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Assessing apple attributes

An interactive showing a range of apple attributes and some methods of assessing these.

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Transcript

Introduction

There are about 45 different traits or attributes that breeders test at various stages of developing a new apple cultivar. Some attributes relate to the tree and how it grows and some to physical and sensory attributes of the apple. A new apple cultivar has to meet quality standards for all these attributes before it’s considered for possible commercialisation. Here, you’ll learn how breeders test some of these attributes.

45 traits

Richard Volz (Plant & Food Research)

The 45 traits – if we talk about a trait as being an individual attribute of a fruit, such as crispness or high yield – all these traits in the fruit, and if we count them all up and the number that we assess during the selection process, there is roughly around 45 traits that we are assessing.

There are different sorts of information that we use to choose breeding parents. Some of the most important is actually the phenotype of the individual parents, and what we are trying to do is not just select the 1 parent but also think about the combination of the 2 parents to develop the new seedling that you have in mind, so you will be looking at the fruit quality and the productivity traits of the potential parent.

The things that we concentrate on particularly at the first stage of cultivar breeding is fruit texture, flavour, attractiveness and freedom from disorders. What we have to remember is that we are dealing with thousands and thousands of fruiting seedlings in any 1 season and so any tests at that stage have to be quite quick.

Ripeness

The starch pattern index test identifies when an apple is ripe and ready for picking. As apples ripen, the starch gradually breaks down to simple sugars, making the apple sweeter to eat, so a riper apple has less starch present. The test involves spraying the apple flesh with an iodine solution, which turns the starch black. The starch level can be measured by comparing the test apple to images that show different levels of starch and define the ideal colour pattern.

Flesh firmness

Richard Volz (Plant & Food Research)

So with flesh firmness, we take an individual apple, we peel the skin from a couple of points on the equator of that apple, and we put it into a machine called a penetrometer. Basically what that penetrometer does is it lowers a probe of set size at a set speed into the flesh part of the fruit to a set distance, and the resistance pressure, if you like, that is exerted by the fruit is recorded by a load cell at the top of that probe, and that is converted to a number measured in kilograms force or newtons force, and that is recorded on the computer. And that’s done usually a couple of times for each fruit and repeated again on a number of fruit in your fruit sample.

Sweetness

Richard Volz (Plant & Food Research)

The soluble solids concentration is a de facto measurement for the sugar concentration in the juice of the fruit, and the sugar concentration is a measurement of the sweetness of the fruit. It’s important in terms of how the consumer perceives the taste of that fruit.

For soluble solids concentration, we use a refractometer. A refractometer essentially measures the refract index of the light, and the bending of that light is proportional to the density of the solution, and in apple juice, the main solute in the juice is soluble solids, which is mostly made up of sugars. So essentially what we do is, as soon as the penetrometer has moved up out of the fruit, there is usually a drop of juice and the drop of juice can go onto the refractometer, a button is pushed and a soluble solids reading comes up.

Acidity

Richard Volz (Plant & Food Research)

Acidity measurements are important along with soluble solids concentration and sweetness because, combined with sweetness, it gives an overall measure of the taste of that fruit, so it’s the balance of sweetness and acidity that we are interested in looking at for any 1 particular apple.

Acidity measurements are carried out on juice samples. Here, what we have to do is realise that, in apple, the major acid is malic acid and so therefore we can use a titration technique to assess the acid concentration in apple fruit. The concentration is quite a bit lower than say soluble solids concentration, so here we have to use a relatively large amount of juice and the way we get that is juice an apple in a conventional juicer.

Essentially what we do is we take a base solution and we put that into our unknown juice sample or unknown acid juice sample and we neutralise that acid according to the amount of base that goes into that solution. We know the concentration of the amount of base going in, we know the volume of juice sample, therefore we can get the concentration of that acid in that juice sample.

Texture

Richard Volz (Plant & Food Research)

One of the quickest ways of assessing fruit characters such as texture and flavour is using an individual person, and so we have a set of assessors that taste the fruit and record various attributes.

Texture – we’ve broken it down into 3 main aspects – fruit firmness, fruit crispness and fruit juiciness. So in order for the assessors to become as consistent as possible, we need to make sure that they are calibrated, and one of the ways in which we do this calibration is to have standards.

So for firmness, we are using a standard which is a carrot, simply, and that will be a firmness of 9 on our scale, whereas a banana would be a 0. Then for firmness, it’s simply the force required when you bite down on a flesh of a particular apple. The force will give you a number on that 0–9 scale.

With crispness, celery would be a standard we use, and it’s the pitch and volume of sound generated when that fruit is crushed within your teeth, and for juiciness, it’s simply the amount of juice released in the mouth on that first bite, and watermelon would be a standard we use for juiciness measurements.

So those are the definitions for those different attributes, and the assessors will then give a number on that 0–9 scale as to where that attribute falls for that particular fruit sample.

Longevity

Richard Volz (Plant & Food Research)

Longevity of apples in our breeding programme is assessed by storing the apples in conventional cold storage usually at 0.5° for a period of time and then assessing it after that time.

So the sort of attributes that we are looking at that can essentially limit an apple’s longevity, it’s poor textural characteristics, the production of off flavours and the production of storage-related disorders.

The symptoms of most storage disorders in the fruit or on the surface of the fruit usually take the form of some blemish, usually a discoloration, usually brown or black. The fruit can go soggy, and there is a whole lot of different symptoms that are prevalent depending upon what storage disorder you are looking at. The essential thing is that they look unsightly, they can themselves produce off flavours in the fruit, and it’s something we don't want in our fruit.

Disease resistance

Richard Volz (Plant & Food Research)

Disease screening is a means of determining whether individual plants or seedlings are susceptible to diseases that occur in the orchard – that is obviously important from the grower perspective. We are wanting to determine which ones are very susceptible, and we want to remove those from the breeding population so that we concentrate on those plants that are more resistant to those particular diseases.

And in this case, we are looking at the important bacterial disease fire blight. And what we have done about 4 weeks ago was inoculate each of these plants with fire blight by simply cutting the leaves in half when the plant was about this high or so. In this case, this plant here is showing no symptoms – it’s continued to grow very well. In this case, the plant was inoculated at this point, and the disease has caused this necrosis. It stopped growing and died back, and you can see all the brown leaves and the lesion occurring on the stem.

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