What Makes Fruit Firm and How to Keep It That Way
Firmness and Consumer Acceptability
Texture and flavor are key quality characteristics that influence consumer acceptability of apples. Research has identified that British consumers tend to segment according to whether they prefer sweet, hard apples or juicy, acidic apples (Daillant-Spinnler et al. 1996). More recently, Jaeger et al. (1998) focused on the impact of meatiness on consumer preferences for apples including cultivars Cox's Orange Pippin (COP), Jonathon, and Boskoop. For each of the three cultivars, the consumers disliked mealy apples. However, consumer's disliking of the flavor of Boskoop had a major influence on the results (Jaeger et al. 1998). From a practical perspective, these studies demonstrate that the apple industry needs to focus on the needs of consumers for flavorsome fruit with an appropriate sweet-acid balance, as well as providing firm fruit.
Apple texture is a function of three food characteristics: the mechanical properties of the tissue, the juiciness of the flesh, and the feel of the tissue in the mouth as it breaks down (mouthfeel). A number of words can be used to describe these texture characteristics in terms that consumers can understand (Table 1). However, the apple industry tends to regulate itself on the basis of instrumental measurements of texture. The most important of these is the penetrometer test (also known as a puncture and/or firmness test), which measures the maximum force required to push a probe into the flesh. We have had a longstanding interest in how these instrumental measurements can be related to apple texture as perceived by people.
Table 1. Sensory texture attributes and their definitions.
|Crispness||Amount and pitch of sound generated when the sample is first bitten with the front teeth|
|Firmness||Force required to bite into the sample|
|Initial juiciness||Amount of juice released from the sample in the first three chews, when chewing with the back teeth|
|Crunchiness||Amount of noise generated when chewing with the back teeth|
|Ease of breakdown||Amount of chewing required to break down the flexh so that it can be swallowed|
|Sustained juiciness||Amount of juice released from the sample during prolonged chewing|
|Pulpiness||Amount of wet, web-like material that develops during chewing|
|Mealiness||Degreee to which the flesh breaks down to a fine lumpy mass|
|Flouriness||Degree to which the flesh breaks down to very fine dry particles|
Influence of Fruit Structure on Firmness
As people bite into an apple, the deformation that occurs tends to emphasize the tensile properties of the flesh. We have found that there are three distinct mechanisms for tissue failure during tensile testing of tissue strength (Harker et al. 1997):
- The cells may break cleanly through the equator and abruptly releases the juice from inside each cell (described as cell fracture)
- A small piece of the cell may be pulled off causing the subsequent collapse and release of juice (described as cell rupture)
- Intact cells may separate from neighboring cells without breaking or releasing juice (described as cell-to-cell debonding).
Examples of cell rupture and cell-to-cell debonding can be seen in Figure 1, which shows the fracture surface following tensile testing of tissue from firm and mealy Braeburn apples. As apples soften, the mechanism of tissue failure changes from cell fracture to cell rupture to cell-to-cell debonding. Fruit firmer than about 15 lb tend to fail by cell fracture, while fruit softer than about 9 lb tend fail by cell-to-cell debonding, and have a mealy texture.
Figure 1. Fracture surfaces of mealy (upper picture) and crisp (lower picture) Braeburn apples. The upper picture shows cell-to-cell debonding, while the lower picture shows cell rupturing.
Correlations Between Instrumental and Sensory Measurements
In our experience, penetrometer measurements are good predictors of sensory attributes "crispness," "firmness," "crunchiness," and "juiciness" as measured by the people we train to assess the texture of apples. In Figure 2, we show the best and worst correlation obtained in a study of texture of apples (the average correlation was about 0.87). When the firmness of two apples differs by more than 1.2 lb, the texture is generally different according to our trained sensory panel. However, differences in texture are often subtle. In some studies, fruit may appear to be similar in texture according to penetrometer measurement (i.e., statistically they are not significantly different), but are different according to evaluation by trained sensory panel.
Figure 2. Correlations between instrumental measurements of firmness and sensory assessment of crispness for individual people (trained sensory panelists). Graphs indicate the best and worst correlations, respectively (total of 16 panelists used in the study).
Instrumental and Operator Influences on Firmness Mmeasurement
The apple industry tends to use a wide range of penetrometers. Some of these machines may provide inherently different firmness values, while some hand-operated machines are greatly affected by the way individual people make a firmness measurement. In our experience, the EPT pressure tester tends to overestimate the firmness of soft fruit, but minimises operator differences, while the Effegi hand-held penetrometer is prone to give different firmness values depending on who makes the measurement (Harker et al. 1996).
Soft Apple Fruit - the International Experience
Over the past 20 years, a number of apple industries have had to face the problem that their fruit was too soft to fulfill market expectations. This has become an increasingly important issue as retail organizations have imposed firmness standards and industries have established there own regulations on minimum acceptable firmness. The problems have been most difficult for those cultivars that have an inherently poor storage potential such as COP apples. A British survey of the quality of COP over an 18-year period between 1966 and 1984 showed that firmness declined by 3 lb even though controlled atmosphere storage (CA) conditions improved (Horscroft 1989). Horscroft (1989) pointed out that the decline in firmness experienced by the British COP industry coincided with the transition to modern intensive growing systems where the yield per hectare of class 1 fruit was the all important factor.
Factors That Influence Firmness of Apples
There have been a number of studies that have considered the impact of preharvest and postharvest factors on the firmness of fruit (reviewed by Harker et al. 1997), but there has been no systematic attempt to quantify the relative importance of these factors. Here, we examine the literature and use firmness difference as a tool to identify the key factors for producing firm fruit. Firmness difference relates to the difference between firmness of apples subjected to a particular treatment and the appropriate control. The information hopefully will provide a useful starting point for the Washington apple industry in its discussion on Red Delicious. Our aim is to consider those factors that can influence the quality of apples growing in existing orchards. Obviously, when planting a new orchard, the choice of cultivar will have a major influence on quality and storage potential of the harvested fruit.
The literature we have reviewed relates to different cultivars, fruit stored in both air and CA, and fruit stored for different durations. There is an inherent risk associated with the variability in experimental protocols that should be noted. One important issue to consider is the influence of storage duration on the firmness difference between treatments. Apple softening does not result in a linear decline in firmness. Rather, there are three phases to apple softening: an initial period during which there is little or no decline in firmness; followed by a period of rapid decline in firmness; and finally another period of little or no decline in firmness (Figure 3). Depending on the time that differences in firmness are assessed, there will be a substantial or little influence of treatment. For example, in Figure 3 the apples remain firmer when stored at 0 °C than at 3 °C for storage duration's less than 50 days, but there was no benefit of storage at 0 °C compared to 3 °C after 75 days.
Figure 3. Softening curves for Royal Gala apples held at different temperatures.
It is often difficult to identify the reasons that a treatment provides firmer fruit. This is often the case for preharvest treatments that result in changes to a number of different quality factors. For example, improvements in firmness associated with the use of rootstocks (Autio 1991) may be attributed to differences in crop load, harvest maturity, and fruit calcium content. The analysis of the literature (Appendix 1 and 2) indicates that the key factors influencing firmness that growers and packers have some control over include:
- Crop load
- Fruit size
- Calcium content
- Harvest maturity
- Storage temperature
- CA storage
Storage temperature and conditions of CA storage have not been considered in detail in this review, since both these issues have been well covered in existing literature (e.g., Kupferman 1997).
Crop Load and Fruit Size
The way the crop is established on the tree can have a profound influence on the final quality of apples. The number of fruit on the tree will influence both fruit size and fruit firmness. Large fruit tend to be softer than small fruit (Blanpied et al. 1978). This influence of size on firmness is generally thought to be a consequence of differences in cell expansion. Large fruit tend to have the same number of cells as smaller fruit, but the cells are larger and exhibit less cell-to-cell contact.
The influence of crop load on firmness cannot be solely explained by differences in fruit size. Results such as those presented in Figure 4 indicate that differences in firmness exist even when fruit are matched for size. Firmness is generally higher in fruit from low crop loads. The time that trees are thinned can also have an influence on firmness. Research on COP demonstrated that improvements in fruit firmness due to low crop load were only evident when fruit thinning occurred between 5 and 15 days after full bloom (Johnson 1992).
Figure 4. Influence of crop load on harvest firmness of Braeburn apples. Crop load is calculated as the number of apples divided by the trunk cross sectional area (TCA). Each point represents mean harvest firmness for 10 apples (each weighing 190 to 210 g) from an individual tree.
Increasing the calcium content of fruit has a beneficial influence on firmness. Postharvest dipping and vacuum infiltration with 4% calcium solutions have had a consistent beneficial effect on maintaining firmness during storage. At out-turn calcium-treated fruit can be as much as 4 lb firmer than control treatments. Indeed, in some cases calcium has prolonged the period of slow softening, or even resulted in slight increases in firmness during the first phase of the softening curve (Abbott et al. 1989).
Apples tend to soften during the maturation period, although the extent of softening may vary from cultivar to cultivar. Gala is a cultivar that tends to loose firmness quite rapidly during the harvest period (Watkins et al. 1993), and a delay in harvest date can have a profound influence on firmness at harvest and during storage. In New Zealand, recent changes in harvest maturity criteria have gone some way to reduce the risk of soft fruit in the marketplace.
Prediction of Firmness
Firmness of fruit after removal from storage tends to be closely related to firmness at harvest. However, it is difficult to predict absolute values of firmness as fruit are removed from cool storage, since the rate of softening varies from year to year and from fruit line to fruit line. The most successful prediction of firmness tends to take into account the factors described above, particularly harvest firmness and maturity (Knee and Farman 1989). In the absence of robust predictors of firmness, it is important to monitor decline in firmness during storage. Non-destructive firmness monitoring of fruit in cool storage may be an important tool in the future.
Temperature Management and CA Storage
In general, the treatment of fruit at harvest and during storage has a far bigger impact on the firmness of fruit than the preharvest factors. Two key factors during storage are temperature and CA conditions. The importance of storage temperature is clearly demonstrated in Figure 3. Recommendations on storage temperature and CA conditions are well covered in existing literature (e.g., Hardenberg et al. 1986; Kupferman 1997). In addition, it is likely that the loss of water from fruit during storage, and the associated decline in torpor (internal pressure of cells) may reduce firmness loss in some cultivars (Hatfield and Knee 1989). However, more research is required on this before recommendations can be made.
Texture attributes, such as crispness, hardness and juiciness, are important to consumers and can be predicted using penetrometer measurements of firmness. Therefore, it is important to optimize firmness through preharvest treatments that make fruit firm, and harvest management and storage conditions that keep fruit firm. The key factors that influence firmness and are under the control of the grower and packhouse include crop load, fruit size, calcium content, harvest maturity, storage temperature, and CA conditions.
Abbott, J.A., W.S. Conway, and C.E. Sams. 1989. Postharvest calcium chloride infiltration affects textural attributes of apples. J. Am. Soc. Hort. Sci. 114:932-936.
Autio, W.R. 1991. Rootstocks affect ripening and other qualities of 'Delicious' apples. J. Am. Soc. Hort. Sci. 116:378-382.
Blanpied, G.D., W.J. Bramlage, D.H. Dewey, R.L. LaBelle, L.M. Massey Jr., G.E. Mattus, W.C. Stiles, and A.E. Watada. 1978. A standardized method for collecting apple pressure test data. Bul. 74 Cornell Expt. Sta.
Conway, W.S., and C.E. Sams. 1987. The effects of postharvest infiltration of calcium, magnesium, or strontium on decay, firmness, respiration, and ethylene production in apples. J. Am. Soc. Hort. Sci. 112:300-303.
Daillant-Spinnler, B., H.J.H. MacFie, P.K. Beyts, and D. Hedderley. 1996. Relationships between perceived sensory properties and major preference directions of 12 varieties of apples from the southern hemisphere. Food Quality Preference. 7:113-126.
Hardenburg, R.E., A.E. Watada, and C.Y. Wang. 1986. The commercial storage of fruits, vegetables, and florist and nursery stocks. USDA Agr. Handbook. U.S. Govt. Printing Office, Washington, D.C.
Harker F.R., J.H. Main Donald, and P.J. Jackson. 1996. Penetrometer measurement of apple and kiwifruit firmness: operator and instrument differences. J. Amer. Soc. Hortic. Sci. 121:927-936.
Harker, F.R., R.J. Redgwell, I.C. Hallett, S.H. Murray, and G. Carter. 1997. Texture of fresh fruit. Hortic. Rev. 20:121-224.
Hatfield, S.G.S., and M. Knee. 1988. Effects of water loss on apples in storage. Int. J. Food Sci. Technol. 23:575-583.
Hipps, N.A., and M.A. Perring. 1989. Effects of soil management systems and nitrogen fertiliser on the firmness and mean fruit weight of Cox's Orange Pippin apples at harvest. J. Sci. Food Agr. 48:507-510.
Horscroft, J. 1989. Production factors influencing the textural qualities of apples. Ph.D. Thesis, Wye College, Univ. London.
Jaeger, S.R., Z. Andani, I.N. Wakeling, and H.J.H. MacFie. 1998. Consumer preferences for fresh and aged apples: a cross-cultural comparison. Food Quality Preference. 9:355-366.
Johnson, D.S. 1992. The effect of flower and fruit thinning on the firmness of 'Cox's Orange Pippin' apples at harvest and after storage. J. Hort. Sci. 67:95-101.
Klein, J.D., S. Lurie, and R. Ben-Arie. 1990. Quality and cell wall components of 'Anna' and 'Granny Smith' apples treated with heat, calcium, and ethylene. J. Am. Soc. Hort. Sci. 115:954-958.
Knee, M., and D. Farman. 1989. Sources of variation in the quality of CA stored apples. Proc. 5th Int. Controlled Atmosphere Research Conference, Wenatchee, Washington, USA. p. 255-261.
Kupferman, E. 1997. Controlled atmosphere storage of apples. Proceedings of the 7th International Controlled Atmosphere Research Conference; Vol 2 (Ed. E.J. Mitcham) pp. 1-30.
Meheriuk, M., and W.D. Lane. 1983. A comparison of fruit from spur and standard ?McIntosh? at harvest and after controlled-atmosphere storage. HortScience 18:220-221.
Neilsen, G.H., M. Meheriuk, and E.J. Hogue. 1984. The effect of orchard floor management and nitrogen fertilization on nutrient uptake and fruit quality of 'Golden Delicious' apple trees. HortScience. 19:547-550.
Porritt, S.W., and P.D. Lidster. 1978. The effect of pre-storage heating on ripening and senescence of apples during cold storage. J. Am. Soc. Hort. Sci. 103:584-587.
Saltveit, M.E. Jr. 1984. Effects of temperature on firmness and bruising of 'Sarkrimson Delicious' and 'Golden Delicious' apples. HortScience. 19:550-551.
Sams, C.E., W.S. Conway, J.A. Abbott, R.J. Lewis, and N. Ben-Shalom. 1993. Firmness and decay of apples following postharvest pressure infiltration of calcium and heat treatment. J. Am. Soc. Hort. Sci. 118:623-627.
Scott, K.J, and R.B.H. Wills. 1977. Vacuum infilration of calcium chloride: A method for reducing bitter pit and senescence of apples during storage at ambient temperatures. HortScience. 12:71-72.
Watkins, C., R. Harker, P. Brookfield, and S. Tustin. 1993. Maturity of Royal Gala, Braeburn and Fuji - the New Zealand experience. Proc. 9th Annual Washington Treefruit Postharvest Conf., p.16-19.
Appendix I - Influence of Preharvest Factors on Firmness of Apples After Storage
Examples showing the influence of preharvest factors on firmness of apples after storage. The values represent the difference in firmness between treatments and controls after removal from storage. The data relates to different cultivars, different storage conditions and different storage durations, but gives an initial indication of the relative importance of different factors.
|Factor||Magnitude of firmness|
|Region/site||3||R. Gala||Watkins et al. 1993|
|Spur type||0.85||McIntosh||Meheriuk & Lane 1983|
COP (at harvest)
|Blanpeid et al. 1978|
Hipps & Perring 1989
Neilsen et al. 1984
|G. Delicious||Neilsen et al. 1984|
|Crop load||4||Braeburn||Volz (unpublished)|
|Time of thinning||1.3||COP||Johnson 1992|
|Size||4.8||G. Delicious||Blanpied et al. 1978|
Appendix II - Influence of Postharvest Factors on Firmness of Apples After Storage
Examples showing the influence of postharvest factors on firmness of apples after storage. The values represent the difference in firmness between treatments and controls after removal from storage. The data relates to different cultivars, different storage conditions and different storage durations, but gives an initial indication of the relative importance of different factors.
|Factor||Magnitude of firmness|
|Maturity||3||R. Gala||Watkins et al. 1993|
|1.5 to 3.3|
|COP and Gravenstein|
|Scott & Wills 1977|
Conway & Sams 1987
Sams et al. 1993
|Water loss||1.3||COP||Hatfield & Knee 1988|
|Assessment temperature||0||G. Delicious||Saltveit 1984|
(4 days @ 38 °C)
|Porritt & Lidster 1978|
Klein et al. 1990
Sams et al. 1993
F. Roger Harker(1), Richard Volz(2), Jason W. Johnston(3), Ian C. Hallett(1), and Nele DeBelie(4)
(1)The Horticulture and Food Research Institute of New Zealand, Mt. Albert Research Centre, Private Bag 92 619
Auckland, New Zealand
(2)The Horticulture and Food Research Institute of New Zealand, Hawke's Bay Research Centre, Private Bag 1401
Havelock North, New Zealand
Private Bag 11222
Palmerston North, New Zealand
(4)Department of Agro-engineering and -economics, Katholieke Universiteit Leuven
B-3001 Heverlee, Belgium
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000