ELSEVIER

Journalof Food Engineering 22 (1994) 433-444 0 1994 Elsevier Science Limited

Printed in Great Britain. All rights reserved 0260-8774/94/$7.00

Combined Methods for the Preservation of Foods in Latin America and the CYTED-D Project

J. M. Aguilera

Department of Chemical Engineering, Universidad Catblica de Chile, PO Box 306, Santiago 22, Chile

J. Chirife

Departamento de Industrias, Universidad de Buenos Aires, Buenos Aires, Argentina

ABSTRACT

Many intermediate moisture foods (IMF) in Latin America are actually foods preserved against microbial spoilage by combined methods (CM) using only a few factors and having low technological development. Water activity (a,j, the main preservation factor used, is controlled through the addition of solutes or in extreme cases by dehydration. Developments in CM technology are illustrated and discussed through two examples: the stabilization of concentrated cheese whey and the long-term, bulk preser- vation offruit pieces.

INTRODUCTION

To commemorate the V Centennial of the Discovery of America, a pro- gram entitled Science and Technology for Development (CYTED-D) was launched by Spain in 1984. The fundamental objective was to pro- mote collaboration and cooperation among countries of Ibero America to achieve scientific and technological developments transferable to the production system, improving the quality of life and the economic development in the region.

433

434 J. M. Aguilera, J. Chirife

The first multinational project of the Food Conservation and Preserva- tion Subprogr~, named ‘Intermediate Moisture Foods Important to Iberian and Latin American Countries’ - CYTED-D AHI - was car- ried out from 1986 to 1989, then extended to January 1991 (Aguilera & Parada, 1992). Intermediate moisture foods (IMF) are ubiquitous in Latin America and many meats, fruits, vegetables and some dairy pro- ducts are preserved by a reduction of the a, via use of sugar or salt, sometimes accompanied by small changes in PH.

As one of the tangible results of this project an inventory with detailed identification, chemical characterization (including a,) and photographs of 246 traditional IMF from the region was published (Aguilera et al., 1991). In this publication consumption modes, quality factors and tech- nological aspects involved in the manufacture of local IMF were also dis- cussed. From this work it became evident that local IMF and those preserved by a combination of factors have been developed as tradi- tional technologies and evolved by trial and error.

THE ‘HURDLE’ CONCEPT AND COMBINED METHODS TECHNOLOGY

The first objective of traditional or newly developed food preservation processes is the inhibition or inactivation of ~croorganisms. This has led to the ‘hurdle’ concept, an extension of the IMF concept where a smaller depression in a, is compensated by the preservation action of other factors such as reduced pH, mild heating, preservatives, competi- tive flora, protective packaging, etc. (Leistner, 1985). Since preservation factors in combined methods (CM} or hurdle technology are used at a low level, changes to the original properties of the food are less than in IMF.

Most IMF (0.6 < a, < 0.97-0.99 and stored without refrigeration) reported in the inventory were stabilized by solute effect of salt or sugar. However, many local foods were actually preserved by CM or hurdle technology, complementing a, depression with lower pH and varying degrees of mild heating. Table 1 shows the main factor and their combi- nations used in the preservation of selected foods in Latin America, according to the CYTED-D AH1 inventory (Aguilera et al., 199 1).

The most widely used hurdle in CM technology is a, (Chirife & Favetto, 1992). Table 2 shows minimal a,,, for growth of foodbo~e bac- terial pathogens at their optimum pH and temperature. With the excep- tion of Staphylococcus aureus, for which a minimum a, of 0% is needed to avoid growth under aerobic conditions, growth of all other foodborne

Prese~ur~on o~foo~ in Latin America 435

TABLE 1 Main Factors used in Latin America for the Preservation of Traditional Foods by

Combined Methods

Product category

a,

Factors

pH F t Smoke Preser. C. flora

Fruits and vegetables x x x - - X X

Meat X X x - X X X

Fish X X X X X - -

Dairy X X X X - X X

Bakery x - x - - X -

Miscellaneous X X x - - X -

Meaning of symbols: a, = water activity; F= mild heat treatment; I = mild refrigeration; Preser. = preservatives; C. flora = competitive flora.

TABLE 2 Minimal u, for Growth of Foodborne Bacterial Pathogens

(Optimum pH and Temperature)

Cumpylobacter jejuni 0.990 Aeromonas hydrophila 0.970 C~os~ridii~m bo~ulinl~m E 0.965 C~ostridium botlilinu~n G 0,965 Shigella spp. 0.960 Yersinia enterocolitica 0.960 Clostridium perfringens 0.945 Ciosrridi~~m botl~linlirn A di B 0.940 Salmonella spp. 0.940 Escherichia coli 0.935 Vibrio parahaemolytic~ 0.932 Bacilius cereus 0.930 Listeria monocytogens 0.920 Staphylococcus aureus (anaerobic) 0.910 Staphylococcus aureus (aerobic) 0.860

pathogenic bacteria can be curtailed by a reduction in a, to approxi- mately O-92. In CM technology where reduction of a, is the main preser- vation factor, food stability during storage is based on successful inhibition of molds and yeasts. However, from the health standpoint the target remains to be Staphylococcus LZUEUS because the growth of this bacteria cannot be ‘practically’ prevented using only a depression in a,.

436 J. M. Aguilera, J. Chirife

In order to reduce the a, to 0.85 a high amount of solute needs to be dissolved in the aqueous phase of the food (Fig. 1). For instance, if sodium chloride was the solute of choice, more than 20 g of sodium chloride per 100 g of water would be needed, which would make the food unpalatable. Other common solutes require even higher amounts on a unit weight basis.

However, an alternative way to control Stu@zylococc~s aureus and some other low a,-resistant bacteria and microorganisms, is to use another preservation factor such as mild heating in combination with a,. This would allow, for instance, to increase the water activity to O-93, with a concomitant ‘drastic’ reduction in the amount of solute added to the food (Fig. 1).

Traditional food preservation methods are primarily based on the utilization of a single hurdle. The use of any hurdle by itself presents several problems and limitations in food preservation, as shown in Table 3. Some of the drawbacks of these technologies relevant to Latin Ameri- ca are the high cost and sophistication of the technology and an under- developed cold chain for frozen and refrigerated foods. It must be kept in mind that storage at ambient temperatures, a goal of IMF and CM preserved foods, means in these countries withstanding temperatures between 15 and 40°C.

EXAMPLES OF DEVELOPMENTS IN CM TECHNOLOGY

Some of the developments and research in this area in Latin America started by the CYTED-D AH1 project will be illustrated through two

160

k t+ 120 3

:I00 8 = 80

Glucose Glycerol PG S. Lactate EtOH NaCl

Fig. 1. ‘Drastic’ reduction in the amount of solute added to the aqueous phase.

Presentation offoo& in Latin America 437

TABLE 3 Some Limitations of Traditional Food Preservation Methods Based on One Hurdle

method

Air drying

Freeze-drying

Canning

Salting

Acidification ( nat Jartif.)

Preservatives Refrigeration/freezing

Hurdle

a,

a,

Thermal inactivation

a,

PH

Antimicrobial action Low temperature

(also a, for freezing)

Li~itat~~~s

Loss of flavor, shape & color Poor texture Slow/incomplete rehydration

cost

Quality loss Cost of the can Energy cost

Very high salt content Poor texture (meats)

Flavor changes due to high acidity

Legal & health problems Energy cost Absence of ‘cold chains’

examples: the stabilization of concentrated cheese whey and the long- term, bulk preservation of fruit pieces.

Stabilization of concentrated cheese whey by combined methods is an alternative to the more expensive spray drying process. Stabilization of cheese whey was carried out according to the flow diagram shown in Fig. 2. The original concentrated whey having an a, of about 0.94 and 0.2% potassium sorbate is subjected to e~yma~c hydrolysis at 37°C. Hydroly- sis reduces the a, to about 0.9 and addition of citric acid brings the pH to about 5.2. During hydrolysis, the growth of Staphylococcus aweus is inhibited by the presence of sorbate and the low a, due to presence of released monomer sugars from lactose. Under these conditions the hydrolysed/concentrated whey (a, = 0.9, pH = 5.2 and 0.2% of potas- sium sorbate) is a shelf-stable product that can be stored for about 3 months at tropical conditions, a period sufficient for some industrial applications such as in a bakery.

Leiras et al. (1991) have studied the effect of sorbate, propionate and pH on the behavior of Staphylococcus aureus during storage of hydro- lysed concentrated whey at 37°C and a, = 0.9. At pH = 55 the presence of either preservative at 0.2% level or in combination at 0.1% showed a preservation effect equivalent to a larger drop in pH to 5.0. These com- binations of preservatives and pH not only inhibited the growth of Staphylococcus uureus but in fact reduced the number of the cells during

438

Raw Material

Fig. 2. Flow diagram for the stabilization of cheese whey concentrate by combined methods.

storage. Addition of ethanol at 3 and 4% not only suppresses growth, but also reduces the number of cells during storage (Shaper0 et al., 1978).

Vegetables and fruits play an important role in the diet of people in the tropics, providing essential minerals and vitamins and adding variety to an otherwise monotonous diet. Postharvest losses of these perishable commodities in less-developed countries exceed 20%. As an alternative to the traditional canning or sugaring of fruits, groups in the CYTED-D AH1 project are developing a CM preservation process whose flow dia- gram is shown in Fig. 3. In this process fruit pieces after blanc~g are combined with sugar, such as glucose or sucrose, citric acid, sorbate or benzoate and sodium bisulfite to achieve final values of a, = 0.93-0.97, pH = 3.1-35, sorbate < 1000 ppm and SO, < 100 ppm, after equilibra- tion. The moisture content of these products is in the range of 5569%. These shelf-stable fruit pieces are different from the low-moisture, high- sugar candied fruits because they have a lower sugar concentration and a higher moisture content, resembling canned fruits. The hurdle combina- tion used in preservation of fruit pieces relies on a slight reduction in a,, control of pH and addition of preservatives such as sorbic acid and sodium bisulfite in small concentrations. Table 4 shows data on the com- position of some of the shelf-sable, ~gh-mois~re fruits (peaches, pine-

Preservation of foods in Latin America 439

Fig. 3. Flow diagram for the production of fruit pieces by combined methods.

TABLE 4 Shelf-Stable High Moisture Fruits (3/4 months at 20/3O”C)

Peach halves

Humectant: Glucose Acidulant: Citric Red. sugars: 28% Moisture: 63% u,: 0.94 pH: 3.5

After 4 months storage at 2OO”C Sorbic: 600 ppm SO,: 32 ppm

Pineapple slices

Glucose Citric 2.5% 69% 0.97 3.1

After 4 months storage at 27°C Sorbic: 376 ppm SO,: 48 ppm

Mango slices

Sucrose Citric

55% 093 3.3

Na benzoate Na bisulfite

apples and mangos). Products may be stored for about 3 or 4 months at room temperature. The low residual content of SO2 after some months of storage assures compliance with most present legal regulations regard- ing the use of this chemical. Applications of CM fruits may include inter- mediate products stored in large containers for further processing into jams, concentrates, juices, etc.

There are high incentives to apply similar CM technologies to increase the availability of protein from low-cost, abundant marine pro- ducts. Foods with good organoleptic properties and adequate shelf-life would be alternatives to the traditional dried, salted fish and assist in increasing per capita consumption of an abundant protein source. Rationale for the microbial stabilization of a washed mince from pelagic

440 J. h4. Aguilera, J. Chir$e

fish by combined methods is shown in Fig. 4 (AguiIera et al., 1992). The concurrent action of sodium chloride to depress u, to 0.9, reduction of pH to 5-O with acetic acid, addition of 0.2% potassium sorbate as a pre- servative and mild heating in the package constitute the hurdles of the process. Protein functionality remains the largest problem to solve in fish preservation by CM technology.

POTENTIAL FACTORS TO BE USED IN CM

As previously discussed a, is one of the most important hurdles used in CM technology. The most widely used solute to depress a, is sodium chloride which has several limitations concerning taste and health- related problems. In an effort to find substitutes or partial replacements for sodium chloride two alternatives have been investigated: sodium lac- tate and ethanol. Figure 5 depicts the ability of sodium lactate and etha- nol to reduce a, in comparison to sodium chloride. The effectiveness of sodium chloride is related to its low molecular weight and the dissocia- tion into two ions. However, the ability of either ethanol or sodium lac- tate to depress a, is not different from that of sodium chloride, so they may be used in comb~ation with sodium chloride to depress the water activity and perhaps avoid some of the problems associated with the use of common salt.

Figure 6 shows that sodium lactate, in addition to lowering the water activity, has inhibitory effects against several bacteria. In particular,

Lowering of stationan/ phase

lime

Fig. 4. Schematic of the effect of combined methods technology on microbial stabiliza- tion of fish mince. -, Fish preserved by combined methbds technology; - - -,

unpreserved fish.

Preservation offoods in Latin America 441

sodium lactate has a good inhibitory effect on Staphylococcus aureus. If a, is depressed to 0.97 with sodium chloride, Staphylococcus aureus grows rapidly but if sodium lactate is used instead to bring down a, to the same level, growth is delayed for over a week. Combination of these two salts can be used in order to produce foods which are shelf-stable but have lower contents of sodium chloride. Hence, sodium lactate has several properties which are important for CM. First, good a, lowering ability because of its low molecular weight; second, it is a GRAS sub- stance, USDA approved for use in meat products at levels up to 3-4%;

1

24

22

20 Na Lactate

6-

Water Activity

Fig. 5. Comparison of the solute effect of Na Cl. Na Lactate and ethanol on water activity.

0 100 200 300 400 50 In~batiot%ns, h” 450

+ control

-B-

&.= 0.97

-A-

+ 0.97

X

&= 0.97

Fig. 6. Effect of Na lactate on the growth of Staphyfococcus aurew C-243. (From Vaamonde et al., 1992.)

442 J. M. Aguilera, J. Chirife

third, it has a mild saline taste, enhancing some sensory properties in meats and finally it has some specific antimicrobial activity, specifically against Staphylococcus aureus as previously shown.

Ethanol may be used in low concentrations to control the growth of several microorganisms in foods. For instance, it is known that small con- centrations of ethanol may be used as antifungal and to control the growth of Staphylococcus aureus. Figure 7 shows data for growth of Staphylococcus aureus at a, = O-9. Addition of ethanol at 3 and 4% not only suppresses growth but also reduces the number of cells during stor- age. Another interesting advantage of ethanol is that due to its high volatility, cooking or heating of the food reduces the residual levels to low values, hence its potential application as a preservative in semi- processed foods should be evaluated. Concentrations of ethanol as low as 0.5 and 1% delay mold growth in bread, increasing shelf life by 50 and 150% respectively (Seiler & Russell, 1991). It is well known that in CM control of the growth of fungi is difficult due to the high a, prevailing in the food and small amounts of ethanol may be used for this purpose.

The popular Latin America cuisine uses several spices, aromatic herbs and condiments. Many of these products have antimicrobial pro- perties which could be exploited as an additional preservation factor. Table 5 summarizes some potential advantages and problems in using these products.

CONCLUSIONS

Many groups are currently working on microbiological aspects of CM foods in Latin America, as a result of the CYTED-D project. Only a few

1 .OE+O9 0%

Incubation time, h Fig. 7. Effect of addition of ethanol on the growth of Staphylococcus aureus F265 at

37°C. (Adapted from Shaper0 et al., 197X.) a, = 090.

Prese~a&~un ~~~oods in Latin America 443

TABLE 5 There is a Renewed Interest in the Use of Spices as Antimicrobials in Foods

Natural substances of plant origin appeal to the public at a time when safety of synthetic food additives is questioned

Many spices or their essential oils (EO) have strong antifungal and antibacterial activity:

The M.I.C. for spices is about l-S% The M.I.C. for most EO is 100-300 ppm

Cinnamon, Clove and Thyme are among the species which have the strongest antimicrobial activity

Problems The M.I.C. for spices (or EO) may be in many cases well above the

‘acceptable’ levels for flavoring foods

EO will tend to concentrate in the fat phase of a food and not in the aqueous phase

TABLE 6 Present Orientations and Prospects for CM Research in

Latin America

Present Many groups active in the area after CYTED-D Use of few traditional hurdles (a,, pH) Too low a,. (high salt/sugar) Use of conventional preservatives Excessive use of ‘trial & error’ approach Lack of fundamental understanding

Future Control of microbial growth at higher a, Use of a larger number of hurdles Utilization of alternative humectants Exploit the ~timicrobial effect of spices Higher use of mild heating More intense use of packaging alternatives (vacuum, MAP) Utilization of predictive models

factors are used in stabilizing and developing new shelf-stable foods, namely a, (through the use of salt and sugars) and pH (using a few acids and preservatives, mostly sorbate). Data on microbial stability of local foods at high temperatures (ZO-30°C) are being generated which are not available from research in industria~zed countries where the tempera- ture range 0- 10°C is more relevant. However, in order to make major

444 J. M. Aguiltm, J. Chin@

inroads in CM or hurdle technology the following aspects should be emphasized:

- research in more basic aspects of water relations in regional foods; - development and use of predictive models for microbial growth

under combined methods conditions; - incorporation of new hurdles such as spices; - integration with other technologies, e.g. packaging, modified atmo-

spheres and mild heating.

Table 6 summaries the main constraints and oppo~nities for com- bined methods technology in Latin America.

ACKNOWLEDGMENTS

Research reported in this paper has been partly supported by FONDECYT (Chile) project No. 382-88 and CONICET (Argentina). This work is part of the CYTED-D Project on Intermediate Moisture Foods Relevant to Ibero America.

REFERENCES

Aguilera, J. M. & Parada, E. (1992). CYTED-D AHI: An Ibero American project on intermediate moisture foods. Food Res. Intl., 25, 159.

Aguilera, J. M., Chirife, J., Tapia, M. S. & Welti, J. ( 199 1). Inventaria de Alimentos de Humedad Zntermedia Tradicionales de Ibero amtrica. Instituto Politicnico National, Mexico.

Aguilera, J. M., Francke, A., Figueroa, G., Bornhardt, C. & Cifuentes, A. ( 1992). Preservation of minced pelagic fish by combined methods. fntl. 3. Food Sci. Technol., 27.17 1.

Chirife, J. & Favetto, G. J. ( 1992). Some physico-chemical basis of food preser- vation by combined methods Food Res. Ml., 25,389-96.

Leiras, M. C., Chirife, J. & Alzamora, S. M. ( 199 1). development of a shelf-stable hydrloyzed concentrated cheese whey. Lebensm. Wiss.u-Technol., 24,12.

Leistner, F. (1985). In Properties of Water in Foods, ed. D. Simatos & J. L. Multon. Martinus Nijhoff, Dordrecht, The Netherlands.

Seiler, D. A. L. & Russell, N. J. ( 199 1). In Food Preservatives, ed. N. J. Russel & G. W, Gould. Elsevier Applied Science Publishers, London.

Shapero, M., Nelson, D. A. & Labuza, T. P. ( 1978). Ethanol inhibition of Staphy- lococcus aureus at limited water activity. J. Food Sci., 43, 1467.

Vaamonde, G., Chirife, J., Brizzio, S. & Favetto, G. J. ( 1992). Unpublished data.