|1.||A method of extending the shelf life of precut salad leaves comprising (a) reducing (18) selected (12) salad leaves to the desired size without unduly damaging the leaves thereby to form cut leaves. (b) washing (22) said cut leaves with an aqueous washing liquid. (c) removing surface water (26) from said washed cut leaves to provide surface waterfree cut leaves, characterized by; (d) uniformly evaporative drying (30) said surface waterfree cut leaves to remove at least 2% by weight of tissue moisture based on the weight of said surface waterfree cut leaves but not sufficient to damage significantly the texture of the cut leaves to provide tissue moisture reduced cut leaves, (e) cooling (34) said tissue moisturereduced cut leaves, and (f) storing (40) said tissue moisturereduced cut leaves under modified atmospheric (38) conditions at a temperature of 010° C.|
|2.||A method as defined in claim 1 wherein said tissue moisturereduced cut leaves are stored (40) under controlled atmosphere (38) conditions.|
|3.||A method as defined in claim 1 wherein said tissue moisturereduced cut leaves are packaged (38) under said modified atmosphere conditions before said storing (40).|
|4.||A method as defined in claim 1, wherein said drying (30) comprises air drying and drying conditions including temperature, time and drying air moisture content are controlled during said air drying not to increase the temperature of said washed cut leaves higher than a temperature of 25° C.|
|5.||A method as defined in claim 2 wherein said drying (30) comprises air drying and drying conditions including temperature, time and drying air moisture content are controlled during said air drying not to increase the temperature of said cut leaves higher than a temperature of 25° C.|
|6.||A method as defined in claim 3 wherein said drying (30) comprises air drying and drying conditions including temperature, time and drying air moisture content are controlled during said air drying not to increase the temperature of said cut leaves higher than a temperature of 25° C.|
|7.||A method as defined in claims 3 or 6 wherein said packaging (38) comprises applying an input modified atmosphere gas mixture containing 3 to 12% 02, 5 to 10% C02 and an inert gas.|
|8.||A method as defined in claims 3 or 6 wherein said packaging (38) comprises packaging said cooled cut leaves in a modified atmosphere which at equilibrium contains 1 3% 02; 3 12% C02; and the remainder an inert gas.|
|9.||A method as defined in claims 3 or 6 wherein said packaging (38) comprises packaging said cooled cut leaves in a modified atmosphere which at equilibrium contains 1 3% 02; 3 12% C02; and the remainder an inert gas.|
|10.||A method as defined in claims 7, 8 or 9 where said equilibrium modified atmosphere contains 5 8% C02.|
|11.||A method as defined in claims 3, 6, 7, 8 9 or 10 wherein said cooling reduces the temperature of said tissue moisturereduced cut leaves to 05° C.|
The present invention relates to improving the shelf life of precut produce fo a salad. More particularly the present invention relates to a process for extendin the shelf life of fresh pre-cut salad leaves by air drying prior to modified atmospher packaging and storage under cool conditions.
During storage of pre-cut salad leaves in packages at refrigerate temperatures, changes in composition and structural features of the tissues of th leaves leads to losses in quality of the leaves due to physiological disorders whic include russet spotting (small brown spots on the ribs and blades), brown stai (pitting and discoloration of the epidermal tissue) and rib and cut-edge pinking Microbial degradation of the tissue occurs during cold storage.
Some systems to extend the shelf life of pre-cut fresh salad leaves by cuttin and surface water removal followed by modified atmosphere packaging (MAP) an cold storage have been developed to minimize quality loss in the leaves durin storage.
In an article entitled "Factors Affecting the Storage Stability of Shredde Lettuce" by H.R. Bolin et al, published in the Journal of Food Science, 42, page 1319 -1321, (1977), a process was discussed for extending the storage stability b minimizing physical damage to the product during cutting, minimizing th microbiological load and storing at a temperature above freezing. This stud indicated that the storage temperature, if too high, was one of the most profoun detriments on the storage life of the shredded lettuce and that samples of shredde lettuce stored 5 or 10° C had a statistically shorter life than samples held at 2° C Bolin et al found that at 2° C, the lettuce remained marketable for 26 day compared with 10 days for the same products stored at 10° C.
In a paper entitled "Improving the Keeping Quality of Cut Head Lettuce" b
Krahn et al in Acta Horticulturae 62, pages 79 - 91, (1977), describes a process fo storing whole vegetables wherein the storage temperature was found to be critical
This paper discusses modified atmosphere packaging (MAP) using 2 to 3% oxyge
(0 2 ) and indicated that MAP produced an improvement in its shelf life.
The article "Modified Atmosphere Packaging of Shredded Lettuce" by Ballantyne et al published in the International Journal of Food Science and Technology, 23, pages 267 - 274, (1988), describes the retail packaging system for packaging shredded lettuce using films of different gas permeabilities and modified atmospheres produced both by respiration and by application of gas flushing techniques and found that an equilibrium modified atmosphere containing 1 to 3% 0 2 and 5 to 6% carbon dioxide (C0 2 ) and nitrogen as the make -up gas was established using 35 μm low density polyethylene film after flushing with 5% 0 2 , 5% C0 2 in nitrogen (N 2 ) extended the shelf life to approximately 17 days when stored at 5° C.
In an article entitled "Effective Preparation Procedures and Storage Parameters on Quality Retention of Salad-cut Lettuce" by Bolin et al in the Journal of Food Science, 56, pages 60 - 67 (1991); describes a technique for preserving cut lettuce by rinsing to remove cellular fluids released during cutting followed by centrifugation to remove added surface moisture. The article indicates that centrifugation can cause desiccation of product which in some cases may be beneficial. Bolin et al. in this paper indicated that lettuce centrifuged at 2000rpm for increasing time periods "resulted in increasing desiccation of the product and increased storage life" which was different from the results he obtained in 1977 on shredded lettuce. The results in 1977 indicated that there were no advantages or increases in the effect of storage life for shredded lettuce by increased centrifuge time and attributed this difference as possibly being "due to the much larger exposed area of the shredded lettuce which would allow for much more reactive system and a greater loss of cellular material during increased centrifugation". Optimum results were obtained by packaging the cut lettuce in pouches that had been slightly evacuated and contained a small amount of C0 2 .
U.S.Patent 3,795,749 issued March 5, 1974 to Cummin et al discloses that heads of lettuce can be stored for prolonged periods by packaging each head in gas permeable polyvinyl chloride film having an 0 2 permeability of at least 2,000 cc/100 sq.in./24 hr/1 atm. and a C0 2 permeability of at least 10,000 cc/100 sq.in./24 hr/1 atm.
U.S.Patent 3,849,581 issued November 19, 1974 to Kubu teaches that the shelf life of shredded lettuce can be extended to about 3 weeks by treating the shredded lettuce with an antioxidant such as sodium bisulphite in solution, buffered to a pH of about 5 to 6.5, subjecting the drained treated shredded lettuce to spin or centrifuging to remove surface water to the extent of about 6 to 10 per cent of the original weight of the wet, treated, shredded lettuce and then packaging in air-tight bags.
U.S.Patent 3,987,208 issued October 19, 1976 to Rahman et al teaches cut lettuce leaves can be stored for about 4 weeks with a minimum loss of overall quality by treating with a dilute aqueous solution of sodium meta-bisulphite, citric acid, ascorbic acid and potassium sorbate, draining, centrifugation at about 1,200 rpm for about 1 minute to remove a substantial amount of surface solution and placing the leaves in bags made up of vinylidene chloride-vinyl acetate or polymer film with a 0 2 permeability of 9-13 cc/100 sq.in./24 hr/1 atm/21° C. U.S.Patent 4,001,443 issued January 4, 1977 to Dave describes a package and method for packaging and storing cut leafy vegetables to prolong the shelf life by subjecting the cut leaves to 70-150 ppm chlorine solution buffered at pH level about 5 to 7, and at a temperature about 7.5° C to 15.6° C; removing of adhered moisture by basket centrifugation; packaging the of cut leaves in packages having high gas barrier properties; partial evacuation of air from package to the extent of package wall collapse to the leaves; and storage of packages at a temperature of about 1.7° C to about 7.2° C.
U.S.Patent 4,711,789 issued December 8, 1987 to Orr et al pertains to the prolonging of the shelf life of pre-cut fresh celery. Orr discloses that raw celery in the cut form can be stored for periods up to 48 days at 3° C by:
(1) selecting cultivars with large petioles;
(2) cutting selected petioles with minimum bruising;
(3) washing the cut petioles in cold water (about 1° C);
(4) removal of surface water by spin drying; (5) packaging petiole pieces in a sealed container with moderately high gas permeability. Orr also describes storing the cut celery in preferably a temperature of about 2 to
U.S.Patent 4,935,271 issued June 19, 1990 to Schirmer states lettuce should be packaged in a packaging material with a high oxygen permeability when extended shelf life is desired. The invention details the construction of a multi-layer laminate which consists of a first perforated film of propylene homopolymer or copolymer, a second film having a bonding layer of ethylene vinyl acetyl copolymer and an outer heat sealable layer.
U.S.Patent 5,097,755 issued March 24, 1992 to Hill teaches the method and apparatus for processing produce by treating the produce with sprayed chlorinated water, slicing and washing the produce, followed by dewatering eg. by vibration, and an air flow drying to remove substantially all water from the surfaces of the cut produce by, for example, air knife drying and fluidized bed drying prior to packaging.
It is a object of the present invention to provide a system for prolonging the shelf life of pre- cut leaves for use in salads (pre-cut salad leaves). Broadly the present invention relates to a method of extending the shelf life of pre-cut salad leaves comprising
(a) reducing selected salad leaves to the desired size without unduly damaging the leaves thereby to form cut leaves.
(b) washing said cut leaves with an aqueous washing liquid. (c) removing surface water from said washed cut leaves to provide surface water-free cut leaves,
(d) evaporative drying said surface water-free cut leaves to remove tissue moisture to reduce the weight of the surface water-free cut leaves by at least 2% but not sufficient to damage significantly the texture of the cut leaves to provide tissue moisture-reduced cut leaves,
(e) cooling said tissue moisture-reduced cut leaves, and
(f) packaging said tissue moisture-reduced cut leaves under modified atmosphere conditions,
(g) storing said packaged cut leaves under said modified atmosphere conditions at a temperature of -1 to 10° C.
Preferably said salad leaves will be lettuce leaves.
Preferably said aqueous washing liquid will comprise an aqueous solution of
a suitable disinfecting chemical.
Preferably said aqueous washing liquid will comprise acidic chlorinated water. Preferably removing said surface water will comprise centrifugation of said washed cut leaves. Preferably said evaporative drying will comprise air drying and more preferably said air drying will be under drying conditions including controlled temperature, time and drying air moisture content so that said washed cut leaves during drying do not have a temperature higher than 25° C.
Preferably said air drying will include a fluidized bed drying. It is also preferred to lower the temperature of said cut leaves prior to packaging to 0-5° C within about 45 minutes of completion of said air drying, more preferably within of 30 minutes of completion of said air drying.
Preferably said cut leaves will be package with an input modified atmosphere gas mixture contained in a gaseous atmosphere containing 3 - 12% 0 2 ; 5 - 10% C0 2 ; the remainder of the atmosphere being an inert gas.
Preferably said containing in a gaseous atmosphere will comprise modified atmosphere packaging (MAP) of said tissue moisture-reduced cut leaves in an atmosphere which under equilibrium conditions will contain 1 to 3% 0 2 (preferably
1 to 2% 0 2 ); 3 to 12% C0 2 (preferably 5 to 8% C0 2 ); and the remainder an inert gas such as nitrogen.
Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawing in which:
Figure 1 is a block diagram illustrating the steps in the process of the present invention.
The process of the present invention is clearly illustrated in Figure 1 with the blocks drawn in solid lines indicating the basic unit processes and those drawn in the dash lines indicating the alternate process steps that may be used in conjunction with the main process should the produce being treated (i.e. used to complete the salad) require different treatments prior to storage.
Clearly preparing salad produce other than salad leaves may be included and these produce items may require different treatments. The present invention is
primarily concerned with cut salad leaves, in particular, cut lettuce leaves which are more difficult to store than most other components of the salad. Even cut lettuce of different types and cultivars require different treatment to obtain the optimum extended shelf or storage life as will be pointed out more specifically herein below. Referring to Figure 1, the fresh salad leaves (or other vegetables if they are to be included in the stored salad) are introduced into the process system as indicated by the arrow 10 and are preferably subjected to a unit operation i.e. sorting as indicated at 12. The rejected material is discarded from the process as indicated at 14 and the selected material is passed on as indicated at 16 into a pre-cutting stage 18.
In the pre-cutting stage 18, the salad leaves are cut using conventional cutting equipment and known techniques to reduce the leaves to the desired size without significantly damaging the leaves by bruising. As is well known, this is normally accomplished by using relatively sharp cutting edges or knives to slice the leaves. The cut leaves pass as indicated by the line 20 into a treatment stage 22 i.e. a washing stage wherein the cut leaves are washed with a suitable aqueous liquid. The liquid will be water but preferably will be water containing a suitable disinfecting agent such as hypochlorite; i.e. the conventionally acidic chlorinated water preferably will be used to inactivate microorganisms. After the washing stage, the washed cut leaves pass as indicated by arrow 24 into the surface water removal station 26 wherein the washing liquid is removed from the surface of the cut leaves in the conventional manner. Generally a centrifuge is used to spin the washed cut leaves for the removal of the major portion of the surface water. Other techniques such as high-speed air treatment may also be used to remove substantially all of the surface water.
Once the surface water is removed, the cut leaves pass as indicated by the arrow 28 into an evaporative drying stage 30 which will normally be air drying controlled to remove a selected amount of tissue moisture. Care must be taken in the drying of the cut leaves to insure that they are not subjected to excessive High temperatures as this will cause quality deterioration. Generally such drying is accomplished by using a low humidity air at a relatively low temperature probably not above 30° C to insure that the leaves themselves do not attain a surface
temperature above 25° C for any significant length of time.
The air or evaporative drying stage 30 must produce a uniformly dried product with no surface wetness. This may be attained by air drying the surfaces of the pre-cut leaves to the extent necessary to reduce the tissue moisture by at least 2% as required for the present invention. To ensure uniform drying the air should circulate over the whole of the surfaces of the cut-leaves and not be obstructed from removing moisture from discrete areas on the cut-leaves.
It will be apparent that the removal of surface water and the air drying as indicated in 26 and 30, respectively, could be combined into a single step; however, it is significantly less expensive to centrifuge the cut leaves to remove surface water than to use high-speed air treatment for surface water removal and air drying.
Normally the tissue moisture-reduced cut leaves leaving stage 30 will have a weight loss of at least 2% (i.e. weight at least 2% less than that of the surface water- free cut leaves leaving station 18). It is important that the drying step remove at least 2% of the tissue moisture but it also is important to ensure that the tissue moisture is not unduly reduced to thereby cause texture damage to the tissue moisture-reduced cut leaves. Generally it has been found that provided the tissue moisture has not been reduced by more than 12%, tissue damage to and quality deterioration of the cut leaves will be minimal.
The optimum amount of tissue moisture that should be removed may differ for different types of salad leaves. Generally satisfactory results are obtained when 2 to 12% weight loss of surface water-free cut lettuce is reached; however, for iceberg and romaine lettuce, better results are obtained when 6 to 10% weight loss of the surface water-free lettuce is achieved.
After the drying operation, the tissue moisture-reduced cut leaves pass as indicated by the arrow 32 to a cooling station 34 where the tissue moisture-reduced cut leaves are cooled preferably relatively quickly to a temperature in the range of 0 - 10° C preferably 0 to 5° C and more preferably 0 to 3° C. This cooling operation should be completed within approximately 45 minutes, preferably less than 30 minutes after completion of the drying step 30. This step may be combined with the packaging step 38.
The cut leaves then pass as indicated by the arrow 36 into a packaging station 38 which may take the form of a modified atmosphere packaging station 38 wherein the microatmosphere surrounding the cut leaves in the package is gassed with a specific gas mixture. Preferably the modified atmosphere gas mixture applied to the cut leaves during packaging will comprise about 3 - 12% 0 2 and about 5 - 10% C0 2 . The remainder being an inert gas such as nitrogen, argon, helium, or other inert gases. The air can be removed from the pouch or the like in which the cut leaves are packaged by gas flushing or by gas compensated vacuum techniques.
It is important that, during storage of the packaged cut leaves, the oxygen surrounding the leaves is not depleted and a minimum amount of oxygen be maintained within the package. Suitable packaging materials or films will be selected in the conventional manner to ensure that, under equilibrium conditions, the microatmosphere around the cut leaves in each package will contain 1 to 3% 0 2 ; 3 to 12% C0 2 ; preferably 5 to 8% C0 2 with the remainder being an inert gas. Generally MAP as opposed to controlled atmosphere storage (CA) will be used for extending the shelf life of the cut leaves. However, if suitable levels of 0 2 and C0 2 are maintained i.e. essentially the same as those described above for equilibrium conditions, CA storage may also be used to extend the shelf life of the cut lettuce. After packaging, the cut salad leaves are stored at a temperature in the range of 0-10° C as indicated at 40. Preferably the storage temperature will be in the range of O to about 3° C.
As indicated above, in some cases, the salad to be stored may include two or more different produce components that require different processing conditions prior to packaging. In such cases, the components may be processed separately through the initial stages of the process; for example, one of the salad components eg. carrot pieces may be introduced as indicated at 10 (line 10) and another component at 10A (or 10B or 10C etc. not shown) (lines 10A, 10B, 10C, etc.) and processed using the same steps as described above for the leaves entering at 10 with the steps for the components processed in lines 10A, 10B, 10C etc. by applying conditions specifically modified or customized the component entering that specific line 10, 10A, 10B, 10C, etc. Where two or more separate processing lines 10, 10A, 10B, 10C, etc., are used
it is necessary to take the salad components or cut leaves from lines 10, 10A, 10B, and IOC etc. after the drying to remove tissue water (steps 30, 30A, etc) to a blending station 42 as indicated by the arrows 44 and 46.
The blended salad components are then conducted as indicated by line 48 to the cooling station 34, packaging station 38 (the cooling and packaging steps 34 and 38 may be combined) and the storage station 40 where the cut leaves blended with other salad materials are stored at the appropriate temperature.
By processing in the manner described above, the shelf life of the salad leaves has been found to be 30 days and in many cases to exceed 30 days. Examples
Three different types of leafy salads having the following compositions were processed;
1. Pre-cut lettuce salad solely pre-cut iceberg lettuce
2. Tossed salad 80% pre-cut iceberg lettuce 10% shredded carrots
10% shredded red cabbage
3. Continental salad 35% pre-cut romaine lettuce
35% pre-cut sui choy 15% pre-cut endive 15% pre-cut raddichio
In each example, the uncut, unwashed salad leaves without surface water were weighed to provide an initial weight. initial weight = W, washed and centrifuged and weighed to provide a centrifuged weight, centrifuged weight = W c the surface water removed and the cut salad leaves weighed to provide a surface water-free weight, surface water-free weight - W d and then tissue moisture was removed to the desired extent and the product weighed to provide a packaging weight packaging weight = W p . In each of the examples the weights <-> W c , δW d and <SW p are given as a percentage
(%) change in weight of the ingredient based on the weight of the ingredient leaving the immediately preceding step and indicated as + for an increase in weight and - for a decrease in weight i.e. <-> W c is a % of Wj ; <SW d is a % of W c ; and <-> W p is a percent of W d . It will be apparent that generally W d is about equal to Wj and thus «SW p provides a reasonable accurate indication of the change in tissue moisture of the leaves being processed.
As will become apparent in all cases it was necessary to remove tissue water to a level where the surface water-free leaves has a weight loss of at least 2% but not more than 12% i.e. $W P is generally in the range of -2 to -10% of Wj. Each of the salads 1, 2 and 3 (defined below) were packaged into 259 gram packages at a temperature of about 10° C and stored at about 2 to 3° C.
The processing conditions (degree of tissue moisture removed) and packaging conditions were varied to determine their influence on the quality of salads over a storage period of 30 days. The salads were examined for quality every 5 days.
The quality evaluation is subjective taking into account appearance (including color), odor, flavor and texture (i.e. examined for russet spotting, brown stain, rib and cut edge pinking and microbiological degradation of the tissue) and each sample was scored on the basis of the known 5-point Hedonic system as follows Points Condition
2 unacceptable 1 very unacceptable
An excellent product possesses no discoloration, no off-odor and had remained fresh, full flavored and crisp. Example 1
Pre-cut lettuce salad (1 above) was subjected to the following weight changes during processing - «SW C = +3%; SW d = +2 - 3%; <SW p = -8%. The salad in each case was packaged in a plastic film pouch of material having permeability to 0 2 of
3,500 ml/m 2 , over a 24 hour period, at atmospheric pressure. The input modified
atmosphere gas composition was 5% C0 2 ; 8% 0 2 and the balance N 2 .
The quality evaluation results obtained by subjecting the leaves packaged at various stages of processing (expressed as W c , W d and W p ) and stored as above indicated at 2-3°C for various time periods are given in Table 1 for the sample indicated as A.
TABLE 1 Quality Evaluation of Cut Leaves in Salad Packaged in Plastic Pouches
SAMPLE PACKAGE STORAGE TIME DAYS
5 10 15 20 25 30
A W c 5 4 3 2 1.5 1
A w d 5 5 3.5 2.5 2 1
A P 5 5 5 4.5 4 3.5
B W c 5 4 3 2 1.5 1
B w d 5 5 3.5 2.5 2 1.5
B W P 5 5 5 5 4.5 4
C W c 5 4 3 2 1.5 1
C w d 5 5 3.5 2.5 2 1.5
C w p 5 5 5 5 4.5 4
Tossed salad (2 above) was subjected to the following weight changes during processing a) lettuce same as example 1. b) shredded carrots: <$W p = ± 1% of Wj c) shredded red cabbage: 6W C = + 1%; SW d = - 1%; and <SW p = - 3%.
The tossed salad after processing was packaged in a pouch of material having
permeability to 0 2 of 4,000 ml/m 2 , over a 24 hour period, at atmospheric pressure. The input modified atmosphere gas composition was 3% C0 2 ; 12% 0 2 and the balance N 2 .
The quality evaluation results obtained by subjecting the salad packaged at various stages of processing (as expressed by W c , W d and W p ) and stored at the above designated storage temperature are given in Table 1 for the sample indicated as B.
It will be apparent that some vegetables such as shredded carrots do not require the same treatment as the cut lettuce. Example 3
Leaf components of the continental salad (3 above) were subjected to the weight changes indicated in Table 2 during processing
TABLE 2 Weight Changes of Pre-cut Salad Leaves at Various Processing Stages
SALAD LEAVES *W C = % SW d = % 5W p = %
ROMAINE LETTUCE + 3 - 2 TO - 3 - 7 TO - 8
CHINESE LEAF + 2 TO +3 - 3 TO - 4 - 10
ENDIVE + 2 - 1 TO - 2 -5 TO - 6
RADDICHIO + 2 TO +3 -2 TO - 3 - 4 TO - 5
The salad was packaged in a pouch made of material having permeability to 0 2 of 4,200 ml/m 2 , over a 24 hour period, at atmospheric pressure. The input modified atmosphere gas composition was 3% C0 2 ; 15% 0 2 and the balance N 2 Ε quality evaluation results obtained by subjecting the salad leaves to various stages of processing (as expressed as W c , W d and W p ) and storing at the storage temperature defined above are given in Table 1 for the sample indicated as C.
In all cases the leaves in the salads were in at least consumer acceptable condition after 30 days of storage which is considerably longer better than may be
obtained using only surface water removal. Example 4
Pre-cut lettuce salad (1 above) was processed as follows; <SW C = = 3%; <SW d = -2 to 3%; $W p = -8% and then packaged in a special corrugated box coated with polymer to provide a container having permeability to 0 2 of 7,800 ml/m 2 , over a 24 hour period, at atmospheric pressure. The input modified atmosphere gas composition was 5% C0 2 ; 8% 0 2 and the balance N 2 . In this case, the package size was a 2 kg box 33.5 X 29 X 12 cm. The results obtained are given in Table 3.
Quality Evaluation of Salad Leaves Packaged in
Corrugated Boxes Coated with Polymer
Storage Appearance Odor Flavor Texture Overall Ηme Days
5 5 5 5 5 5
10 5 5 5 4.75 4.75
15 4.75 5 5 4.75 4.75
20 4.5 5 5 4 4
25 4 5 5 4 4
30 3.5 5 5 4 3.5
It is apparent that over the full 30 days of storage at the temperature defined above the product remained acceptable in overall quality.
Having described the invention modifications will be evident to those skilled in the art without departing from the scope of the invention as defined in the appended claims.
Next Patent: TREATMENT OF FRUITS AND VEGETABLES BY CALCIUM SALTS