This invention relates to the packaging of bananas.

Respiring biological materials consume oxygen (O₂) and produce carbon dioxide (CO₂) at rates which depend upon temperature and the stage of their development. Ideally, a respiring material should be stored in a container whose permeability to O₂ and CO₂ is correlated with (i) the atmosphere outside the package, (ii) the rates at which the material consumes O₂ and produces CO₂, and (iii) the temperature, to produce the desired atmosphere within the container. This is the principle behind the technology of controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP), as discussed, for example, in US-A-3,450,542, 3,450,544, 3,798,333, 4,734,324, 4,830,863, 4,842,875, 4,879,078, 4,910,032, 4,923,703, 5,045,331, 5,160,768, 5,254,354 and 6,013,293, WO-A-94/12040, WO-A-96/38495 and WO-A-00/04787 and EP-A-0,351,115 and 0,351,116.

Bananas are respiring biological materials whose storage and ripening present the most serious problems because

• (i) bananas are grown in locations far distant from the locations at which they are consumed;
• (ii) they are damaged by storage at temperatures below about 14.4 °C, with the extent of the damage depending upon the time spent below that temperature and how far the temperature is below 14.4 °C (58 °F);
• (iii) they go through a climacteric when they ripen, thus producing a very large increase in respiration rate and the generation of heat;
• (iv) they generate ethylene as they ripen, and they ripen at a rate which increases with the concentration of ethylene around them -- as a result, a single prematurely ripe banana can trigger premature ripening of many others; and
• (iv) once they have ripened, and have been exposed to air, they rapidly become overripe.

A serious disadvantage of the conventional procedure is the need to harvest the bananas a good while before they are fully grown. It would be desirable to harvest the bananas at a later time, when they are larger. However, the later the bananas are picked, the greater the propensity for their climacteric to be triggered by small concentrations of ethylene, and experience has shown that if the bananas are harvested later than the presently established timetables, this results in prematurely ripe bananas when the bananas are shipped in vented bags, and in so-called "green-ripe" bananas when the bananas are shipped in sealed bags. Green-ripe bananas soften, but remain green, and have an unpleasant flavor.

Another serious disadvantage of the conventional procedure is that, in order to ripen the green bananas by exposing them to ethylene, it is necessary to open each of the shipping bags if, as in most cases, the bags have been sealed during shipping.

Another serious disadvantage of the conventional procedure is that the bananas, once ripened, must be sold within a few days, or scrapped.

Another serious disadvantage of the conventional procedure is that the heat generated by the ripening of the bananas is generated over a relatively short period of time, which heats the bananas to an extent that causes dehydration of the bananas and/or increases the demand on the refrigeration equipment used to keep the bananas cool.

The present invention mitigates or overcomes one or more of these disadvantages by packaging green bananas in a sealed container having designed permeabilities to oxygen (O₂), carbon dioxide (CO₂) and ethylene. Thus, this invention provides a method of ripening green bananas which comprises

• (A) providing a sealed package which comprises
  • (a) a sealed container, and
  • (b) within the sealed container, green bananas and a packaging atmosphere around the green bananas:
  the sealed container
  • (i) including at least one atmosphere control member which provides a pathway for O₂ and CO₂, and which comprises a gas-permeable membrane comprising
    • (a) a microporous film, and
    • (b) a polymeric coating on the microporous film; and
  • (ii) having an O₂ permeability at 13 °C, per kg of bananas in the container (OP13/kg), of at least 700, preferably at least 1000, particularly at least 1500, ml/atm.24 hrs, an R ratio at 13 °C of at least 2, preferably at least 3, and an ethylene permeability at 13 °C, per kg of bananas in the container (EtOP13/kg) which is at least 3 times, preferably at least 4 times, the OP13/kg of the container; and
• (B) placing the sealed package in an atmosphere containing ethylene.

In this specification, including the Examples and the Claims below, reference is made to particular features of the invention. It is to be understood that the disclosure of the invention in this specification includes all appropriate combinations of such particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment of the invention, or a particular claim, that feature can also be used, to the extent appropriate, in the context of other particular aspects and embodiments of the invention, and in the invention generally.

In describing and claiming the invention below, the following abbreviations, definitions, and methods of measurement are used. OTR is O₂ permeability. COTR is CO₂ permeability. EtTR is ethylene transmission rate. OTR, COTR and EtTR values are given in ml/m².atm.24 hrs; in some cases, the equivalent in cc/100 inch².atm.24 hrs is given in parentheses. OTR and COTR values referred to herein can be measured using a permeability cell (supplied by Millipore) in which a mixture of O₂, CO₂ and helium is applied to the sample, using a pressure of 0.7 kg/cm² (10 psi) except where otherwise noted, and the gases passing through the sample were analyzed for O₂ and CO₂ by a gas chromatograph. The cell could be placed in a water bath to control the temperature. The abbreviation P₁₀ is used to denote the ratio of the oxygen permeability at a first temperature T₁°C to the oxygen permeability at a second temperature T₂, where T₂ is (T₁-10)°C, T₁ being 10°C and T₂ being 0°C unless otherwise noted. The abbreviation R or R ratio is used to denote the ratio of CO₂ permeability to O₂ permeability, both permeabilities being measured at 20°C unless otherwise noted. Pore sizes given in this specification are measured by mercury porosimetry or an equivalent procedure. Parts and percentages are by weight, except for percentages of gases, which are by volume; temperatures are in degrees Centigrade, and molecular weights are weight average molecular weights expressed in Daltons. For crystalline polymers, the abbreviation T_o is used to denote the onset of melting, the abbreviation T_p is used to denote the crystalline melting point, and the abbreviation ΔH is used to denote the heat of fusion. T_o, T_p and ΔH are measured by means of a differential scanning calorimeter (DSC) at a rate of 10°C/minute and on the second heating cycle. T_o and T_p are measured in the conventional way well known to those skilled in the art. Thus T_p is the temperature at the peak of the DSC curve, and T_o is the temperature at the intersection of the baseline of the DSC peak and the onset line, the onset line being defined as the tangent to the steepest part of the DSC curve below T_p.

Where reference is made herein to sealing bags containing bananas, it is to be understood that the sealing can be, but generally is not, hermetic sealing. Conventional methods for sealing bags of bananas can conveniently be used in this invention. Such conventional methods include, for example, the use of a cable tie to seal the neck of the bag. A seal made by conventional methods is not a hermetic seal, and has the advantage that it permits equilibration of the pressures inside and outside the bag. If the bag is sealed hermetically, it will generally be desirable to include one or more pinholes in the bag, to achieve such equilibration.

The containers used in this invention include an atmosphere control member as defined above, preferably a control member as described in one or both of WO-A- 96/38495 and WO-A-00/04787. The microporous polymeric film preferably comprises a network of interconnected pores having an average pore size of less than 0.24 micron, with at least 70% of the pores having a pore size of less than 0.24 micron. Preferably the pores in the microporous film constitute 35 to 80% by volume of the microporous film. Preferred microporous films comprise a polymeric matrix comprising (i) an essentially linear ultrahigh molecular weight polyethylene having an intrinsic viscosity of at least 18 deciliters/g, or (ii) an essentially linear ultrahigh molecular weight polypropylene having an intrinsic viscosity of at least 6 deciliters/g, or (iii) a mixture of (i) and (ii). The microporous film may contain 30 to 90% by weight, based on the weight of the film, of a finely divided particulate substantially insoluble filler which is distributed throughout the film.

The polymeric coating on the control member preferably comprises a crystalline polymer having a peak melting temperature Tp of -5 to 40 °C, e.g. 0 to 15°C, or 10 to 20 °C, an onset of melting temperature T_o such that (Tp - T_o) is less than 10 °C, and a heat of fusion of at least 5 J/g. The polymer preferably comprises a side chain crystalline polymer moiety comprising, and optionally consisting of, units derived from (i) at least one n-alkyl acrylate or methacrylate (or equivalent monomer, for example an amide) in which the n-alkyl group contains at least 12, preferably at least 14, for example 16-50, preferably 16-22, carbon atoms, for example in amount 35-100%, preferably 50-100%, often 80-100%, and (ii) one or more comonomers selected from acrylic acid, methacrylic acid, and esters of acrylic or methacrylic acid in which the esterifying group contains less than 10 carbon atoms. The polymer can be a block copolymer in which one of blocks is a crystalline polymer as defined and the other block(s) is crystalline or amorphous. Preferred block copolymers comprise polysiloxane polymeric blocks, and (ii) crystalline polymeric blocks having a T_p of -5 to 40°C. Such a polymer can be prepared by copolymerizing a mixture of reactants which comprises (i) at least one n-alkyl acrylate or methacrylate in which the n-alkyl group contains at least 12 carbon atoms and (ii) a polysiloxane having a copolymerizable group at one end thereof.

Other polymers which can be used to the coat the microporous film include cis-polybutadiene, poly (4-methylpentene), polydimethyl siloxane, and ethylene-propylene rubber.

The gas-permeable membrane preferably has one or more of the following properties

• (i) a P₁₀ ratio, over at least one 10°C range between -5 and 15 °C or between 10 and 20 °C, of at least 2.0 to 2.8;
• (ii) an oxygen permeability at all temperatures between 20° and 25°C of 2,480,000 to 7,000,000 ml/m².atm.24 hr. (160,000 to 450,000 cc/100 in².atm.24hr); and
• (iii) an R ratio of at least 2.0, preferably at least 3.0, particularly at least 3.5.

In one embodiment, the control member is as described in US-A-6,013,293.

The permeability of the container, can be influenced by perforating the container in order to make a plurality of pinholes therein.

In the method of the invention, green bananas are ripened while in a sealed container. The ripening can be carried out in a conventional ripening room containing ethylene, typically but not necessarily at a concentration of 500 to 1000 ppm. It was surprising to discover that, when using suitable containers, it was unnecessary to follow the conventional practice of opening the bags, and that the bananas would ripen satisfactorily in this way. An important advantage of this method of ripening bananas is that the ripening takes place in a more controlled fashion, resulting in lower peak temperatures in the bananas, which in turn results in reduced dehydration of the bananas and, when the ripening is carried out at temperatures below room temperature, reduced demand upon the refrigeration equipment.

The temperature at which ripening is carried out and the concentration of ethylene in the atmosphere influence the rate at which ripening takes place. In general, slower ripening results in bananas which remain in a desired range of color stage for a longer period. On the other hand, this must be balanced against delivery dates required by retail outlets and inventory constraints. Generally the ethylene-containing atmosphere will be maintained at the temperature less than 22 °C, preferably less than 20 °C, for example 16-21 °C.

The atmosphere within the bags will change substantially during the ripening process, as the bananas consume O₂ and generate CO₂. Preferably, the packaging atmosphere, for at least part of the period before the bananas reach their climacteric, contains at least 10% preferably at least 12%, particularly 14 to 19%, of O₂, and less than 10%, preferably less than 4%, of CO₂, with the total quantity of O₂ and CO₂ being less than 20 %, preferably less than 17 %. For at least part of the period after the bananas have passed their climacteric, the packaging atmosphere preferably contains at least 0.8%, preferably 1.5 to 6%, especially 1.5 to 3%, of O₂, and less than 15%, preferably less than 7%, of CO₂, with the total quantity of O₂ and CO₂ being less than 16%, preferably less than 10 %.

The invention can in principle be used for any quantity of bananas. However, it is particularly valuable when relatively large quantities are involved. Thus it is generally preferred that the sealed container contains at least 4 kg, preferably at least 15 kg, especially 16 to 22 kg of bananas.

The invention is illustrated in the following Examples, a number of which are comparative Examples, designated by the letter C before the number of the example. The bananas, bags and control members used in the Examples were as follows.

The bananas were Cavendish bananas, from Ecuador in Examples 2A-B, C21-22, from Costa Rica in Examples 3A-C and C3, and from Colombia in the other Examples.

The large bags were about 0.96 m (38 in.) wide and about 1.2 m (50 in.) long, and were made from polyethylene film about 0.056 mm (2.2 mil) thick (available from Roplast Industries under the tradename RA 3030). The polyethylene film had an OTR at 13 °C of about 2915 (188) and at 22 °C of about 4,650 (300), and EtTR at 13 °C of about 11,400 (735) and at 22 °C of about 18,100 (1,170), an R ratio of about 4.5, and a P10 ratio (between 0 and 10 °C.) of about 1.76. The small bags were about 0.3 m (12 in.) wide and about 0.46 m (18 in.) long, and were made from the same polyethylene film.

The Type S control members were as described in WO-A- 00/04787 and comprised a microporous polyethylene film coated with a polysiloxane/SCC block copolymer. The Type S members had an OTR at 13 °C of about 3,803,850 (245,410) and at 22 °C of about 5,000,000 (324,000), an EtTR at 13 °C of about 16,280,000 (1,050,300) and at 22 °C of about 19,500,000 (1,260,000), an R ratio of about 3.8, and a P10 ratio (between 0 and 10 °C.) of about 1.8. The microporous polyethylene film contained 50-60% silica, had a thickness of about 0.18 mm (0.007 inch), a tear strength of about 90g, a porosity of about 65%, an average pore size of about 0.1 micron and a largest pore size of 4-10 microns (available from PPG industries under the tradename Teslin SP 7). The block copolymer was prepared by the reaction of a polydimethyl siloxane terminated one end only by a methacryloxypropyl group (available from Gelest under the tradename MCR M17), 40 parts, dodecyl acrylate, 26.8 parts and tetradecyl acrylate, 33.2 parts, as described in Example A7 of WO-A- 00/04787.

The Type A control members were as described in WO-A- 96/38495, and comprised the same microporous polyethylene film coated with an SCC polymer of dodecyl acrylate, 42 parts, tetradecyl acrylate, 53 parts, and acrylic acid, 5 parts. The Type A members had an OTR at 22 °C of about 1,705,000 (110,000), an R ratio of about 4, and a P10 ratio (between 0 and 10 °C.) of about 1.4.

In each Example, the control member was secured to a portion of the bag in which one or more round holes had been cut. In Examples 1 and C11-14, the periphery of the control member was heat sealed to the interior of the bag, thus creating a control member of the kind described in US-A-6,013,293. In the other Examples, the control member was secured to the exterior of the bag by means of a layer of a pressure sensitive adhesive on the peripheral margin of the control member, and the effective area of the control member was about equal to the area of the hole or holes in the portion of the bag to which the control member is attached.

The color stages referred to in the Examples are those accepted by the industry and as shown below. Color stage Description 1 95% green 2 80% green, 20% slightly yellow 3 50% yellow, 50% green 4 80% yellow, 20% light green 5 95% yellow, with slight green color at stem and blossom end 6 100% yellow 7 100% yellow with brown sugar spots Bananas are preferably at color stage 3.5 to 5 when put on retail sale.

Many of the Examples are summarized in Tables 1-5 below. In the Tables, when more than one result is given for a particular Example, this reflects the fact that more than one test was carried out under the same conditions.

Each of these Examples uses a large bag. In Examples 1 and C11-13, each bag has one S-type control member placed under one or more holes in the bag. In Example C11, the control member had an area of 967 mm² (1.5 in²) and was placed under a single hole of diameter 20.6 mm (0.81 in.). In Example C12, the control member had an area of 1935 mm² (3 in²) and was placed under 2 holes, each of diameter 20.6 mm (0.81 in.). In Example C13, the control member had an area of 3225 mm² (5 in²) and was placed under 4 holes, each of diameter 19 mm (0.75 in.). In Example 1, the control member had an area of 12,900 mm² (20 in²) and was placed under 6 holes, each of diameter 25 mm (1 in.). In Example C14, the bag did not have a control member. Each bag was packed with about 18.1 kg (40 Ib) of green bananas. The bananas had been harvested at week 13, and maintained at 13-14°C for about 11 days after harvest before being packed. Except in Example C14, excess air was extracted from the bags using a vacuum pump, and then securely tied. In Example C14, the bags were left open. The sealed bags were cooled to about 13 °C and shipped to Gulfport, Mississippi, and then to San Francisco, California, maintaining the temperature at about 13°C. In San Francisco, 36 days after packing, half the bags in each Example were opened, and the other half left intact. All the bags were then exposed to ethylene (500-1000 ppm) in a commercial ripening room for about 24 hours. The bananas in the opened bags ripened rapidly in the expected way; thus by day 43, their color was 6, by day 46 their color was greater than 7, and by day 49, they were overripe. The bags which were still sealed were opened on day 49. The results for the bags opened on day 49 are shown in Table 1 below. These Examples demonstrate that bananas harvested at 13 weeks can be transported in a suitably designed bag, and can be ripened into an excellent product by exposure to ethylene through the bag. Example No. C11 C12 C13 1 C 14 Control member yes yes yes yes no Total area of hole(s) in bag under control member (mm²) 335 670 1140 3040 - Days to change from color stage 3.5 to color stage 5 > 8 > 8 > 8 5.5 DDU Taste and texture on day 49 SGU SGU SGU Exct DDU % O₂ (approximate) at day 23 8.6 9.8 12.7 15.5       at day 46 2.9 0.6 1.8 2.2 % CO₂ (approximate) at day 23 4.45 3.65 3.3 2.85       at day 46 13.8 11.4 5.0 9.0 SGU soft, green and unpalatable
DDU dehydrated, decayed and unpalatable by day 47 (day 11 after exposure to ethylene)
Exct excellent taste and texture

Each of these Examples uses a small bag. In Examples 2A-B, each bag has one A-type control member placed over four or five holes in the bag. In Example 2A, the control member had an area of 145 mm² (5.7 in²) and was placed over four holes each of diameter 19 mm (0.75 in.). In Example 2B, the control member had an area of 4516 mm² (7 in²) and was placed over 5 holes, each of diameter 19 mm (0.75 in.). In Example C21, the control member and the holes under it were as in Example 2A, except that the control member was an uncoated microporous film. In Example C22, the bag was intact except for 200 pinholes each about 0.5 mm (26 gauge) in diameter. Each bag was packed with about 1.35 kg (3 Ib) of green bananas which had been maintained at 13-14 °C for about 11 days after harvest. Except in Example C22, excess air was extracted from the bags using a vacuum pump, and the bags were then securely tied. In Example C22, the bags were left open. After three days, to allow the packaging atmosphere to equilibrate, the bags were exposed to ethylene (500-1000 ppm) in a ripening room. The results are shown in Table 2 below. These Examples demonstrate that small quantities of bananas can be ripened in a suitably designed bag, and can remain in the bag in excellent condition for several days longer than bananas exposed to the air.

These Examples show that the bananas generate heat more evenly when ripened in a container including an atmosphere control member. In each Example, a large bag was packed with about 18.1 kg (40 Ib.) of green bananas. The green bananas had been harvested 13 days previously and had been stored at 13-14 °C since harvest. A temperature sensor (available from Sensitech, Beverly, Massachusetts, under the tradename Template P) was inserted into one banana in each bag. In each of Examples 3A, 3B and 3C, the bag had two S-type control members, each having an area of 11,300 mm² (17.5 in²). Each control member was placed over a single hole in the bag, the hole having an diameter of 70 mm (2.75 in.) in Example 3A, 74.4 mm (2.93 in.) in Example 3B, and 78.7 mm (3.1 in.) in Example 3C. In Example C3, the bag was perforated so that the bananas were surrounded by air. The bags were then sealed with rubber bands. The sealed bags were placed in a refrigerated room at about 13 °C. After about 84 hours, the temperature of the room was raised to about 16.7 °C and after about 12 hours, an ethylene generator was used to provide an initial ethylene concentration in the room of 500-1000 ppm. About 24 hours after the generation of ethylene had begun, the room was vented. The temperature of the bananas was monitored for about 15 days, and reached a peak at about 60 hours after the generation of ethylene had begun. At that time, the concentration of O₂ and CO₂ was measured. The results are shown in Table 3 below. It will be seen that the peak temperature was substantially lower in the bags containing control members than in the perforated bag. Example No. 3A 3B 3C C3 Control member yes yes yes no Total area of holes in bag under control members (mm²) 7700 8700 9700 - Temperature (°C) of bananas 12 hrs after temperature of room was set to 16.7 °C 16.3 15.9 15.7 16.6 Peak Temperature °C 21.2 21.1 20.9 23.9 Difference between peak temperature and 16.6 °C 4.9 5.3 5.2 7.3 % O₂ 60 hours after injection of ethylene 2.2 1.75 1.9 20.95 % CO₂ 60 hours after injection of ethylene 7.95 6.1 7.4 0.03

Each of these Examples uses a large bag and two S-type control members, each control member having an area of 11,300 mm² (17.5 in.²). In Example 4, there was a single hole, diameter 82.5 mm (3.25 in.), under each control member. In Example C4, there were seven holes, each hole of 25.4 mm (1 in.), under each control member. The total area of the holes was 10,700 mm². About 18.1 kg (40 Ibs.) of green bananas were placed in each bag. In Example 4, the bag was sealed with rubber bands. In Example C4, the bag was not sealed. The green bananas had been maintained at 13-14 °C for about 11 days after harvest. The bags were left in a cold room at 13-14 °C. Three days after packing, the bags were exposed to ethylene for 24 hours in a conventional ripening room at 16.7 °C and containing 500-1000 ppm of ethylene.

Table 4 shows the number of days taken to reach various color stages. Example No. 4 C4 Control member yes no Days to color stage 4 6.5 4.2 6.5 4.5 7.1 4.5 Days to color stage 5.5 11.5 6.6 12 7 12.3 7.2 Days from color stage 4 to color stage 5.5 5 2.4 5.5 2.5 5.2 2.7 Table 5 below shows, for each of the bags in Examples 1, C11, C12, C13, 3A, 3B, 3C and 4 the permeability of the bag to O₂ and to ethylene ("Et" in Table 5), and the respective contributions of the control member and the remainder of the bag. For this calculation, the size of the bag, after sealing, was assumed to be 0.96 x 1.04 m (38 in. x 41 in.), i.e. to have a total area of 2 m² (3115 in²). Example No. Perm. of bag (mUatm.24hr) at 13°C Perm. of bag at 13°C /kg of bananas Hole area (m² ) Perm. Of ACM at 13°C Perm. of rest of bag at 13°C C₁₁ O₂ 7,200 O₂ 395 0.000335 O₂ 1,300 O₂ 5,900 Et 30,650 Et 1,695 Et 5,500 Et 25,100 C12 O₂ 8,500 O₂ 470 0.000670 O₂ 2,550 O₂ 5,900 Et 36,000 Et 2,000 Et 10,900 Et 25,100 C₁₃ O₂ 10,250 O₂ 565 0.001140 O₂ 4,350 O₂ 5,900 Et 43,650 Et 2,400 Et 18,550 Et 25,100 1 O₂ 17,450 O₂ 965 0.003040 O₂ 11,550 O₂ 5,900 Et 74,600 Et 4,120 Et 49,500 Et 25,100 3A O₂ 35,000 O₂ 1,935 0.007700 0₂29,100 O₂ 5,900 Et 149,800 Et 8,280 Et 124,700 Et 25,100 3B O₂ 39,000 O₂ 2,155 0.008700 O₂ 33,100 O₂ 5,900 Et 166,650 Et 9,200 Et 141,550 Et 25,100 3C O₂ 42,900 O₂ 2,370 0.009700 O₂ 37,000 O₂ 5,900 Et 183,550 Et 10,150 Et 158,450 Et 25,100 4 O₂ 46,500 O₂ 2,570 0.010700 O₂ 40,600 O₂ 5,900 Et 199,200 Et 11,000 Et 174,100 Et 25,100