Applications fully dehydrated intermediate and high moisture foods

Improved knowledge of the relation of water in foods led to the rediscovery and optimization of old preservation techniques and to a renewed interest in foods that are shelf-stable by control of water activity. This applies for fully dehydrated, intermediate and high moisture traditional foods with inherent empiric hurdles, and also for novel products, especially high water activity foods, for which the hurdles are intelligently selected and intentionally applied. Traditional fully dehydrated and intermediate moisture foods (IMF) can be regarded as some of the oldest foods preserved by man. However, in the quest for quality, the importance of considering the combined action of decreased aw with other preservation factors as a way to develop new improved foodstuffs moved along almost simultaneously with modern food processing to the point that, currently, consumers are searching for fresh-like characteristics in many processed products. The food industry has responded to these demands with the so-called minimally processed foods, which have become a widespread industry that is receiving a lot of attention lately.

Therefore, the control of aw for food design is being used in many ways according to needs:

• At various stages of the food distribution chain, it is used during storage, processing, and/or packaging as a 'back-up' hurdle in existing minimally processed products with short shelf-life to diminish microbial pathogenic risk and/or increase their shelf-life (i.e., a slight reduction in aw in addition to refrigeration).

• Traditionally, it is used for obtaining long shelf-life products (fully dehydrated and intermediate moisture ones). Actual trends in these applications are to obtain very high sensory quality products by using the advances in the knowledge of water sorption phenomena, aw prediction, deleterious physico-chemical reactions and polymer science, as well as more controlled and/or sophisticated drying techniques.

• As one of the preservative factors (together with other emerging and/or traditional preservative factors), it is used to obtain high moisture novel foods by hurdle techniques.

In industrialized countries, with ready availability of energy and infrastructure and wide use of refrigeration, the control of aw has been mainly applied to develop a great variety of mild thermally processed, chill- and frozen-distributed foods. Topical applications include fermented meats (fermented sausages, raw hams) and shelf-stable mild heated meats (ready-to-eat fresh-like meats); sous vide and cook-chill dishes, health foods (low-fat and/or low-salt and functional foods); and foods processed by emerging techniques (e.g., hydrostatic high pressure) (Leistner and Gould, 2002).

On the contrary, in many developing countries, refrigeration is expensive and not always available. Thus, the emphasis of the aw lowering approach has been on the development of ambient-stable foods, which have minimal energy, machinery and infrastructure requirements for processing, storage and distribution (Leistner and Gould, 2002). Common applications entail foods with reduced aw (achieved by partial drying or addition of salt or sugar), usually combined with acidification (i.e., a reduction of pH) and addition of preservatives; fermented foods, and fully dehydrated foods.

Most of the traditional foods that remain stable, safe and tasty during long-term storage without refrigeration in the developing countries of Africa, Asia and Latin America are intermediate moisture foods, in which lowering of aw is one of the main preservative factors or hurdles (Leistner and Gould, 2002). Many of the manufacturing processes for IMF were empirically developed, but now the hurdles and their specific roles are better understood and can be rationally selected to design or to optimize the preservation system. There are actually two categories of foods with reduced aw whose stability is based on a combination of factors: intermediate moisture (IM) foods and high moisture (HM) foods.

IM foods range generally from 0.60 to 0.90 aw and 10-50% water by weight (Jayaraman, 1995). Additional hurdles provide the margin of safety against spoilage by microorganisms resistant to aw (mainly moulds and yeasts, which can grow at aw as low as 0.60), and also against some bacterial species that are likely to grow when the aw value of the IM food is near the upper limit of water activities (i.e., aw 0.90). With these targets, the lowering in aw is often combined with chemical preservatives (i.e., nitrites, sorbates, sulphites, benzoates, antimicrobials of natural origin, smoke components) and a reduction of pH (that usually inhibits or decreases bacterial growth, accentuates the action of preservatives and increases the minimum aw values for bacterial growth), and sometimes with competitive microorganisms. Other IM products receive a thermal treatment during manufacturing process that inactivates heat-sensitive microorganisms, while the subsequent hot filling in sealed containers further improves microbial stability (Leistner and Gould, 2002).

Most of IM foods are designed to be storable for several months at ambient temperature even in tropical climates and to be eaten 'as is' without rehydration. They are moist enough to be ready to eat without giving rise to a sensation of dryness but dried enough to be ambient-stable (Karel, 1973, 1976; Jayaraman, 1995). Many IM products, due to the addition of very high amounts of solutes (such as sugar or salt) to reduce aw to the desired level, are too sweet or too salty, becoming undesirable from the nutritional and sensory point of view. Therefore, this category of products has been subjected in the last decade to continuous revision and discussion.

On the other hand, high-moisture foods have an aw value well above 0.90. Thus, in this category, the reduction of aw is a hurdle of less relative significance because most of the microorganisms are able to proliferate (Leistner and Gould, 2002). Stability at ambient temperature is reached by applying intentional and carefully designed hurdle technology. HM fresh-like fruits and cooked meat products, preserved by the interaction of aw - mild heat treatment - pH - preservatives and storable without refrigeration, represent a rational application of the combined approach (Alzamora et al., 1995, 2000, Leistner and Gould, 2002).

In 1994, within the Science and Technology for Development (CYTED) Program, Project 'Development of intermediate moisture foods (IMF) from Iberoamerica', a survey was conducted in eleven countries, collecting information on 260 traditional IM and HM foods. Table 8.3 shows the main factors used in Spain and Latin America for the preservation of the traditional

Table 8.3 Main factors used in Iberoamerican countries for the preservation of traditional food by the combined methods technology

Preservation factors

Table 8.3 Main factors used in Iberoamerican countries for the preservation of traditional food by the combined methods technology

Preservation factors

Product category

aw

pH

F

t

Smoke

Preser.

C.F.

Fruits & 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

-

F = mild heat treatment; t = mild refrigeration; Preser. = preservatives; C.F. = competitive flora.

F = mild heat treatment; t = mild refrigeration; Preser. = preservatives; C.F. = competitive flora.

foods evaluated (Welti et al., 1994; Tapia et al., 1994). Many of these products, which are also common in different parts of the world, are safe and storable without refrigeration and require inexpensive packaging. Selected representative products of each category, the process parameters involved and their contribution as microbial stability factors (hurdles) are shown in Table 8.4. Most shelf-stable foods do not rely solely on aw for microbial control but on other preservation factors. The binary combination of aw and pH acts as a relevant hurdle in many of these products preventing proliferation of pathogenic microorganisms, while the rest (antimicrobials, thermal treatment, etc.) play a secondary role, mainly against spoilage flora (Tapia et al., 1994).

Different approaches have been explored for obtaining shelf-stability and freshness in fruit products. Commercial, minimally processed fruits are fresh (with high moisture), and are prepared for convenient consumption and distribution to the consumer in a fresh state. Minimum processing includes minimum preparation procedures like washing, peeling and/or cutting, packing, etc., after which the fruit product is usually placed in refrigerated storage where its stability varies depending on the type of product, processing, and storage conditions. However, product stability without refrigeration is an important issue not only in developing countries but in industrialized countries as well. The principle used by Leistner for shelf-stable high moisture meats (aw > 0.90), where only mild heat treatment is used and the product still exhibits a long shelf life without refrigeration, can be applied to other foodstuffs. Fruits would be a good choice. Leistner states that for industrialized countries, production of shelf-stable products (SSP) is more attractive than IM foods because the required aw for SSP is not as low and less humectants and/or less drying of the product is necessary (Leistner, 2000).

If fresh-like fruit is the goal, dehydration should not be used in processing. Reduction of aw by addition of humectants should be employed at a minimum level to maintain the product in a high moisture state. To compensate for the

Table 8.4 Preserving factors in selected food products of reduced aw (adapted from Tapia et al., 1994)

PRESERVING FACTORS

LEVEL OF HURDLE RELEVANCE

Product

aw

pH

Antimicrobial

Thermal

Refrigeration

Competitive

Most relevant

Seconc

treatment

requirement

flora

Meat products

Sausage

0.92

5.6

Sodium nitrite

No

No

Yes

aw

A, CF

Sausage

0.74

4.5

Sodium nitrite

No

No

Yes

aw, pH

A, CF

Spanish ham

0.85

6.2

Sodium nitrite

No

No

No

aw

A

Beef foie grass

0.87

6.3

Sodium nitrite

Yes

Yes

No

aw, R

T

Vegetable products

Ketchup

0.94

3.8

Potassium sorbate

Yes

No

No

pH, aw

A, T

Garlic cream

0.84

4.0

Essential oils of

No

No

No

aw, pH

A

natural occurrence

Garlic sauce

0.96

3.7

Essential oils of

Yes

No

No

pH, aw

T

natural occurrence

Chili cream

0.84

4.2

Essential oils of

Yes

No

No

aw, pH

A, T

natural occurrence

Fruit products

Candied papaya

0.70

4.6

No

Yes

No

No

aw

pH, T

Candied pineapple

0.80-

4.5-

No

Yes

No

No

aw

pH, T

0.87

5.6

Dehydrated plum

0.77

3.9

No

No

No

No

aw

pH

Dehydrated banana

0.62

5.1

No

No

No

No

aw

No

Peach jam

0.83

3.1

No

Yes

No

No

aw, pH

T

Mango jam

0.81

5.0

Sodium benzoate

Yes

No

No

aw, pH

T, A

Guava paste

0.780.88

3.5

Sulphite

Yes

No

No

aw, pH

A, T

Sweet potato paste

0.84

3.4

Sodium benzoate

Yes

No

No

aw, pH

A, T

Table 8.4 continued

PRESERVING FACTORS

LEVEL OF HURDLE RELEVANCE

Table 8.4 continued

PRESERVING FACTORS

LEVEL OF HURDLE RELEVANCE

Product

aw

PH

Antimicrobial

Thermal treatment

Refrigeration requirement

Competitive flora

Most relevant

Secondary

Fishery products

Brined anchovies

0.75

6.2

No

No

No

No

aw

Dry-salted anchovies

0.71-

5.6-

No

No

Yes

No

aw

R

0.74

5.8

Cod-type dry fish

0.74-

7.5-

No

No

No

No

aw

0.75

8.6

Anchovies in oil

0.76-

6.1-

No

No

Yes

No

aw

R

0.80

6.2

Smoked trout

0.96

5.4

Smoke

No

Yes

No

aw

R, S

Smoked salmon

0.96-

5.7-

Smoke

No

Yes

No

Refrigeration

S

0.98

6.2

Dairy products

Sweet condensed milk

0.84

6.6

No

Yes

No

No

aw> T

Melted cheese

0.97-

5.7-

No

Yes

Yes

No

T, refrigeration

0.98

6.0

Milk jam

0.81-

5.6-

No

Yes

No

No

aw

Maillard pr

0.85

6.0

Goat cheese

0.91

5.6

No

Yes

Yes

No

aw

T, PH, R

Reggianito cheese

0.86

5.5

No

Yes

No

No

aw

T, pH

Miscellaneous products

Mayonnaise

0.93-

3.8-

Potassium sorbate

Yes

No

No

pH, aw

A

0.94

3.9

Honey

0.62-

3.1-

No

No

No

No

aw, pH

0.69

3.3

Soy sauce

0.79

4.7

No

Yes

No

No

aw, pH

T

Soy sauce

0.79

4.8

Sodium benzoate

Yes

No

No

aw, pH

A, T

A: Antimicobial; CF: Competitive flora; R: Refrigeration; S: Smoke; T: Thermal treatment.

A: Antimicobial; CF: Competitive flora; R: Refrigeration; S: Smoke; T: Thermal treatment.

high moisture left in the product (in terms of stability), a controlled blanching can be applied without affecting the sensory and nutritional properties; pH reductions can be made that will not impair flavour; and preservatives can be added to alleviate the risk of potential spoilage microflora. In conjunction with the above-mentioned factors, a slight thermal treatment, pH reduction, slight aw reduction and the addition of antimicrobials (sorbic or benzoic acid, sulfite), all placed in context with the hurdle technology principles applied to fruits, make up an interesting alternative to IM preservation of fruits, as well as to commercial minimally processed refrigerated fruits. Considerable research effort has been made within the CYTED Program and the Multinational Project on Biotechnology and Food of the Organization of American States (OAS) in the area of combined methods, geared to the development of shelf-stable high-moisture fruit products. Over the last two decades, use of this approach led to important developments of innovative technologies for obtaining shelf-stable 'high-moisture fruit products' storable for 3-8 months without refrigeration. These new technologies are based on a combination of inhibiting factors to combat the deleterious effects of microorganisms in fruits, including additional factors to diminish major quality loss. Slight reduction of water activity (aw 0.94-0.98), control of pH (pH 3.0-4.1), mild heat treatment, addition of preservatives (concentrations < 1,500 ppm), and antibrowning additives were the factors selected to formulate the preservation procedure (Alzamora et al., 1989, 1993, 1995; Guerrero et al, 1994; Cerrutti et al, 1997).

Novel (in their application) and refined impregnation techniques exist for developing minimal processes. Pulsed vacuum osmotic dehydration, a new method of osmotic dehydration that takes advantage of the porous microstructure of vegetable tissues, is a technique that uses vacuum impregnation (VI) to reduce process time and improve additives incorporation. During VI of porous materials, important modifications in structure and composition occur as a consequence of external pressure changes. VI shows faster water loss kinetics in short-time treatments as compared with time-consuming atmospheric 'pseudo-diffusional' processes, due to the occurrence of a specific mass transfer phenomenon, the hydrodynamic mechanism (HDM), and the result produced in the solid-liquid interface area. Many fruits and vegetables have a great number of pores and offer the possibility of being impregnated by a predetermined solution of solute and additives. Thus, product composition as well as its physical and chemical properties may be changed to improve its stability. An important advantage of using low pressures (approx. 50 mbar) in the minimal preservation of fruit is that equilibration times are shorter than at atmospheric pressure (e.g., 15 minutes under vacuum versus a few hours in forced convection at atmospheric conditions, or a few days in media without agitation for reducing aw to 0.97) (Alzamora et al., 2000). This process could be appropriate in the development of new minimally processed fruit products or in the development of improved pretreatments for such traditional preservation methods as canning, salting, freezing or drying, and also in high-quality jam processes (Alzamora et al., 2000).

At present, especially physical, non-thermal processes (high hydrostatic pressure, mano-thermo-sonication, oscillating magnetic fields, pulsed electric fields, light pulses, etc.), receive considerable attention, since in combination with other conventional hurdles they are of potential use for the microbial stabilization of fresh-like food products with little degradation of nutritional and sensory properties. With these novel processes often not a sterile product but only a reduction of the microbial load is intended, and growth of the residual microorganisms is inhibited by additional, conventional hurdles. Interesting results have been reported by the research group of the Universidad de las Americas (Mexico) for obtaining minimally processed avocado sauce, avocado puree and banana puree. These fruit products were preserved by the interaction of blanching, high-pressure, reduction of pH and aw and preservatives, and the combination of heat treatment and high pressure significantly decreased browning reactions (Alzamora et al., 2000). Another group of hurdles which is at present of special interest in industrialized as well as in developing countries are 'natural preservatives' (spices and their extracts, hop extracts, lysozyme, chitosan, pectine hydrolysate, etc.) (Leistner, 2000). As an example, high-moisture strawberry can be preserved for at least three months by combining mild heat treatment, 3,000 ppm vanillin (instead of synthetic antimicrobials), 500 ppm ascorbic acid, and adjustment of aw to 0.95 and pH to 3.0 (Cerrutti et al., 1997).

Lastly must be mentioned the excellent recopilation of traditional and artisanal combined methods employed around the world (many of them involving the control of aw) by the world's two leading authorities on hurdle technology: Professor Lothar Leistner and Dr Grahame Gould (Leistner and Gould, 2002). This overview covers hurdle techniques applied in developed countries and also in Latin America, India, China and Africa. Basic principles underlying preservation procedures are critically discussed for many popular products. Among them, it is interesting to cite the following:

• Paneer, a cottage cheese-type Indian product (hurdles: aw 0.97; pH 5, Fo value 0.8), stable for several weeks without refrigeration.

• Dudh churpi, an Indian dairy product (preparation: heating, acid coagulation, addition of sugar and potassium sorbate, smoking, drying).

• Meat (preparation: marination in salt, glycerol, nitrite, acidulants and ascorbate, cooking and packaging; aw 0.70 or 0.85, pH 4.6) storable at room temperature for one month or at 50C for more than four months.

• Rabbit meats, quite popular in China, marinated and cooked; fried; brined and cooked; or smoked (hurdles: aw 0.92-0.98, refrigeration).

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