At the packinghouse, after washing, sorting is done to elimi- nate fruit with visual defects and sometimes to divert fruit of high surface color to a high-quality pack (sizing segregates fruit by either weight or dimension). Ideal storage conditions for plums are a temperature of 0◦C and 90–95% relative humidity; under such conditions, storage life is 2–4 weeks (Crisosto and Kader 2000). Martinez-Romero et al. (2003) reported that forced-air cooling after harvesting and before transportation to packinghouse, handling in packinghouse, and during storage helped maintain fruit quality and prolong shelf life. Their research indicated that forced-air cooling led to a reduction in respiration rate of mechanically damaged plums that otherwise deteriorate rapidly.
Guerra et al. (2009) conducted a study to determine storage capacity and changes in quality and consumer acceptance of plums harvested at different dates, and to assess the instru- mental parameters that showed good correlation with sen- sory quality and consumer acceptance. The results showed that both harvest date and storage time had significant impact on quality and sensory characteristics. Furthermore, a high degree of linear regression was observed between color pa- rameter, a*, and total soluble solids and acidity ratio, which suggested that nondestructive measurement of instrumental color a* value could be used as an estimate of consumer acceptance.
Generally, during postharvest storage, concentration of sugars increases and organic acids decrease. Figure 31.2 shows changes in concentrations of sugars and organic acids during 0◦C storage of ‘Amber Jewel’ plums harvested on two different dates (D1 and D2) 1 week apart (Singh et al. 2009).
The increase in total sugar levels between two harvesting dates 7 days apart (129 and 136 days after full bloom) was not significant, but the concentrations of individual sugars changed. It was also reported that fruit harvested on both dates showed a similar increase in concentration of total sug- ars during storage at 0◦C.
During postharvest storage of fruits, ethylene triggers many, though not all, aspects of fruit ripening. Abdi et al.
(1998) reported that within a fruit species, such as plum, high-ethylene producers would soften and ripen at a faster
Total sugars
Fructose
Glucose
Sorbitol
g/100 g FWg/100 g FWg/100 g FW mg/g FWmg/g FWmg/g FWmg/g FWmg/g FW
g/100 g FWg/100 g FW
Sucrose Tartaric acid
Citric acid Succinic acid Malic acid Total organic acids
D1 D2
16
12
8 14
11
8
2 5
1
0.2 0
0.1
0.0 0.06
0.05
1
0 2 3
Storage (week) Storage (week)
4 5 6 7 0 1 2 3 4 5 6 7
0.04
0.03 1
2 3 1 2 3 1 2 3 1 2 3 4 8 9 10 11
Figure 31.2. Changes in concentrations of sugars and organic acids during 0◦C storage of “Amber Jewel” plums harvested on two different dates (D1 and D2) 1 week apart. (From Singh et al. 2009).
rate than low ethylene producer cultivars. The ethylene action inhibitor 1-methylcyclopropene (1-MCP) has been shown to delay ripening and improve postharvest quality of climacteric fruits (Abdi et al. 1998; Dong et al. 2002).
Menniti et al. (2006) reported that an application of 1-MCP before air storage of plums could be the best way to reduce the ripening process for short or medium storage periods (40 and 60 days) while controlled atmosphere (CA) storage plus 1-MCP treatment could be used for long-term (80 days) qual- ity retention. Ozkaya and Dundar (2009) studied the effect of 1-MCP on different quality parameters and respiration rate of
‘Black Diamond’ plums during storage that were harvested at commercial maturity. Treatment with 1-MCP was effective in reducing weight loss and maintaining firmness during 30- day storage at 0◦C and 5 days of shelf life. The other quality parameters (titratable acidity, color, total soluble solids, and individual sugars) were not affected significantly by 1-MCP.
Traditional use of 1-MCP has been in the gaseous form but a recent study (Manganaris et al. 2008a) also reported using this treatment in the form of an immersion method for plum fruit to extend their shelf life.
Edible coatings have been used for a variety of fruits to extend their shelf life. Eum et al. (2009) treated ‘Sapphire’
plums with a coating based on carbohydrate (Versasheen) with sorbitol as plasticizer and stored at 20◦C and 85% RH.
The effect of coating on the gas transmission rates was es- timated to assess coating efficiency. The coating treatment reduced the transmission rate of CO2, O2, and H2O. The loss of firmness was delayed as a result of coating treat- ment thereby improving keeping quality of plums. Navarro- Tarazaga et al. (2008) studied the effects of the composition of hydroxypropyl methylcellulose-beeswax edible coatings on the postharvest quality of coated ‘Angeleno’ plums; the coating treatments reduced plum softening and bleeding but were not effective in reducing fruit weight loss.
A number of other methods of improving postharvest qual- ity of plums have been reported in the literature. Treatment of plums with nitric oxide effectively reducing oxidative damage during postharvest storage at 25◦C storage for 13 days (Yao et al. 2010). Luo et al. (2010) explored the po- tential of hot-air treatment to delay ripening of ‘Qingnai’
plums as a potential technology to expand the marketing of green plums was determined. Heat-treated fruits changed from green to yellow at a slower rate than control fruits. The hot-air treatment extended the postharvest life of ‘Qingnai’
plums by up to 6 days, from 12 to 18 days. Perez-Vicente et al. (2002) investigated the role of exogenously applied putrescine, a polyamine, during postharvest storage of me- chanically damaged plums; exogenous putrescine inhibited and delayed ethylene and CO2production rates.
Chilling injury and internal browning are the two physi- ological disorders reported in plums. Most plum and fresh prune cultivars exhibit flesh translucency associated with flesh browning as chilling injury, or “gel breakdown” a term used in South Africa for chilling injury (Taylor et al. 1995;
Crisosto et al. 1999;). Internal browning is a physiological disorder that originates before the harvest and is associated with high temperatures during fruit maturation and delayed harvest. Polyphenol oxidase can initiate flesh discoloration in injured or bruised fruits (Siddiq et al. 1993; 1996). This enzyme not only affects sensory color but can also be detri- mental to nutritional quality. Guerra and Casquero (2009) showed that prompt cooling is important for the physiochem- ical and sensory quality of plums and that delayed cooling can increase internal breakdown symptoms in fruit that can lead to chilling injury. Plums are also susceptible to flesh reddening, a disorder associated with chilling injury (Man- ganaris et al. 2008b). Delayed storage can increase severity of flesh reddening, which is accompanied by an increase in anthocyanins content. It was also suggested that continuous exposure to ethylene resulted in particularly marked increases in reddening (Manganaris et al. 2008b).
PROCESSED PRODUCTS
Plums have a great potential as a fresh market and/or process- ing crop, which can be harvested between cherry and apples in many areas, especially in the Unites States. About one- half of the plums are consumed fresh while the rest are pro- cessed. The major processed plum products are dried prunes, prune juice, and whole canned plums. The amount of plums processed as canned has decreased consistently and signif- icantly during the last two decades; from 39,100 pounds in 1990–1991 to 6,400 pounds in 2008–2009 (Fig. 31.3).
400 350 300 250 200 150 100 50 0
1990–1991 1995–1996 2000–2001 2005–2006 2008–2009
227
39
17 16 7 6
159
233 363
Canned Dried 310
‘000’ pounds
Figure 31.3. US processing of canned and dried plums for selected years since 1990–1991. (Adapted from USDA-ERS 2010).
Processing of dried plums had about 60% increase between 1990–1991 and 2008–2009 (to 363,200 pounds); however, the amount of plums dried in 2008–2009 has dropped back to slightly above 2005–2006 levels (USDA-ERS 2010).
Other processed forms, such as paste, sauce, and plum juice have not been developed and marketed on a scale similar to these products from other fruits such as apples, cherries, citrus, pears, apricots, etc. (Chang et al. 1994; Espie 1992).
However, with new scientific claims of many health benefits of the phytochemicals (plums being rich in many), plums have a great potential for a variety of new processed products than are currently available in the market.
Dried Plums/Prunes
Dried plums or prunes represent over one-half of the pro- cessed plum products consumed on a per capita basis in the United States (Table 31.4). Fruit is harvested at full maturity when soluble solids contents reach at least 22% (22◦Brix).
Other maturity indices include fruit firmness, flesh, and skin color. Modern dehydrators have replaced the old methods of drying prunes in the sun in the United States (Somogyi 1996). However, in many of the plum-producing developing countries, sun drying still continues to be one of the cheapest sources of drying plums.
Typical processing steps involved in the production of prunes are listed as follows (Somogyi 1996):
r Plum quality analysis at harvest (Brix, titratable acidity, color, % initial moisture).
r Air classification to remove leaves, stems, and extrane- ous matter followed by a wash with approximately 20 ppm chlorinated water.
r Washed plums loaded onto drying trays and trays trans- ferred to dryers.
r Drying at 140–165◦F for 24–36 hours (to a moisture of about 16±3% from initial moisture of∼80%).
r Dried prunes stored at ambient temperature for about 1–2 weeks for moisture equilibration.
r Packaged in high density polyethylene-line corrugated r box.Stored at dry, cool, condition (preferably refrigerated,
40–55◦F).
Prunes are dried to about 18% moisture, which has suf- ficiently low water activity to avoid problems of microbial spoilage allowing long-term storage (Newman et al. 1996).
Generally, forced-draft tunnel dehydrators are used for drying plums, with a total drying process time of 24–36 hours, de- pending on the size and soluble solids contents of the prunes.
The operating temperature for the tunnel is 145–165◦F (dry bulb temperature) with wet bulb 15◦F lower than the dry bulb at the cool end. Yield of dried prunes is about 33%.
For the prunes that are marketed as “Dried Prunes with Pits,”
dried fruit from above step are rehydrated to 24–30% mois- ture, pasteurized, inspected and bulk- or retail-packaged for
food service or consumer use, respectively. Potassium sor- bate is the most commonly used preservative for prunes when moisture contents of the finished prunes are higher than 25%. Some products made from dried prunes, on a smaller scale, are prune juice, juice concentrate, whole pitted prunes, canned prunes (pitted), and various forms of dry and low- moisture products (diced prunes, prune bits, prune paste, low-moisture prune granules, low-moisture prune powder, prune fiber, prune fillings and toppings).
Most of recent research related to prune processing has been focused on ways to improve efficiency of the drying methods with due consideration given to the retention of best quality characteristics (both chemical and nutritional) in the finished product. In order to increase the drying rate, Jazini and Hatamipour (2010) employed a new physical pre- treatment of plums that consisted of piercing fruit with a thin needle. The effect of physical pretreatment on drying time was compared with chemical pretreatment (dipping of plums in hot 1% NaOH solution). Their results showed that pierced plums dried faster than chemically pretreated plums. Koocheki and Zarpazhooh (2010) investigated wa- ter loss (WL), solid gain (SG), weight reduction (WR), and shrinkage during osmotic drying of plums, using response surface methodology. In most cases, increases in sucrose con- centration, temperature and immersion time increased WL, SG, WR, and shrinkage. Immersion time and temperature were the most significant factors that affected WL during osmotic dehydration of plums followed by the concentra- tion of sucrose solution. The partial replacement of sucrose and monosaccharides by fructooligosaccharides reduced the calorific value of carbohydrates in osmotically dried material by 12–37% depending on process conditions (Klewicki and Uczciwek 2008).
Doymaz (2004) studied the effect of a dipping treatment on air-drying of plums, and observed that dipping of plums in 5% potassium carbonate and 2% ethyl oleate for 1 minute was effective in removing the natural wax coating and hence reduced the drying time by about 30% as compared to un- treated fruit. Cinquanta et al. (2002) reported that physical (abrasion) and chemical (alkaline ethyl oleate dip) pretreat- ments before drying significantly reduced losses of total phe- nols. Gabas et al. (2002) studied the rheological properties of prune as a function of drying conditions. Their data showed that prunes exhibited a more pronounced elasticity at low moisture content and drying temperature. Higher moisture content and temperature resulted in more viscous and less rigid prunes. Sabarez et al. (2000) used solid phase micro- extraction (SPME) in conjunction with GC–MS to monitor changes in some major volatile flavor compounds under sim- ulated commercial drying conditions (80◦C air temp. 35%
RH, 5 m/s air velocity) for ‘d’Agen’ plums. They observed that aroma profile was significantly modified during drying and substantial loss of the original volatile flavors. Piga et al.
(2003) reported that drying had a detrimental effect on an- thocyanins and ascorbic acid in plum cultivars they studied.
Table 31.5. Functional Attributes of Dried Plums Functional Attribute Reduction/Replacement Of Humectancy Mono- and diglycerides Natural color Caramel color, molasses Flavor enhancement Salt, artificial flavors Natural sweetness Refined sugars Natural preservative Calcium propionate
Fat replacement Emulsifiers, modified starches, fats Source: American Society of Bakery Engineers (Sanders 1993).
Stier (2008) reported a number of properties of dried plums with potential benefit in processed food products (high fiber content, natural source of sorbitol, high levels of malic acid, low glycemic index, and flavor enhancer). These nutritional and functional properties make dried plums, plum powders, or juice concentrates suitable for use in meat and poultry products, bakery products, sauces, marinades, etc. Sanders (1993) noted a variety of functional attributes of dried plums (Table 31.5); that include natural preservative and a fat sub- stitute, which could be of special benefits owing to more emphasis on low-fat foods and products processed without added preservatives by the consumers.
Dried plum products are suitable for use in many bak- ery products as shown in Table 31.6, which in addition to added nutritional benefits, can also improve desirable sensory attributes.
Prune Juice
Prune juice is essentially a water extract of dried prunes with a Brix of 18.5◦. It is a brownish to reddish brown liquid having taste and flavor of prunes. The process involves di- rect heating of the dried prunes in appropriate volumes of water (typically 4–5 times the fruit weight) to extract fruit solids without burning and affecting flavor and color. Tradi- tionally, the process can take several hours (1 h boil, 10 h
Table 31.6. Dried Plum-Based Bakery Ingredients
Product Form Suggested Uses
Diced dried plums and extruded bits
Breads, muffins, cookies, cakes, fillings
Dried plum paste Fillings, breads, bagels, cookies Dried plum juice
concentrate
Breads, pastries, cakes, muffins, cookies, fillings
Dried plum powder/
granules/flakes
Dry mixes, bagels, reduced-fat/
fat-free mixes
Dried plum puree Reduced-fat/fat-free bakery cakes, cookies, muffins, fillings Source: American Society of Bakery Engineers (Sanders 1993).
simmering) and under atmospheric cooking. A short dura- tion (10–15 minutes) pressure-cooking may speed-up juice extraction. After extraction, other unit operations of filtration (to remove pits and undissolved solids), pasteurization (88◦C for 1 minute) and packaging can be similar to processing of other juices. The juice thus made can also be concentrated in a vacuum evaporator. For canning and bottling, the product should be hot filled (88◦C) and sealed before processing in boiling water for 20–35 minutes depending on the size of a can. As per FDA regulations, prune may contain citrus (lime and lemon juices) and enriched with vitamin C. The Federal regulations require that the following label declaration ap- pear on the container below the words Prune Juice: “A water extract of dried prunes.” Luh (1980) reported that prune juice differed from other juices, which are squeezed from fresh fruits.
Prune Juice Concentrate
Prune juice, processed as earlier, can be concentrated to 60◦–72◦Brix depending on the intended end use. Concentrate with lower soluble solids is frozen and used for reconstitu- tion into single strength juice. The high Brix (≥65% soluble solids) is shelf-stable/self-preserving without any need for freezing or added preservatives; the latter quality is of great benefit for shipping long distances, for export markets.
For better process efficiency, prune juice before concen- tration is depectinized by the addition of commercial pectic enzymes (Somogyi 1996). Juice is concentrated in a high vac- uum evaporator; many types of such evaporators are available commercially. Process is carried at temperature of 48◦C or lower. Water in the juice is evaporated and the juice sugars and other solids are concentrated to the desired level.
Prune juice concentrate offers many benefits and applica- tions in different food systems (Anon 2010); it (1) extends the shelf life of bread products, serves as an antistaling agent;
(2) sweetens and colors natural-baked goods; (3) is a natural substitute for preservatives; (4) can be a good sugar substi- tute, and natural color/flavor enhancer; (5) can be used as filling for hard candies and chocolate; (6) maintains moisture in chewy cakes and cookies; and (7) can be used as binding agent in cereal bars.
Canned Prunes
Dried prunes make an excellent product when canned. Pro- cessing consists of washing dried prunes, blanching for 4 minutes in hot water (to start softening of prunes), filling in cans, and adding syrup. Cans are exhausted for 12–15 min- utes at 170◦F before sealing. Thermal processing is carried out at 212◦F for 12 minutes. Canned prunes can be more prone to form hydrogen springer than other canned fruits, which can be minimized with high vacuum as a result of extended exhaustion before can closure (Downing 1996).
Canned prunes, which are moist, have no added preser- vatives, and a long shelf life, can be used for ready-to-eat snacking. Studies on canned prunes in syrup showed that use of syrup previously employed in the osmotic dehydration process did not impair quality of the canned product (Silveira et al. 1984). Bolin et al. (1971) reported that when apple juice was used as stewing medium for canned prunes, a desirable flavor was produced. Flavor did not improve by the addition of citric acid and ascorbic acid; both had rather an adverse effect instead.
Plum Juice
Most of the research on plum juice production was under- taken in the 1970s and 1980s. Plum juice has not been pro- cessed to a scale similar to prune juice, probably due to its high acidity. Siddiq et al. (1994) developed a method us- ing pectinase enzymes to press juice from ‘Stanley’ plums that increased juice yields significantly. Chantanawarangoon et al. (2004) evaluated antioxidant capacity, total phenolics, and total anthocyanins in juices prepared from 10 plum cul- tivars and suggested that plum juice with high antioxidant capacity, total phenolics, and total anthocyanins could be new healthy beverages for consumers and potential sources of nutraceutical supplements as well.
Plum Puree/Paste
Plum paste is another product with limited commercial pro- duction, however, with recent research on health benefits of polyphenols and dietary fiber, it offers a great potential for increasing production and uses in many food formulations.
Wang et al. (1995) developed a procedure to produce pastes from ‘Stanley’ plums and studied the effects of processing conditions on chemical, physical, and sensory properties of the pastes. Plums were processed by heat concentration into two pastes of 25◦ and 30◦Brix. Soluble solids of these two pastes were increased to 40◦ and 45◦Brix, respectively, by addition of sugar. Heat concentration resulted in a signif- icant decrease in titratable acidity, total anthocyanins, and total pectin. Pastes showed pseudoplastic behavior within the shear rate range of 20–100 rpm. Sugar addition had a darkening effect on color, but no noticeable effect on rhe- ological properties of the pastes. Sensory evaluation indi- cated that preference could be adequately predicted by flavor and color under suitable◦Brix/acid ratio. Raina et al. (1999) also reported that concentration of plum paste had signifi- cant effects on the rheological properties and acidity of the resultant paste. It was not feasible to concentrate the paste beyond 35◦Brix. Sweetened paste had higher acceptability scores than unsweetened paste.
Yildiz-Turp and Serdaroglu (2010) studied the effects of using different amounts of plum puree (5%, 10%, or 15%), as an extender, on some properties of low-fat beef patties.
Moisture content of patties decreased as the concentration
of plum puree was increased; however, the highest cooking yield and moisture retention were found in samples with 5%
plum puree. Overall, the results showed that 5% or 10% plum puree can be used as an extender in low-fat beef patties with increased juiciness and higher texture scores.
Jam and Jelly
Plum jam and jelly have not gained enough consumer accep- tances to result in commercial production. However, there is potential for the production of jams and jelly in combination with other fruits, which are lower in phenolic compounds and antioxidant activity. Kim and Padilla-Zakour (2004) in- vestigated the changes in total phenolics, antioxidant capac- ity, and anthocyanins from fresh fruits (cherries, plums, and raspberry) to processed shelf-stable jams, with the objective of finding if high sugar and acid levels may provide protec- tion to phenolic compounds. Their results showed that jam processing had no consistent effect on phenolics, although an- tioxidant capacities generally decreased. Nonetheless, plums can be used as a source of phenolics in jams prepared from other fruits that may be low in these phytochemicals.
Fresh-Cut Plums
“Fresh-cut produce” is defined as any fresh fruit or veg- etable or any combination thereof that has been physically altered from its original form, but remains in a fresh state.
Regardless of commodity, it has been washed, trimmed, peeled, and cut into 100% usable product that is subsequently bagged/packaged to offer consumers with high nutrition, con- venience and value while still maintaining freshness (IFPA, 2004). In recent years, a market for fresh prunes has devel- oped; however, less than 1% of prunes produced in California are marketed through this channel due to volatile market con- ditions (Anon 2006). Fresh-cut plum can keep good quality for 2–5 days, depending on the cultivar and ripeness stage (firmness) when stored at 0◦C in packages that minimize loss of water (Crisosto and Kader 2000). Cisneros-Zevallos and Heredia (2004) studied the role of ethylene and methyl jas- monate on the changes in health-promoting antioxidant com- pounds in different fresh-cut produce that included plums.
They found that these two plant hormones combined with wounding (cutting) could be used to enhance the health pro- moting antioxidant content of fresh-cut produce.
Other Products
Yang et al. (2010) optimized a process for preparation of green plum vinegar: green plums were pitted, washed, mashed into a slurry, and pectinase enzyme added for pectin hydrolysis. Dry yeast was used for the fermentation of plum juice at 28◦C for 72 hours. The vinegar thus produced had an aroma typical of green plums and vinegar.