresh foods are more readily available today, due in part to more effective agricultural programs and delivery systems, canned foods remain an important component of the food supply. Given the ready availability of fresh foods, consumers have come to prefer and demand foods that have quality factors associated with fresh products. Thus, consumers prefer food products that more closely resemble fresh foods. Conversely, products having extended shelf life are very much in demand.

To meet the needs of consumers there have been numerous approaches adopted to improving the quality of the canned product while maintaining microbiological safety. More specifically, there have been a number of approaches to increasing the firmness of thermally processed canned vegetables. For example, addition of calcium salts to a brine in which the vegetables are suspended for canning, often combined with low- temperature blanching prior to canning, has been used with varying degrees of success. It has been suggested that canned green beans may be firmer after a low- temperature blanching treatment prior to canning. Another approach to solving the problem of excessively softened canned vegetables is to acidify the vegetables prior to thermal processing. This may allow for a less severe thermal processing treatment since bacterial cells are more easily inactivated at a lower pH. This approach, however, has drawbacks as well. For example, the amount of a food-grade acid (i.e., an "acidulant") needed to be added to a low-acid food product in order to lower the pH to less than 4.6 is normally relatively large, which can lead to a sour taste. Such sourness may result in a perception by the consumer of a "pickled" rather than simply canned product. It is known that different food-grade acids cause varying degrees of sourness. For example, citric acid generally is the least "sour" of the commonly used food acids and tartaric acid is the most sour of the common food-grade acids. Pangborne, Relative Taste Intensities of Selected Sugars and Organic Acids, Journal of Food Science. 2^:726-733, (1963). Other reports have likewise ranked tartaric acid as the most acidic in flavor of the food grade acids. In one example of the acid flavor impact of various acids in pure aqueous solution, the acids were ranked from most to least acidic as follows: tartaric > acetic > lactic > citric. Amerine et al.. Acids and the Acid Taste . I. The Effect of pH and Titratable Acidity, Am. Journal of Enoloαv and Viticulture. .16. (l) , 29-37 (1965).

Thus, while tartaric acid is approved for use as an acidulant in food-processing applications, it is not known to be used in the canning of low acid products, due to sensory considerations. On the other hand, citric acid, which as noted above has been ranked as the least acidic in flavor of common food acids, has been noted as an acidulant for canned vegetables. Acidulants in Food Processing , Gardner, W. H. , in Handbook of Food Additives, Furia, T. , ed. , (6), 263 (1968). Indeed, lemon juice, which contains citric acid as the primary organic acid, is a permitted optional ingredient in various canned vegetables. See 21 C.F.R. § 155 et seq.

There have been other approaches adopted to improve the texture of canned vegetables. There has been some success through the use of calcium salts and low-temperature blanching to produce firmer vegetables. One exception is with asparagus, which consumers may find particularly objectionable if excessively softened by processing. Prior studies have shown that the addition of calcium salts and/or the use of low- temperature blanching prior to canning does not markedly improve the texture of canned asparagus. There is a need, therefore, for a thermal- processing method for canning low-acid food products that enhances the desirable qualities in the canned product yet ensures microbiological safety. gmnmayγ of the Invention

It is an object of the present invention to provide a method by which the consumer acceptability of canned low-acid foods may be improved. More specifically, the present invention provides a method by which canned low-acid foods such as asparagus, green beans, broccoli, cauliflower and carrots and the like can be thermally processed to attain microbiological safety yet retain desirable qualities such as flavor and texture. It is a further object of the invention to provide a processing method that reduces the amount of energy required to fully process canned low-acid foods.

These and other objects are accomplished by a process in which low-acid foods are acidified with food- grade tartaric acid to a pH below 4.6. After sealing in containers, the acidified product is then thermally processed for a time sufficient to kill or inactivate substantially all vegetative microbes and their spores. That is, the containers are commercially sterilized. In one embodiment, the thermal processing time of one-pint containers in boiling water (100°C, 212°F) was about 20 minutes. The containers are then cooled to room temperature.

Brief Description of the Drawings Fig. 1 is a plot of the pH of several samples of asparagus versus the amount of several acids added to the samples.

Fig. 2 is a plot of the pH of several samples of green beans versus the amount of several acids added to the samples.

Fig. 3 is a plot of the pH of several samples of broccoli versus the amount of several acids added to the samples. Fig. 4 is a plot of the pH of several samples of cauliflower versus the amount of several acids added to the samples.

Fig. 5 is a plot of the pH of several samples of carrots versus the amount of several acids added to the samples.

Fig. 6 is a plot of the effect of a reduced thermal process (P) compared to traditional thermal processing (R) on the texture of the asparagus. Detailed Description of a Preferred Embodiment According to the present invention, tartaric acid is used as an acidulant to decrease the pH of certain foods (such as low-acid vegetables) before canning. After canning, the resulting product pH is within a target range of 4.1 to 4.4. This pH range is below the pH at which C. botulinum growth occurs, but allows for a less severe thermal processing treatment to attain microbiological safety of the canned food. Because the canned food is subjected to less severe processing, it has a firmer texture than an otherwise similar but traditionally canned food. A food canned according to the present invention also has a surprising and unexpected, yet desirable, non-acid (i.e., not sour) flavor compared to otherwise similar products acidified with other food-grade acids.

Also, in view of the decreased severity of thermal processing required, processes according to the present invention beneficially consume significantly less energy than more traditional canning processes. While one embodiment of the inventive process is described herein with reference to a preferred embodiment for canning asparagus, it will be recognized that the process is equally applicable to other low-acid foods such as green beans, broccoli, cauliflower, Brussels sprouts, peas, corn, zucchini, carrots and the like. It is also envisioned that the process according to the present invention may be applied, but not limited to many canned, water-containing foods, such as fruits, cereal grains and cereal grain-based products, and such foods canned in sauces.

It will be appreciated that the process of canning foods to attain microbiological safety depends on any number of factors well known to those of ordinary skill in the art. Examples of only a few of the factors that affect the thermal processing necessary to ensure a safe product include the type of food, particle size, container size, type of heating (convective versus conductive) , cleaning prior to canning, blanching, solute content in the fluid within the can, post¬ processing cooling, and pH. Moreover, the process of canning foods destined for commercial distribution in the United States is regulated by the United States Department of Agriculture and the United States Food and Drug Administration under strict guidelines.

It has been surprisingly found that, of the organic acids that are approved for food use, tartaric acid is particularly well suited to acidify low-acid canned vegetables to facilitate reduction of the thermal processing required to commercially sterilize the food, yet retain a desirable flavor. This result is unexpected in view of the association of tartaric acid with increased sour flavor. Of the organic acids approved for use in food, tartaric acid is a relatively strong acid. It is a non¬ volatile, diprotic acid having a pK_a of 2.93 and 4.23. The pk_a/ which generally may be defined for purposes herein as the pH at which one half of the acid is dissociated, also represents the pH at which the buffering capacity of the acid is greatest.

However, because it is a relatively strong acid and, as noted above, has been commonly thought to have a highly acidic flavor, tartaric acid has not been a likely candidate for acidifying low-acid canned foods. Nonetheless, because it is a relatively strong acid, a relatively smaller amount of acid is needed to reduce the pH of the canned system to below 4.6, thereby allowing a less severe thermal processing.

Figure 1 illustrates the ability of tartaric acid to decrease the pH of canned asparagus. As shown in the Figure, a lesser amount of tartaric acid is required, relative to other organic acids, to decrease the pH of the system to below the critical level of 4.6.

To prepare the samples tested in Figure 1, asparagus spears and water were combined at a ratio of 60% to 40% by weight. The asparagus and water were then blended to make a homogeneous puree, or slurry. Fifty grams of the slurry was then added to separate containers. The amounts of acid indicated in Fig. 1 were then added to the containers, which were then heated in boiling water for twenty minutes. The containers were then cooled and the pH of each was measured.

It is clear from Figure l that less tartaric acid was required to decrease the pH of the slurry to below 4.6 than the other acids tested. This result arises from the diprotic nature of tartaric acid, which enables it to simultaneously reduce pH and function well as a buffer in the target pH range of about 4.1 to about 4.4. Figs. 2 through 5 are similar plots for green beans, broccoli, cauliflower and carrots, respectively. The titration curves for each of these vegetables are consistent with the corresponding curves of Fig. 1. Table 1, below, demonstrates how the use of tartaric acid according to the present invention affects the flavor intensity of the acid in an asparagus system. In this table six parts of asparagus were combined with four parts of water. The combined asparagus and water were blended to make a slurry, and the acid indicated in the table was added. The containers were then heated in boiling water for twenty minutes and cooled. Five panelists were independently required to taste each of the samples and rank the perceived acidity of each sample on a scale of 1 to 5. Table 1

Heated asparagus slurry pH values as a function of added organic acid.*

Volume Acid

Acid Added Acid Added^b pH Taste Rank^d

None — 6.21 1

Tartaric 1.5 ml 4.27 2

Gluconic 1.8 ml 4.24 3

Citric 2.5 ml 4.21 4

Succinic 3.2 ml 4.28 5

^•Acid added to 50g of asparagus slurry, which was then heated 20 minutes in a boiling water (100°C, 212°F) bath. nl IN acid added ^cpH after heating ^dl = least acid; 5 = most acid

All of the panelists ranked the samples in the order indicated in the table. Thus, of the four common organic food acids tested in an asparagus slurry, tartaric acid was perceived to have the least acid flavor impact. This effect is surprising and unexpected.

Example l A preferred canning process according to the invention will now be described with regard specifically to the canning of asparagus.

Fresh asparagus spears were sorted and cut to a length of approximately four inches as in a conventional asparagus canning process. The cut spears were thoroughly washed to remove soil and other contaminants.

Conventional washing techniques such as multiple immersions in cold water were used and are sufficient.

The cut spears were then transferred to canning jars with the tips of the spears oriented downward. An aqueous solution of sodium chloride and an aqueous solution of tartaric acid, or tartaric acid/citric acid

(3:1) were then added to the jars according to Table 2.

The jars were filled to a level standard in the canning process. As detailed below, sufficient acid was added to reduce the pH to below 4.6, and preferably within a target range of about 4.1 to 4.4.

Although the process detailed in Table 2 used aqueous solutions of tartaric acid and sodium chloride, crystalline tartaric acid and sodium chloride could be used.

The filled jars were then sealed and thermally processed. Thermal processing comprised immersing the sealed jars in boiling water at atmospheric pressure for sufficient time to commercially sterilize the contents of the jars,that is, to achieve microbial safety. The thermal processing time varies depending on numerous factors, as noted above. One of the most critical factors is the size of the container. In the example of Table 2, 16-fluid-ounce containers were boiled for 20 minutes at atmospheric pressure. The processed containers were subsequently cooled to room temperature.

Based upon these data the target pH was accurately reproduced in each of the sample containers. These results again reflect the buffering capacity of tartaric acid in the tested systems.

Blends of tartaric acid and citric acid were also evaluated, as reflected in Table 2. Among vegetables, asparagus is somewhat unique in that the edible portion of the plant contains a phenolic compound commonly referred to as rutin. During thermal processing, the rutin present in the plant tissue may be extracted into the canning brine. When iron is present in the brine there is the possibility that an iron-rutin complex may be formed, which unacceptably darkens the color of the canned product. Citric acid, which is well known for its ability to chelate iron, was added in the examples reported in Table 2 to eliminate the problems with such darkening. However, there was no reported darkening in the samples acidified with tartaric acid only.

The determination of thermal processing time is well known to those skilled in the art and is beyond the scope of this invention. For purposes herein, the thermal processing time is generally determined by the well known concepts of the thermal death point (i.e., the temperature required to completely destroy, within a stated time, a specified concentration of spores in a medium of known pH) , and thermal death time (i.e., the time required at a given temperature to accomplish the same objective) . General texts discussing canning are readily available, and the Code of Federal Regulations sets forth the parameters and requirements of safe thermal processing.

Asparagus canned according to the method described above has improved texture over traditionally canned asparagus. Figure 6 is a plot of the effect of acidification on the texture of asparagus spears. In the Figure, glucono-delta-lactone (GDL; a powder) was used to prepare aqueous solutions at the concentrations indicated, and to reach the indicated pH. Samples of canned asparagus spears at each pH were retorted according to traditional methods of commercial processing (retorted samples are identified in the graph of Fig. 6 with the designation "R") . Separate corresponding samples were thermally processed according to the method of the present invention, as discussed above (identified in Fig. 6 with the designation "P") . That is, the samples (in sealed 16-oz jars) were immersed in boiling water at atmospheric pressure for 20 minutes. A control sample was not acidified.

The canned asparagus was prepared for testing to evaluate texture using an Instron apparatus. To prepare the samples, approximately 2 cm were cut off the butt end of each asparagus spear. The immediately adjacent 3-cm portion was then cut off the spear and tested on the Instron apparatus. The results from these 3-cm portions are displayed in Fig. 6 as open horizontal bars. Then, the next immediately adjacent 3-cm portion was cut off the spear and similarly tested. The results from these portions are displayed in Fig. 6 as solid horizontal bars.

Figure 6 illustrates generally that a decrease in the heat treatment, which is allowed by the weak acidification of the product, results in a firmer product when the product is processed according to the present invention. Although GDL is used in the example of Figure 6, it is used for illustrative purposes only and demonstrates the effect of a reduced heat treatment (allowed by acidification) on texture.

Table 2

Container* Asparagus Acid^b Salt^c pH pH⁴*

(g) Added Brine Target Blended

1 212 10.6 134 4.2 4.23

2 212 10.6 134 4.2 4.18

3 213.8 8.6 135 4.4 4.34

4 209.4 8.4 132 4.4 4.34

5 208 10.4 128 4.2 4.29

6 211.9 10.6 131 4.2 4.25

7 225.9 9.0 142 4.4 4.40

8 316 12.6 197 4.4 4.44

"Pint jars nL IN tartaric acid in containers 1-4; mL 1:3 IN citric acid/lN tartaric acid in containers 5-8 ^c2% (W/W) aqueous sodium chloride solution ^dEntire contents of container blended after cooling

Example 3 Table 3 provides four alternative ingredient formulations used for canning asparagus according to the present invention.

Table 3

Example weight percent

Component 1 2 3 4

Asparagus 51 52.7 52.2 52.0

Water (mL) 45.9 44.1 43.7 43.8

Sodium Chloride 0.5 0.5 0.5 0.5

(g) Tartaric Acid 2.6 2.6 2.6 2.6

(IN solution)

(mL)

Asparagus Flavor* — 0.1 — 0.1

(natural extract)

(mL)

Sugar* (g) 1^0 1.0

Total 100 100 100 100

^•Optional

While the present invention has been described in accordance with preferred embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the claims.

Claims

We Claim:

1. A method of thermally processing a low-acid vegetable in a sealed container comprising the steps of: adding an amount of tartaric acid to the vegetable in the containers prior to sealing the containers such that the pH of the solution in the container is within a range of about 4.1 to 4.4 after the containers are thermally processed; sealing the containers; and thermally processing the vegetable within the sealed containers sufficient to kill or inactivate substantially all vegetative microbes and their spores.

2. The method according to claim 1 wherein the vegetable comprises asparagus.

3. The method according to claim 1 wherein the vegetable comprises green beans.

4. The method according to claim 1 wherein the vegetable comprises carrots.

5. The method according to claim 1 wherein the vegetable comprises broccoli.

6. The method according to claim 1 wherein the vegetable comprises cauliflower.

7. The method according to claim 1 wherein the tartaric acid solution is added as a IN aqueous solution thereof.

8. The method according to claim 1 wherein the tartaric acid is added in a solid form.

9. The method according to claim 1 including the step of adding sugar to the aqueous solution in the containers.

10. A method of canning low-acid vegetables, comprising steps of: preparing the vegetables for canning; adding the vegetables to canning containers; adding to the vegetables in the containers tartaric acid in an amount sufficient to decrease the pH of the contents of the container to at least below 4.6; sealing the containers; heating the containers for a time sufficient to kill or inactivate substantially all vegetative microbes and their spores.

11. A method for preserving a low-acid water- containing food comprising the steps of:

(a) adding an amount of tartaric acid to the food sufficient to cause the food to have a pH of about 4.1 to about 4.4 after completion of step (d) ;

(b) placing the food in a sealable container; (c) sealing the container;

(d) heating the sealed container and the contents to at least 100°C for a time sufficient to kill or inactivate substantially all microbes inside the container.

12. The method according to claim 11 in which the food is a cereal-based food.

13. A method for preserving a low-acid vegetable, comprising the steps:

(a) suspending the vegetable in a liquid comprising a solution of tartaric acid, the tartaric acid being at a concentration in the liquid sufficient to cause the liquid to have a pH of about 4.1 to about 4.4 after completion of step (d) ;

(b) placing the vegetable and aqueous liquid in a sealable container;

(c) sealing the container;

14. A method of thermally processing a low-acid vegetable so as to render the vegetable capable of being stored without spoiling, the method comprising the steps of: (a) adding tartaric acid to the vegetable so as to acidify the vegetable to a pH of about 4.1 to 4.4; (b) heating the vegetable to a temperature sufficient to either kill or inactivate substantially all microbes and spores thereof in and on the vegetable.

15. A method of thermally processing a low-acid vegetable so as to render the vegetable capable of being stored without spoiling, the method comprising the steps of:

(a) placing the vegetable in a container;

(b) adding tartaric acid to the vegetable in the container so as to acidify the vegetable to a pH of about 4.1 to 4.4;

(c) sealing the container;

(d) heating the sealed container to a temperature and for a time sufficient to at least inactivate substantially all microbes and spores thereof in the container.

16. The method according to claim 15 wherein step (b) comprises adding an aqueous solution of tartaric acid to the vegetable in the container.

17. The method according to claim 15 wherein the tartaric acid is added in a solid form.

18. A low-acid food having improved texture and contained in a sealed container, the product being thermally processed so as to render the food capable of being stored without spoiling according to the method comprising the steps of: adding an amount of tartaric acid to the food in the container prior to sealing the container such that the pH of the contents of the container is within a range of about 4.1 to 4.4 after the container is thermally processed; sealing the container; and heating the food within the sealed container sufficient to kill or inactivate substantially all microbes and spores thereof contained within the container.

19. The product according to the process of claim 18 wherein the food comprises a vegetable.

20. The product according to the process of claim 18 wherein the food comprises a cereal based food.