1. boundary layer resistance [s/cm]
An integrated approach to indicate freshness of horticultural produce
M. Linke, W. Herppich, and M. Geyer
Institute of Agricultural Engineering Bornim, Potsdam, Germany
Freshness of horticultural produce as an important component of postharvest quality combines a number of extrinsic and intrinsic produce properties. This is one of the main reasons for the difficulties to objectively determine freshness without great efforts. The objective of the present study was to develop a method for the determination of freshness based on easy-to-use measuring procedures.
MATERIAL AND METHODS
To describe the new developed method, greenhouse-grown tomatoes of the variety ‘Clarance’ were used as an example.
Tomato fruits (n=88) were placed in two different types of containers and stored in two typical postharvest environments (21.5°C; 13.0°C) for up to 25 days.
As marketability thresholds, visible or sensible extrinsic properties (wilting, colour, firmness, visible microbial infestation) can be chosen, to allow the consumer to inspect the produce at the time of decision to buy.
Two limit values for the marketability of the produce are established. Depending on the environmental conditions after harvest either water loss or the degradation of valuable compounds will finally induce loss of freshness. In a particular measure the flow conditions close to the produce surface, characterised by a superimposed boundary layer resistance, will affect which of the two limits is reached first.
If the state of the produce on the one hand at the time of harvest is fixed as FI=1.0 and on the other hand is fixed as FI=0.0 when crossing a limit value the freshness indicator can be expressed in the form of :
In the case of tomatoes the current water content related to the water content at harvest was chosen as the limit value 1. The 2nd limit value is characterised by the temperature and time dependent progress of internal degradation of compounds. The produce stiffness was chosen as an applicable
measure that can be verified by the consumer to a certain extent.
Several test persons examined this sensible limit value. The
subjective determination of the stiffness was supplemented
by objective measurement of the elastic produce properTies
using compression tests with a universal testing machine.
RESULTS AND DISCUSSION
According to references in literature the limit value 1 due to water loss was assumed to be 7% related to the water content at the date of harvest.
As the limit value 2 a stiffness of 1.5 N/mm was determined as a result of compression tests. In the figure the time after harvest stands for this limit value. At a constant mean temperature, the time when reaching the limit value 2, varied in dependence of the water status of the produce.
The tomatoes in the containers 3 and 4 with the higher boundary layer resistances showed lower keeping quality. In this context, it is important to note that tomatoes in the containers 3 and 4 had approximately 20 to 30% higher respiration rates during the entire postharvest chain in comparison to the relevant fruits in the containers 1 and 2.
container
In one type of containers the super-imposed boundary layer resistance was determined in the range of 20 s/cm. The value, characterising the flow conditions close to the produce, corresponds to a flat open transport packaging unit at low air movement. The other type of containers with 520 s/cm shows a typical value for a nearly closed packaging under restricted natural convection.
container number
water loss [%]
boundary layer resistance [s/cm]
mean temp. [°C]
FI [-] 1
4.2 20
21.5
0.12 2
3.5
13.0
0.21 3
1.5
520
0.09 4
1.4
0.27
Changes in freshness components of tomatoes subjected to different climatic conditions in the postharvest chain (T…mean temperature; rB…boundary layer resistance)
Freshness indicator FI of tomatoes 300 hours after harvest stored under different climatic conditions.
Using this approach, both the temperature history and information about the water status of the produce are needed. Further investigations have to be carried out to include the effects of microbial activities on the freshness indicator.
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