Stabilizing color in kiwi fruit and product

A method for producing a dried fruit product in which certain physical, chemical and/or sensory features such as green color, flavor and Vitamin C content are stabilized wherein pieces of fruit are produced by peeling and slicing from a green chlorophyll-containing fruit, the pieces of fruit are infused with a low molecular weight carbohydrate, preferably in an infusion solution comprising one or more of glucose, sucrose and fructose together with a buffering agent, and then the pieces are dried. The infused dried product is suitably packaged in an oxygen, water vapor and ultraviolet light impermeable medium for prolonged storage. A dried fruit product produced by the method is also claimed.

This invention relates to a method of producing a dried fruit product in 
which certain physical, chemical and/or sensory features such as taste and 
color are substantially stabilized, and/or a dried fruit product formed by 
such a method. The invention is intended particularly, though not 
necessarily solely, for use in relation to green chlorophyll-containing 
fruit, especially kiwifruit. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method of stabilizing color 
in green-fleshed chlorophyll-containing fruit such as kiwifruit during 
drying of said fruit which will at least provide the public with a useful 
choice. 
Accordingly in one aspect the invention consists in a method of stabilizing 
color in green fleshed chlorophyll-containing fruit during drying of the 
fruit by preventing degradation of chlorophyll in the fruit comprising: 
infusing pieces of said fruit with a low molecular weight carbohydrate; 
and drying the infused fruit pieces moisture content in the fruit pieces of 
between about 2% and about 4% by weight to a predetermined extent. 
In a further aspect the invention consists in a method of producing a dried 
fruit product according to any of the examples herein. 
In a still further aspect the invention consists in a dried fruit product 
when prepared by a method according to any of the preceding paragraphs. 
In a still further aspect the invention consists in a dried fruit product 
when prepared by a method according to any of the examples herein. 
When used herein "low molecular weight carbohydrate" includes 
monosaccharides, disaccharides, oligosaccharides, and mixtures thereof. 
To those skilled in the art to which the invention relates, many changes in 
construction and widely differing embodiments and applications of the 
invention will suggest themselves without departing from the scope of the 
invention as defined in the appended claims. The disclosures and the 
descriptions herein are purely illustrative and are not intended to be in 
any sense limiting. 
The invention consists in the foregoing and also envisages constructions of 
which the following gives examples only.

DETAILED DESCRIPTION 
One preferred form of the invention will now be described. 
In the preferred form the invention provides a method of producing a dried 
fruit product in which certain physical, chemical and/or sensory features 
such as taste and color are stabilized. 
Pieces of the fruit to be processed are cut to a suitable size, which may 
be adjusted according to the rate of infusion desired in a particular 
case, as obviously infusion will take place more rapidly into pieces of 
small volume and large surface area than the reverse. It is preferable 
that all of the pieces to be infused at one time be of a similar 
thickness, for example 5-6 mm slices or 10-11 mm slices. 
The cut pieces are then infused with a carbohydrate to a predetermined 
extent. In the preferred form, this is carried out by vacuum infusion of 
the fruit pieces in an aqueous solution of a carbohydrate, preferably 
comprising between about 10% and about 70% carbohydrate by weight. We have 
obtained good results using a 50% carbohydrate solution by weight. The 
choice of carbohydrate is quite broad, and we have obtained good results 
with a mixture of glucose and fructose and especially with invert sugar. 
However, it is envisaged that many other forms of carbohydrate such as 
sucrose, and mixtures of sucrose with fructose, glucose, or both, could be 
used within the scope of the invention. 
The length of time for infusion to take place will obviously be adjusted by 
one skilled in the art according to the volume: surface ratio of the 
pieces of fruit, the exact concentration of the carbohydrate infusion 
solution, and the extent of infusion desired for a particular fruit. It is 
necessary to ensure that the degree of infusion is sufficient to give the 
required stabilizing effect, but without making the fruit too sweet to the 
taste. Using a 50% sugar solution, with 5 to 6 mm thick slices of fruit, 
we have found that an infusion period of 15 to 20 minutes produces a good 
result. 
By way of example, ripe kiwifruit generally has a pH of about 3.2 to 3.3 
and a titratable acidity of about 1.2%. This can give rise to an acidic 
flavor in the final finished product. We have found that the acidic flavor 
can be reduced considerably by incorporating a buffering salt in the 
carbohydrate solution which buffers the acidity in the fruit at about pH 
4.5. This can reduce the harshness of the perceived acidity and improve 
the flavor of the finished product. We have found that the presence of the 
buffering salt also increases the stability of the chlorophyll pigments. A 
suitable buffering salt for this purpose is a salt of citric acid, most 
preferably sodium or potassium citrate used in low concentrations. 
After the fruit pieces have been infused with the carbohydrate to a 
predetermined extent appropriate to the fruit, the infused fruit pieces 
are dried to a predetermined extent. 
It has been found that a good shelf life is obtained with the product by 
drying the fruit pieces to a moisture content of between about 2% and 
about 4% by weight. 
The infused fruit pieces are preferably vacuum dried, preferably at about 
40.degree.-50.degree. C. until the moisture content is between about 2% 
and 4% by weight. 
The drying step may alternatively be carried out in two steps, the first 
being an air drying step which is carried out at an elevated temperature, 
preferably between about 40.degree. and about 50.degree. C., until the 
pieces of fruit have a minimum moisture content of about 40% and most 
preferably have a moisture content of about 45%. At the temperature given, 
this has been found to take about 4 hours. The air dried fruit pieces are 
then further dried under vacuum, preferably at about 40.degree.-50.degree. 
C. until the final moisture content is between about 2% and 4%. The 
residual moisture content is removed by storing the vacuum dried material 
over a suitable dessicant in a sealed container for a period of 1-2 weeks. 
This residual drying process can be enhanced by passing air through the 
dessicator in a closed loop. The moisture content of the fruit pieces is 
reduced by this method to about zero. 
For extended shelf life the dried product is then packaged in an oxygen, 
water vapor and UV light impermeable medium. In order to protect the 
product from oxygen degradation, the packaging may be vacuum packed, or 
nitrogen flushed, or packed with an oxygen scavenger, such as will be 
known to those skilled in the art. 
It has been found that this method in accordance with the invention is 
particularly suitable for producing a dried kiwifruit product having 
stabilized color, flavor and Vitamin C content over an extended shelf 
life, but it is envisaged that it could also be applied to other fruit 
particularly other chlorophyll-containing green fruit, such as kiwano. 
An example of a method in accordance with the invention for producing a 
stabilized dried fruit product will now be given, but it is not intended 
that the precise parameters given within the example should be in any way 
limiting. 
EXAMPLE 
A quantity of kiwifruit is peeled, and sliced to give slices of about 5 to 
6 mm in thickness. An infusion solution is also prepared comprising an 
aqueous solution of invert sugar containing 50% sugar by weight and 1% 
sodium or potassium citrate. 
500 grams of the peeled sliced kiwifruit is then placed in a container and 
500 grams of the prepared infusion solution is added and it is ensured 
that all fruit pieces are completely immersed in this solution. The fruit 
in the infusion solution is then vacuum infused for 15 to 20 minutes. 
The air dried fruit pieces are then transferred to a vacuum drier and 
vacuum dried at about 40.degree.-50.degree. C. until a final moisture 
content of 2% to 4% by weight is reached. Alternatively, the infused 
pieces are first transferred onto mesh trays and air dried at 40.degree. 
to 50.degree. C. until a moisture content of about 45% by weight is 
reached (approximately 4 hours at 40.degree. to 50.degree. C.), and then 
vacuum dried at 40.degree.-50.degree. C. to a 2%-4% moisture content by 
weight. The residual moisture content of the vacuum dried kiwifruit slices 
was removed by storing over silica gel in a dessicator for a period of 1-2 
weeks to give a final moisture content of about zero. 
The infused dried fruit pieces are then packed under vacuum or nitrogen 
flush or with an oxygen scavenger in an oxygen, water vapor, and 
ultra-violet light impermeable packaging medium such as will be known to 
those skilled in the art. It would also be possible to pack the fruit 
pieces in transparent packs having shelf appeal, but for long term storage 
these would preferably themselves be retained in an ultra-violet light 
impermeable package. 
The green color of many green fruit, such as kiwifruit, is due to the 
presence of a group of natural pigments known as chlorophylls. They are 
magnesium chelated tetrapyrroles each having a propionate esterified with 
phytol alcohol at position 7 in the ring structure. In green plant 
material, chlorophylls are known to occur within the plastid bodies known 
as chloroplasts. Chlorophylls are insoluble in water but soluble in 
organic solvents such as alcohol, ether, benzene and acetone. 
Chlorophyll pigments are generally susceptible to degradation due to the 
action of certain enzymes as well as to heat, light and acid. It is known 
that during the drying of green fruit and vegetables the chlorophyll 
pigments are converted to brown pheophytins by the replacement of the 
magnesium ion with hydrogen. This conversion is brought about by organic 
acids that are released when cells are ruptured as well as those that are 
formed during heating. It is also known that lipid degrading enzymes such 
as lipase, lipoxygenase and linoleate hydroperoxide isomerase have a 
bleaching effect on chlorophylls. Dehydrated products in clear packaging 
may undergo photo-oxidation and subsequent loss of green colors. 
In dried kiwifruit in particular the chlorophyll pigments are degraded 
quite rapidly during the drying process and subsequent storage. The 
temperature of drying is very critical with chlorophyll conversion to 
brown pheophytin taking place above 50.degree. C., depending on the time 
of drying and relative humidity of the air. Usually dried kiwifruit is 
olive green to brown in colour and turns to dark brown on storage within a 
few weeks. This change in color is usually accompanied by the development 
of unpleasant odors and flavors. 
It is an advantage of the method of the present invention and of the 
product of the present invention that a dried fruit product may be 
provided which has enhanced stability of certain physical chemical and/or 
sensory features, in particular flavor, color and Vitamin C content and an 
extended shelf life. It is advantageous to produce a green dried fruit 
product from green-fleshed fruit which can be stored for extended periods, 
and which may be used in, for example, cereals containing mixed dried 
fruits, baking, and confectionery products. The product of the present 
invention in at least the preferred form has a shelf life of up to nine 
months or more under suitable packaging, retaining good flavor, color and 
Vitamin C characteristics over this time. In at least the preferred form 
the product of the invention has palatable sweetness and reduced tartness, 
and is also of a desirable texture following the drying sequence described 
herein. Thus it can be seen that the invention provides a method of 
producing a dried fruit product in which certain physical, chemical and/or 
sensory features are stabilized and/or a dried fruit product formed by 
such a process and/or a method of stabilizing color in fruit containing 
chlorophyll during drying, which is advantageous, uses biologically 
acceptable, naturally occuring treatment ingredients, and which can be 
readily applied to produce a useful, palatable product.