WO2010010059A1

WO2010010059A1 - Method for the production of fresh-cut fruits and vegetables based on the combined use of ultraviolet light and acidification

Info

Publication number: WO2010010059A1
Application number: PCT/EP2009/059286
Authority: WO
Inventors: Lara Manzocco
Original assignee: Universita degli Studi di Udine
Current assignee: Universita degli Studi di Udine
Priority date: 2008-07-21
Filing date: 2009-07-20
Publication date: 2010-01-28
Anticipated expiration: 2011-01-21
Also published as: ITPD20080214A1

Abstract

The present patent application describes a method for the production of ready-to-eat or cook fresh-cut fruits and vegetables. With the method described, fresh-cut fruits and vegetables can be prepared maintaining a low microbial load and avoiding deterioration of the sensory properties of the fruits and vegetables and/or browning thereof. For said purpose the method entails treatment of the cut fruits and vegetables via the combined use of acidifying solutions and ultraviolet radiation.

Description

Method for the production of fresh-cut fruits and vegetables based on the combined use of ultraviolet light and acidification Field of the invention The invention relates to a method for the production of fresh-cut fruits and vegetables ready for sale without requiring further treatment before manipulation for cooking and/or consumption. The method entails treatment of the freshly-cut fruits and vegetables with at least acid solutions and ultraviolet radiations. State of the art Growing consumer awareness of the importance of healthy eating habits combined with the evolution in lifestyles has significantly increased the demand for ready-to-eat fresh fruits and vegetables. The fresh-cut fruit and vegetable market is designed to meet these needs, delivering refrigerated fruit and vegetables in sealed containers after being cut and cleaned, so that they can be eaten raw or utilized for cooking both at home and in catering as well as in industrial processes as semi-manufactured products.

For the companies in the field, the production of fresh-cut fruits and vegetables (washed, peeled, grated and cut) is a challenging market as these products deteriorate much faster than when they are whole (Nguyen-The, C; Carlin, F. The microbiology of minimally processed fresh fruits and vegetables. Crit. Rev. Food Sci. Nutr., 34, 371 -401 ,1994; Watada, A.E.; Qi, L. Quality of fresh-cut produce. Postharvest Biol. Technol., 15, 201 -205, 1999). In fact, the cellular lesions caused by processing inevitably lead to the development of several and complex degradation phenomena, fundamentally due to the activity of endogenous enzymes (e.g. polyphenoloxidase, polygalacturonase and esterase) with consequent modification in colour and texture, in addition to the formation of off- odours and off-flavours. Among the main enzymes responsible for the quality decay of fresh-cut fruits and vegetables are:

- polyphenoloxidase which induces the appearance of off-colours and/or browning on the surface of the product;

- pectolytic enzymes which cause texture modification of fruits and vegetables. The modifications induced by these enzymes significantly affect the sensory quality of the product which, in the course of a few days (and in some cases only a few hours), assumes characteristics which make it unacceptable to the consumer and, therefore, no longer suitable for sale. Beside enzymatic alterations, microbiological alterations are also possible due to contamination of the cut tissues as well as of the leached cellular liquids. These products must be sold refrigerated and have a very short shelf life. If good manufacturing procedures are performed, the risk of microbiological contamination can be controlled, although it should not be underestimated, while the risk of enzymatic activity represents the main cause of quality deterioration as it determines rapid loss of the essential characteristics of freshness of fresh-cut fruits and vegetables.

It follows that actual processing of fresh-cut fruits and vegetables, based on the application of good manufacturing practices and refrigeration of the product packed in conventional or modified atmosphere, are not sufficient to guarantee enzyme inactivation which is essential for maintaining product freshness and the high quality standards required by the consumers.

Traditionally, enzymatic inactivation of fresh-cut fruits and vegetables is performed by blanching. Although very effective, this treatment does not fit with the production of fresh-cut fruits and vegetables as it determines almost complete loss of the sensory characteristics typical of the fresh product (colour, texture, juiciness, firmness, etc.). To avoid these undesired effects, some alternative solutions have been proposed based on the use of non-thermal physical treatment such as irradiation with gamma rays, X rays or accelerated electrons, high pressures (Boynton, B.B.; Welt, B.A.; Sims, C.A.; Balaba, M.O.; Brecht, J. K.; Marshall, M. R. Effects of low-dose electron beam irradiation on respiration, microbiology, texture, color, and sensory characteristics of fresh-cut cantaloupe stored in modified- atmosphere packages, J. Food Sci., 71 , 149-155, 2006; Torres, J. A.; Velasquez, G. Commercial opportunities and research challenges in the high pressure processing of foods. J. Food Eng., 67, 95-1 12, 2005). It should be noted, however, that these treatments have substantial drawbacks since, although effective, they are not fully accepted by the consumers or too expensive. The solution most commonly adopted by producers to obtain enzymatic stabilisation of fresh-cut fruits and vegetables is therefore based on the addition of substances that act as enzymatic inhibitors, acidulants or molecules that consume the precursors of the enzymatic reactions (Lyengar, R.; McEvily, A.J. Anti- browning agents: alternatives to the use of sulfites in foods. Trends Food Sci. Technol., 3, 60-64, 1992; Sapers, G. M.; Miller, R.L. Browing inhibition in fresh-cut pears. J. Food ScL, 62, 342-346, 1998; Buta, J. G.; Molne, H. E.; Spaulding, D.W., Wang, CY. Extending the storage life of fresh-cut apples by using natural products and their derivatives. J. Agric. Food Chem., 47, 1 -6, 1999; Lee, J. Y.; Park, H.J.; Lee, CY. ; Choi, W. Y. Extending shelf life of minimally processed apples with edible coatings and antibrowning agents., Lebens-Wiss.Und-Technol., 36, 323-329, 2003; Belloso O.M. et al., EP 0937406,1999; Paaimaans G.J., EP 1010368, 2000; Campana S. et al, EP 1254600, 2002; Lidster P. D. et al., WO 2004/086872, 2004). There are many applications of chemical substances aiming to browning prevention by inhibition of polyphenoloxidase. Most widely used molecules include citric and ascorbic acid and their salts. Usually these substances are used to prepare aqueous solutions in which the fresh-cut fruits and vegetables are dipped for a variable time. This ensures uniform treatment of the surface of the product. Generally speaking, to obtain an effective anti-browning action, these substances must be used in concentrations which often modify the sensory characteristics of the product. For example, the widespread use of acidifiers can significantly modify perception of the product flavour by the consumers. Nothing is known, on the other hand, about the possibility of using molecules able to inactivate pectolytic enzymes responsible for modifications in vegetable/fruit texture.

Since ultraviolet radiations are well-known both as effective antimicrobial agents and as physical agents able to modulate the biochemical processes underlying the vegetable ripening process, they are widely used in the sector of post-harvest preservation of whole fresh fruits and vegetables. To this regard, they are used to reduce the microbial surface load and control the ripening of fruit, vegetables, dried fruit, mushrooms and seeds (Lu, J. Y.; Stevens, C; Yakubu, P.; Loretan, P. A.; Eakin, D. Gamma electron beam and ultraviolet radiation on control of storage rots on quality of Walla Walla onions, J. Food Proc. Pres., 12, 53-62,

1988; Stevens, C; Khan V.A.; Tang, A.Y.; Lu, J.Y. The effect of ultraviolet radiation on mold rots and nutrient of stored sweet potatoes. J. Food Protect. 53, 223-226, 1990; Maharaj, R.; Ami, J.; Nadeau, P. Effect of photochemical treatment in the preservation of fresh tomato {Lycopersicon esculentum cv. Capello) by delaying senescence, Postharv. Biol. Technol., 15, 13-23, 1999; Gonzalez-Aguilar, G.; Wang, CY. ; Buta, G.J. UV-C irradiation reduces breakdown and chilling injury of peaches during cold storage. J. Sci. Food Agric, 84, 415-422, 2004; Gonzalez- Aguilar, G.; Zavaleta-Gatica, R.; Tiznado-Hernandez, M. E. Improving postharvest quality of mango "Haden" by UV-C treatment. Postharv. Biol. Technol. 45, 108- 116, 2007).

Recently the use of ultraviolet light has also been proposed for maintaining the quality of fresh-cut fruits and vegetables. Hankinson and Herickhoff {US 7258882, 2007) proposed a solution for the production of fresh mushrooms ready for distribution, based on successive washing operations in water with the addition of chemical agents which act as antioxidants and preserve the texture (sodium erythorbate, calcium chloride) followed by treatments with ozone and ultraviolet light in order to reduce the microbial load of the mushrooms. In this case, the ultraviolet light is applied for few seconds (between 5 and 60 seconds) on mushrooms t previously washed and eventually cut and has the purpose of reducing the high microbial load of this type of vegetable which is well known to be very high. It has also been suggested that ultraviolet light can modify the metabolism of cut fruits and vegetables, inducing activation of the cell defence mechanisms. Lamikandra et al. (Lamikandra, O.; Kuenema, D.; Ukuki, D.; Bett- Garber, K.L. Effect of processing under ultraviolet light on the shelf life of fresh-cut cantaloupe melon. J. Food Sci., 70, 534-539, 2005) showed that exposure of slices of melon to ultraviolet light can increase the activity of some enzymes such as peroxidase, esterase and lipase. However, according to these authors, if these fruits and vegetables are cut under ultraviolet light, the enzymatic activity of the enzyme esterase could be reduced, resulting in melon slices with improved texture. It is therefore clear that there is a substantial lack of processes aimed at enzymatic stabilisation of fresh-cut fruits and vegetables since currently food companies are necessarily using various substances with antioxidant action which provide only a partial function (i.e. they inactivate the enzymes responsible for browning but not the pectolytic enzymes) and often considerably modify the sensory characteristics of the product.

The object of the present invention is therefore to identify process parameters able to preserve the sensory qualities of fresh-cut fruits and vegetables, mainly avoiding enzymatic deterioration and at the same time guaranteeing a reduction in the microbial load. Summary

The proposed invention provides enzymatic stabilisation of fresh-cut fruits and vegetables via the combination of two factors: i) exposure to ultraviolet light at any time during the processing operations of fresh-cut fruits and vegetables preparation; ii) treatment of the cut fruit/vegetable with an acidulated solution. Given the known antimicrobial properties of ultraviolet light, the process results not only in enzymatic stabilisation but also in reduction of the microbial load of the fresh-cut fruits and vegetables. The invention therefore provides a method for the production of fresh-cut fruits and vegetables characterised by treatment of said cut fruits and vegetables with an acid aqueous solution in combination with exposure to ultraviolet light. In comparison to the known techniques, the proposed technical solution allows inhibition of enzymatic activity and reduction of the microbial load of minimally processed fruits and vegetables, reducing the use of chemical substances. The product obtained has sensory characteristics comparable with those of the fresh product. The process is economically sustainable as it does not exploit changes of physical state, it is performed at usual vegetable/fruit processing temperatures and requires only minimal investment. Detailed disclosure of the invention The invention consists in the combined use of ultraviolet light and treatment with an at least acidulated aqueous solution in order to obtain the enzymatic stabilisation of fresh-cut fruits and vegetables. While exposure times to the ultraviolet light necessary for obtaining an antimicrobial effect are in the order of a few seconds, a longer treatment is necessary to obtain enzymatic inactivation. During this time, the vegetable tissues can undergo enzymatic alteration involving loss of the colour and texture characteristics typical of fresh-cut fruits and vegetables. In order to limit the development of the deteriorative enzymatic reactions during exposure to the ultraviolet light, the latter must be applied in combination with a treatment for temporary enzymatic inhibition, which is obtained by dipping in an acidulated aqueous solution. Although these are the minimum conditions for achieving the purpose of the invention, the acidulated solution can be enriched with other substances able to further limit browning of the vegetable and/or improve its texture.

The method for the production of minimally processed fresh-cut fruits and vegetables, according to the invention, comprises some process steps which can be performed under exposure to ultraviolet light. In particular, the ultraviolet irradiation can be applied at any time during such a process of preparation, and at least during one of the process steps hereinafter mentioned or at the end of the same. Said process steps are substantially: a) cutting the fruits and vegetables; b) treating with an acid aqueous solution; c) optionally, draining of the fresh-cut fruits and vegetables; d) packaging of the fresh-cut fruits and vegetables.

Preferably all these operations are performed under exposure to ultraviolet light, and the same can be optionally preceded, according to the type of vegetable/fruit to be treated, by washing and/or grading and/or peeling operations. In case that the ultraviolet irradiation is performed on the packaged fresh-cut fruits and vegetables, the packaging must be permeable to the ultraviolet radiation. For the purpose of the invention, the ultraviolet light must be characterised by an emission spectrum whose power is for at least 30% at a wavelength of 253.7 nm. The light source can be positioned at a variable distance from the product surface, but must ensure that the irradiance on the product surface is higher than 1 W/m² and preferably comprised from 8 and 20 W/m² for a time of at least 30 seconds and preferably between 1 and 10 min. The area under ultraviolet light, must be maintained at the usual fresh-cut vegetable/fruit processing temperature, i.e. between 4 and 20 °C.

In detail, after the preliminary treatment of washing and/or grading, the fruits or vegetables enter into the processing area where the ultraviolet radiation emitting source is present. Fruits or vegetables are submitted to cutting operations and subsequently treated with an aqueous solution, which is at least acidulated, and if necessary added with other substances able to stabilise the colour and texture characteristics of the product, by means of dipping into or spray with said aqueous solution. The time of contact between the fresh-cut fruits and vegetables by dipping or spray and the at least acid solution must be at least 30 seconds and preferably between 1 and 2 minutes.

The aqueous solution must be maintained at a pH value at least one unit below that of the vegetable/fruit tissues to be treated. For example, in the case of slices of banana having a pH of 4.8, the pH of the aqueous solution must be below 3.8 and preferably between 2 and 3. For pieces of apple, pear, peach or pineapple having pH between 3.5 and 4.0, the pH of the aqueous solution must be below 2.5-3.0. In general, therefore, the pH of the at least acid aqueous solution is preferably no higher than 3 and more preferably between 2 and 3. The acidification can be obtained by the addition of appropriate known acidifying agents permitted for use in food and preferably selected from the group consisting of citric acid or its salts, D-gluconolactone, hydrochloric acid and oxalic acid. The substances suitable for said purpose are preferably citric acid and its salts. Said substances can also be enriched with other known substances permitted for use in food which are specifically directed to maintain colour and texture (preferably selected from calcium or magnesium salts, sulphur compounds such as sulphites, vegetable extracts). Preferably the substances used for maintaining texture are calcium salts, which are used in concentrations suitable to achieve a Ca²⁺ ion concentration of no less than 0.2% (w/w) and preferably between 0.3 and 2% (w/w).

In case the acidulate solution is added with said substances the solution will be indicate herein as "stabilising solution", having the purpose to inhibit at least the major enzymatic processes involved in the degradation of the fresh-cut fruits and vegetables.

If the contact between the fruit/vegetable pieces and the at least acidulate solution is realised by dipping, said step is followed by the step of draining. It should be noted that the fresh-cut fruits and vegetables leave preferably the processing area exposed to ultraviolet light only after packaging.

The method subject of the present invention provides fresh-cut fruit and vegetables having sensory properties similar to those of the fresh products but characterised by a greater enzymatic stability and hygienic quality which translate into a longer shelf life at refrigeration temperature.

The examples given below refer to the production of packaged slices of banana and apple for use as fresh fruit ready to eat; they are provided for illustrative purposes and do not in any way limit the invention. The method can in fact be usefully applied also to other fruits and vegetables, both leaf and non-leaf including tubers, roots, inflorescences and fruit. Example 1

Bananas of the Cavendish variety were washed with water at 16°C and submitted to four different processes. The pH of the banana pulp was 4.8. Process A (Control) The bananas were peeled and sliced into 0.5 cm thick slices. The temperature of the processing environment was 16°C. The banana slices were packed in plastic containers and kept at 4°C for increasing periods of time up to 4 days. Process B (Dipping in the stabilising solution) The bananas were peeled and sliced into 0.5 cm thick slices. The temperature of the processing environment was 16°C. The banana slices were dipped in an aqueous solution kept at pH 2.0 by addition of citric acid and containing 2% (w/w) of calcium chloride. After 1 minute of dipping, the banana slices were removed from the solution, drained, packed in plastic containers and kept at 4°C for increasing periods of time up to 4 days. Process C (Exposure to ultraviolet light)

The bananas were peeled and sliced into 0.5 cm thick slices. The temperature of the processing environment was 16°C. The banana slices were exposed to ultraviolet light for 3 minutes (characteristic wavelength 257.3 nm). The irradiance on the surface of the banana slices was 10 W/m². The banana slices were then packed in plastic containers and kept at 4°C for increasing periods of time up to 4 days. Process D (Exposure to ultraviolet light and dipping in the stabilising solution) The bananas were peeled and sliced into 0.5 cm thick slices. These operations were performed under ultraviolet light (characteristic wavelength 257.3 nm). The irradiance assessed on the processing surface of the banana slices was 8 W/m². The temperature of the processing environment was 16°C. The banana slices were dipped in an aqueous solution kept at pH 2.0 by addition of citric acid and containing 2% (w/w) of calcium chloride. During the dipping treatment, the solution was kept under ultraviolet light. After 1 minute of dipping, the banana slices were removed from the solution, drained and packed in plastic containers. The packaged banana slices were removed from the area under ultraviolet light and kept at 4°C for increasing periods of time up to 4 days. Overall the banana slices were exposed to ultraviolet light for 3 minutes.

During refrigerated preservation of the banana slices obtained by means of processes A, B, C and D, colour and texture were analytically determined as reported hereinafter. Tables 1 and 2 show, respectively, the lightness and the red-green point values of the banana slices during preservation after the various treatments. It can be observed that neither the dipping treatment in the stabilising solution (Process B) alone or exposure to the light (Process C) alone were able to satisfactorily stabilise browning of the banana slices. On the contrary, the chromatic changes in the product were minimised by combination of the two treatments (Process D). Analogous results were obtained also as regards the texture of the banana slices (Table 3). Table 1 : Lightness (L^*)

Table 2: Chromatic parameter a^* (Red-green point)

Table 3: Texture (N)

In order to evaluate the antimicrobial effect of the ultraviolet light towards the eventual microbial flora present on the surface of the banana slices, additional tests were performed. It should be noted that, if appropriately processed, banana slices have a microbial load on average below 100 FCU/g. To simulate undesired contamination, the microbial load was brought to 2.5-10⁶ ± 0.5-10⁶ FCU/g by dipping in a suspension containing lactic bacteria. The inoculated banana slices were then exposed to the ultraviolet light for 3 minutes at 16°C. The microbial count, performed by classical analysis, of the banana slices exposed to the ultraviolet light was 7-10⁴ ±1 -10⁴ FCU/g. These results confirm that, in addition to the enzymatic inhibition, the proposed process also reduces the microbial load present on the vegetable surface.

Lastly, tests were performed to ascertain whether process D provides banana slices that are not only colour stable but also sensory pleasant. For said purpose, the banana slices obtained via process D underwent sensory analysis with judges expert in sensory methodologies. The judges were asked to attribute a score for colour, sweetness, acidity, astringency, banana flavour and texture in relation to a control sample represented by non-processed banana slices. The sample that underwent process D was perceived as comparable with the control as regards colour, sweetness, astringency, banana flavour and texture. In this regard it should be underlined that the treatment with ultraviolet light did not determine sensory perceivable changes in flavour. On the contrary, the banana slices obtained via process D were perceived as significantly (p<0.5) more acid. However, the modest increase in acidity was not perceived as a negative characteristic. In fact, no significant differences were encountered in the preference expressed by a group of 50 consumers for the two samples (p>0.1 ). Example 2

Apples of the Golden delicious variety were washed with water at 10°C and underwent two different processes. The pH of the apple pulp was 4.0. Process A (Control)

The apples were peeled and sliced into 1 cm thick slices. The temperature of the processing environment was 10 °C. The apple slices were packed in plastic containers and preserved at 4 °C for increasing periods of time up to 6 days. Process B (Exposure to ultraviolet light and dipping in the stabilising solution) The apples were peeled and sliced into 1 cm thick slices. These operations were performed under ultraviolet light (characteristic wavelength 257.3 nm). The irradiance assessed on the processing surface of the apple slices was 8 W/m². The temperature of the processing environment was 10°C. The apple slices were dipped an acidulate aqueous solution kept at pH 3.0 by addition of citric acid. During the dipping treatment, the solution was kept under ultraviolet light. After 1 minute of dipping, the apple slices were removed from the solution, drained and packed in plastic containers. The packaged apple slices were removed from the area under ultraviolet light and kept at 4°C for increasing periods of time up to 6 days. Overall the apple slices were exposed to ultraviolet light for 5 minutes. During refrigerated preservation of the apple slices obtained via processes A and B, colour, texture and total viable microbial count were analytically determined as reported hereinafter. Tables 4 and 5 show, respectively, the values of the chromatic parameters a^* (red-green point) and b^* (yellow-blue point) of the apple slices during refrigerated preservation after the various treatments. It can be observed that process B produced apple slices with very limited colour changes during the subsequent refrigerated preservation compared to those observed in the control samples. In fact, the control samples showed very evident browning after only one day of preservation. On the contrary the apple slices obtained with process B showed negative red-green point values and a practically constant yellow-blue point, during the entire period of preservation. Table 6 shows the texture values of the apple slices that underwent processes A and B during the refrigerated preservation. It can be seen that the control sample showed a progressive loss of texture during preservation, whereas the one treated with process B was characterised by high texture values comparable with those of the freshly prepared apple slices. Finally, as shown in table 7, the process is also capable to decrease the microbial load present on the surface of the fresh-cut apple slices.

Table 4: Chromatic parameter a^* (Red-green point)

Table 5: Chromatic parameter b^* (Yellow-blue point)

Analytical methods used in the examples 1 and 2

The colour was measured by tristimulus colourimeter (Colorlab Chromameter 2 Reflectance, Minolta, Japan) provided with CR 200 measuring head and illuminant "C" (6.774 K) adopting the standard conditions of the Commission Internationale de I'Eclairage (CIE). Before each measurement the instrument was calibrated to a known colour reference. The data were expressed as luminosity (L^*), red point (a^*) and yellow point (b^*). The texture was determined using the penetrometric technique by means of a probe. An lnstron Universal Testing Machine (Instron International Ltd., High Wycombe, United Kingdom) model 4301 was used for the analysis, equipped with 15 mm probe which penetrated the fruit for a depth of 7 mm at a speed of 100 mm/min. The tests were carried out in triplicate performing 5 measurements for each sample. The data were processed by means of the Automated Materials Testing System Version 5.25 software (Instron International Ltd., High Wycombe, United Kingdom).

The lactic acid bacteria count was performed by classical analysis on deman Rogosa Sharpe medium.

The total viable microbial count was performed by classical analysis on Plate Count Agar medium.

Claims

1. A method for production of fresh-cut fruits and vegetables combining a treatment with an at least acid solution and an ultraviolet light exposure comprising the steps of: a) cutting fruits and vegetables; b) treating said fresh-cut fruits and vegetables with the at least acid solution; c) optionally, draining the fresh-cut vegetables; d) packaging thereof, wherein the ultraviolet light exposure has an irradiance onto the surface of the fresh-cut fruits and vegetables higher than 1 W/m² for a time of at least 30 seconds and is performed at any time of the process or at the end thereof on the packaged product.

2. The method according to claim 1 , wherein the irradiance is comprised from 8 to 20 W/m² for a time comprised from 30 seconds to 10 minutes.

3. The method according to claims 1 and 2, wherein the ultraviolet light has an emission spectrum whose power is for at least 30% at a wavelength of 253.7 nm.

4. The method according to claims 1 , wherein all the steps are performed under ultraviolet light.

5. The method according to claim 1 , wherein the at least acid solution is an acid aqueous solution whose pH is at least one unit lower than that of the fruits and vegetables to be treated.

6. The method according to claim 5, wherein the pH of the at least acid solution is lower than 3.

7. The method according to claim 5, wherein the acidifying agent of the at least acid solution is selected in the group consisting of citric acid, D- glucono-lactone, hydrochloric acid, oxalic acid and salts thereof.

8. The method according to claim 1 , wherein the at least acid solution is added with substances suitable for preserving the colour and/or the texture of said fresh-cut fruits and vegetables.

9. The method according to claim 8, wherein said substances are selected in the group consisting of calcium or magnesium salts, sulphur compounds, vegetable extracts.

10. The method according to claim 9, wherein said substances are calcium salts in a Ca²⁺ concentration higher than 0.2 % (w/w).

11. The method according to claim 9, wherein said Ca²⁺ concentration is comprised from 0.3 to 2 % (w/w).

12. The method according to claim 1 , wherein the treatment with the at least acid solution is performed by dipping in or spray with the same for a time of at least 30 seconds.

13. The method according to claim 12, wherein the dipping or spray are for a time comprised from 1 to 2 minutes.