Measuring texture quality of fresh jujube fruit using texture analyser

Abstract：【Objective】Traditionally, some adjectives such as compact, loose, crisp, soft and hard are used to define fruit texture, which are highly subjective and of low precision. Texture analyser has been used to measure the rheological properties of the whole fruit and quantify fruit texture indexes including hardness, strength, toughness, brittleness and firmness at the same time, which reflect the fruit properties objectively and accurately. Different probes and puncture speeds of the texture analyzer, puncture site of the fruit were studied on fresh jujube fruit at different maturity in order to establish unified method for measuring texture quality of fresh jujube fruit.【Methods】 (1) For selection of probe, the crisp ripe half red fruit of‘Hupingzao'was used as the material and tested with a TA.XT plus texture analyser. The displacement mode was used and two probes including P/2 (cylindricality, 2 mm in diameter) and P/2 n (needle-like, 2 mm in diameter) were compared. The test speed of the probe was 2 mm· s-1 to a depth of 5 mm; the pre-test speed was 1 mm· s-1 and the post-test speed 10 mm· s-1; the minimum induction force was 5 g; and the data acquisition rate was 400. Fruit samples were placed at the appropriate position on the loading table. The smooth surface at the middle part was used as the test surface. (2) For comparison of test speed, a P/2 probe was used. Different test speeds of 1 mm· s-1, 2 mm· s-1 and 5 mm· s-1 were set to puncture the crisp ripe fruits of three varieties including‘Hupingzao'‘Xiangzao'and‘Shandonglizao'to determine the best test speed. The middle part of the fruit was punctured during thetest. The pre-and post-test speeds, minimum induction force, data acquisition rate and sample placement were the same as for probe selection. (3) For comparison of puncture sites of fruit, a P/2 probe and2 mm· s-1 test speed were used, and texture parameters were measured at three sites including fruit shoulder, middle part and bottom part of the crisp ripe half red fruits of‘Hupingzao'and‘linyilizao'. The other instrument parameters and sample placement method were the same as before. (4) For determination the best fruit ripening stage for the measurement, fruits at white ripe stage, crisp ripe half red stage, crisp ripe full red stage, full ripe stage (slightly soft full red) of four varieties including‘Linyilizao'‘Lengbaiyuzao'‘Fengmiguanzao'and‘Popozao'were measured at middle part.【Results】 (1) The puncture force of P/2 n was concentrated at a point on the peel, and the force value of pericarp breaking could be used as the pericarp strength value. Therefore, the results from the P/2 n probe did not fully reflect the pericarp texture. On the contrary, P/2 probe obtained more consistent values of mean flesh firmness and flesh fiber index. Therefore, the P/2 probe was more suitable for analyzing the texture quality of fresh jujube fruit. (2) Differences in the six textural parameters including pericarp strength, pericarp break distance, pericarp toughness, pericarp to pulp brittleness, mean flesh firmness and flesh fiber index of three varieties indicated that when the test speed was too small, the puncture time was too long, which increased the relative error. When the test speed was too high, relatively large impact force concealed subtle differences in fruit texture quality among varieties. Taking this into consideration, we recommended test speed of 2 mm · s-1 for measuring the texture properties of different varieties of jujube fruit. (3) The order of texture difference was inconsistent between the two test sites of fruit, i.e. fruit shoulder and fruit bottom. The puncture in the middle part of fruit was not only easy to operate, but also reflected the true characteristics of varieties. Therefore, the middle part of the fruit was the best puncture site. (4) The absolute values of pericarp strength, pericarp break distance, pericarp toughness, pericarp to pulp brittleness and mean flesh firmness decreased gradually with the ripening of the fruit; the pericarp strength, pericarp break distance and pericarp toughness increased again in the mature period;flesh fiber index showed a low-high-low tendency as the fruit became ripe. From white ripe stage to fully ripe stage, the fruit turned soft, and the texture parameters changed accordingly. At the full red crisp ripe stage, the accumulation of nutrients was the highest and was the best time for measuring texture properties of fresh jujube fruits.【Conclusion】Texture quality of fresh jujube fruit is suggested to be measured at full red crisp ripe stage using a P/2 probe, a pre-test speed of 1 mm· s-1, a test speed of 2 mm· s-1, a post-test speed of 10 mm· s-1, a minimum induction force of 5 g, and a data acquisition rate of 400. The smooth surface of the middle fruit part is chosen as the test site and the opposite side placed against the loading table was cut flat close to the stone. The obtained parameters including pericarp strength, pericarp break distance, pericarp toughness, pericarp to pulp brittleness, mean flesh firmness and flesh fiber index can accurately reflect the texture characteristics of flesh jujube fruit.