Postharvest Storage
Optimum storage conditions for peaches and nectarines are –1◦C to 0◦C and 90–95% RH with airflow of approximately 50 ft3/min. Fruit tissue softening is accelerated at elevated temperatures where respiration rate can be as high as 10 times at 20◦C compared to 0◦C. At elevated temperatures, depending on fruit maturity, ethylene production rate in- creases significantly from less than 5.0 μL/kg/h at 0◦C to as high as 160μL/kg/h at 20◦C (Crisosto and Kader 2004).
Polygalacturonase enzyme is responsible for the softening of fruit tissue as a result of depolymerization of pectic polysac- charide chains during postharvest storage (Tijskens et al.
1998). Togrul and Arslan (2004) reported that the use of car- boxymethylcellulose (CMC), from sugar beet pulp cellulose as a hydrophilic polymer, in an emulsion coating containing beeswax, triethanolamine, and oleic acid, was shown to ex- tend the shelf life of peaches from 12 to 16 days at 25◦C and 75% RH.
Zhang et al. (2010) reported that the firmness of peach fruit was closely related with the contents and nanostructural characteristics of carbonate-soluble pectin, which might be hydrolyzed by enzymes in fruit flesh. Lysiak et al. (2008) evaluated the storability of peaches after dipping in a 2%
CaCl2 solution at 20◦C for 30 minutes and storing them at 4◦C for 2 weeks in boxes covered with polyethylene bags.
Overall, there were distinct improvements in storability re- sulting from the CaCl2and polyethylene barrier. The CaCl2
treatment improved firmness, largely maintained the solu- ble solids content and increased sugar–acid ratio; use of polyethylene bags minimized fruit weight loss.
The effect of gamma irradiation (0.5–2.0 kGy) on the mi- crobiological, physicochemical, and sensory properties of
“Dangeumdo” peaches during 6-day storage at 20◦C was investigated by Kim et al. (2009). The gamma irradiation was effective in achieving microbiological safety; it also im- proved the antioxidative activity but had somewhat negative effect on the color and texture of the peaches.
Caprioli et al. (2009) studied the effect of postharvest treat- ments (1-methylcyclopropene or 1-MCP, CO2, and N2) fol- lowed by storage at 10◦C, and hydrocooling at 1◦C followed by storage at 0◦C on fruit quality, carotenoids, and ethylene and CO2 production in peaches. Ethylene production was reduced by all the treatments, and hydrocooling in combina- tion with low-temperature storage was the best treatment for maintaining fruit firmness.
Chilling Injury
Peaches (most mid- and late-season) and nectarines (some mid- and late-season cultivars) are susceptible to chilling injury during storage. Chilling injury develops faster and more intensely in fruit that is stored at 2.2–7.6◦C (peaches) and 2.2–7.8◦C (nectarines) than that stored at 0◦C or below (Crisosto and Kader 2004). Brovelli et al. (1998) showed that, based on the development of flesh mealiness, the qual- ity of nonmelting flesh peaches was not as severely affected as a result of chilling exposure as was the case with melt- ing flesh peaches. This quality of nonmelting flesh peaches can be a major advantage for refrigerated storage and long- distance transportation. Modified atmosphere (MA) storage (12% CO2and 4% O2) of peaches in unperforated polypropy- lene bags could be useful as it is associated with lower weight loss, less senescence and chilling injury, reduced incidence of decay, and delayed ripening of the fruit beyond normal shelf-life period (Fernandez-Trujilio et al. 1998).
High CO2, controlled atmosphere (CA) storage is a proven technology to overcome chilling injury, while prestorage heat treatment appear like an emerging alternative although show- ing some undesirable side effects (Murray et al. 2007). Com- bined treatments were useful for improving juiciness and were the only alternative to reach 4 weeks with commercial quality Flavorcrest peach fruit. Although heat-treated fruit had generally redder flesh than others, this side effect was reduced by CA (Murray et al. 2007). Application of salicylic acid is helpful in alleviating the symptoms of chilling injury in peaches during cold storage (Wang et al. 2006). Ortiz et al.
(2009) indicated that CA storage at 2◦C, in addition to pre- serving other quality attributes, showed better retention of peach aroma as compared to fruit stored under air at 2◦C.
Zhu et al. (2010) studied the effect of fumigation with nitric oxide gas, intermittent warming, or a combination of both in preventing chilling injury of “Feicheng” peaches.
Chilling injury index, firmness, ethylene production, cell wall fractions, and cell wall metabolism-associated enzymes were evaluated. Their results showed that three treatments
significantly prevented mealiness in peaches. Although inter- mittent warming increased the activity of polygalacturonase, it was suggested that nitric oxide fumigation could offset the side effect of intermittent warming.
Methyl jasmonate application is also shown to be beneficial for maintaining quality by alleviating chilling injury symp- toms of peach fruits under low-temperature stress (Meng et al.
2009; Ziosi et al. 2009).
PROCESSED PRODUCTS
About one-half of peaches and almost all of nectarines, pro- duced are consumed fresh. Canned and frozen peaches are the two major processed peach products. Other products, like dried peaches, peach jam, jelly, and juice are processed on a much smaller scale (Reiger 2004). With increasingly new scientific claims of health benefits of phytochemicals, yellow-flesh peaches, which are rich in phytochemicals (Gil et al. 2002), have great potential for processing into a variety of new products.
Canned Peaches
About 40% of peaches produced in the United States are processed into canned products, mostly halves and slices.
Clingstone peaches are commonly used for commercial can- ning, as they are able to retain flavor and consistency. In most cases, peaches are canned within 24 hours of delivery to the processing plant, which ensures that the peaches maintain nutritional value and flavor (Reiger 2004; CCPB 2010). The optimum maturity of peaches for canning purposes is when the color is orange–yellow and the fruit is still firm. A brief description of processing steps (Downing 1996) for canning peaches is given below:
r Grading: Since peaches received at the cannery usually include a wide range of sizes, it is necessary to grade fruits using a mechanical grader. In cases where fruit is to be stored before canning, this pregrading is done before the fruit has reached canning maturity (this mini- mizes bruising of the fruit).
r Halving and pitting: Peaches are usually canned as halves or slices. The halving and pitting of peaches is accomplished using automatic machines.
r Peeling and washing: Clingstone peaches are lye-peeled either by spray or immersion method. Conditions used for lye peeling are: 5–11% lye solution at a temperature of 215–220◦F for 45–60 seconds. The treatment time, strength, and temperature of the lye solution depend on the maturity of the fruit. Following the hot lye treatment, fruit pieces are thoroughly sprayed with cold water for complete removal of the peel and the lye residue. Free- stone peaches are peeled by (a) steaming, (b) scalding in water, (c) lye peeling, or (d) combination steam and lye.
r Quality grading and slicing: An inspection belt is provided for the grading and sorting of poorly sized, off-color, partially peeled, and otherwise imperfect fruit. It is recommended to grade and fill the halves di- rectly from the inspection belt into cans as mechanical grading/filling often results in additional injury to the fruit. Fruit that is not canned as halves goes directly from the belt to the slicing machine.
r Filling and syruping: The peach halves should be filled as rapidly as possible after peeling and grading as ex- tended exposure to the air results in discoloration. Slices should go over an inspection belt for removal of de- fective pieces. The slices are then discharged to hand- pack fillers. The standards of identity for canned peaches specify the densities of cut-out syrup that correspond to label names such as “heavy syrup” or “light syrup,” etc.
Syruping of canned peaches is accomplished by rotary and straight-line syrupers or by prevacuumizing syru- pers.
r Exhausting and closing: The cans are given a steam ex- haust for about 6 minutes at 190–195◦F. Cans are closed immediately after exhausting; a gross headspace of 5/16 of an inch is recommended.
r Processing and cooling: It is recommended to process cans immediately after exhausting and closing of cans so that the heat of the exhaust is not lost before pro- cessing begins. Clingstone peaches are processed using continuous reel type cookers at 212◦F for 20–35 min- utes depending on the diameter of the cans and the type of product—halves or slices. Some processors also use commercial processes of 14–18 minutes at 240◦F. After processing, the cans are water-cooled immediately to 95–105◦F.
Firmness of canned peaches is an important quality at- tribute. In an attempt to improve the firmness of canned peaches, Wang (1994) treated peach slices by submerging them in pectinase solution containing 100 mg/L CaCl2under vacuum for 0.5–2 hours. This treatment was effective in im- proving the firmness of peach slices from 7 to 25 J/kg; also, calcium content increased from 280 to 430 mg/kg. Canning is shown to have negative effect on the retention of procyani- dins in canned “Ross” clingstone peaches as well as in the syrup used in the canning (Table 30.4, Hong et al. 2004).
Styles of Canned Clingstone Peaches
The styles of canned clingstone peaches classified per US standards are: (a) “Halves or Halved” canned peaches- peeled and pitted, cut approximately in half along the su- ture from stem to apex; (b) “Quarters or Quartered” canned peaches–halved peaches cut into two approximately equal parts; (c) “Slices or Sliced” canned peaches–peeled and pit- ted peaches cut into sectors smaller than quarters; (d) “Dice or Diced” canned peaches–peeled and pitted peaches cut into
Table 30.4. Content of Procyanidin Oligomers in Frozen and Canned Peaches (mg/kg, Wet-Weight Basis)
Canned Peach Oligomer
Fraction
Frozen Peach
0 month
1 month
2 months
3 months
P1 19.59 17.50 17.18 15.77 15.85
P2 39.59 36.50 34.84 29.03 30.55
P3 38.81 34.77 31.33 19.23 19.06
P4 17.81 16.97 10.07 6.26 3.61
P5 12.43 11.85 7.61 2.83 –
P6 10.62 7.87 3.52 – –
P7 3.94 2.76 – – –
P8 1.75 – – – –
Source: Hong et al. (2004).
approximate cubes; (e) “Whole” canned peaches—peeled, unpitted, whole peaches with or without stems removed; and (f) “Mixed pieces of irregular sizes and shapes” are peeled, pitted, and cut units of canned peaches that are predominantly irregular in size and shape, which do not conform to a single style of halves, quarters, slices, or dice (USDA-AMS 1985).
Liquid Media and Brix Measurements
Cut-out requirements for liquid media in canned freestone peaches are not incorporated in the grades of the finished product since syrup or any other liquid medium is not a factor of quality for the purpose of these grades. The cut-out Brix measurements for the respective designations are shown in Table 30.5.
Frozen Peaches
About 6–8% of peaches produced are processed as frozen peaches. Processors usually need three different types of peaches: (1) for retail packages of slices, varieties with red around the pit cavity, such as ‘Rio Oso Gem,’ are desirable;
(2) peaches frozen in bulk for later processing into preserves should have good flavor and should not have red color around the pit cavity for good color/appearance of the jam or mar- malade. ‘Fay Elberta’ peaches picked on the immature side are ideal for preserves; and (3) for the institutional market, mainly pies, a highly flavored, firm-textured variety with re- sistance to browning is best suited. For freezing peaches, an ideal variety preferably should have the shape, bright color, and firmness of ‘Rio Oso Gem,’ the flavor of ‘Elberta,’
and the nonbrowning characteristic of ‘Sunbeam’ (Boyle et al. 1977). Fruit preparation steps of washing, peeling, and slicing are the same as discussed under Section ‘Canned Peaches.’
Table 30.5. Cut-out Brix Levels of Syrups Used in Canned Peaches
Designations
Brix Measurements
• “Extra heavy syrup” or “Extra heavily sweetened fruit juice(s) and water” or
“Extra heavily sweetened fruit juice(s)”
22◦or more but less than 35◦
• “Heavy syrup”; or “Heavily sweetened fruit juice(s) and water” or “Heavily sweetened fruit juice(s)”
18◦or more but less than 22◦
• “Light syrup”; or “Lightly sweetened fruit juice(s) and water” or “Lightly sweetened fruit juice(s)”
14◦or more but less than 18◦
• “Slightly sweetened water”; or “Extra light syrup” or “Slightly sweetened fruit juice(s) and water” or “Slightly sweetened fruit juice(s)”
10◦or more but less than 14◦
Source: USDA-AMS (1985).
Air-blast tunnel freezing is the most commonly used sys- tem by most processors. In this method, the prepared product ready to be frozen is placed on wire mesh trays and loaded on to racks. The tray racks are moved into freezing tunnel.
Cold air is usually introduced into the tunnel at the opposite end from the one where product to be frozen enters. The temperature and velocity of the air is of critical importance in the freezing process. The temperature of the air is usually between 0◦F and –30◦F (–18◦C and –34◦C, respectively). Air velocity can range from 100 ft/min to 3500 ft/min. The length of time a product is subjected to cold air blast in the tunnel depends on the product size (Boyle et al. 1977).
Otero et al. (2000) compared classical methods of freez- ing and high-pressure shift freezing (HPSF) with respect to their effect on modification to the microstructure of peach and mango using a histochemical technique. With HPSF method, samples were cooled under pressure (200 MPa) to –20◦C without ice formation, and then pressure was re- leased to atmospheric level (0.1 MPa). The high level of super-cooling led to uniform and rapid ice nucleation. They concluded that problems associated with thermal gradients were minimized in HPSF method that prevents quality losses due to freeze-cracking or large ice crystal presence, thus, their method was helpful in maintaining the original tissue structure.
Hong et al. (2004) used normal-phase liquid chromatography–mass spectrometry (LC-MS) to deter- mine the levels and fate of procyanidins in frozen and canned
‘Ross’ clingstone peaches as well as in the syrup used in the canning over a 3-month storage period. Retention of these health beneficial compounds was better in frozen peaches as compared to canned peaches (Table 30.4). Storage of canned peaches for 3 months demonstrated a time-related loss in high-molecular-weight oligomers (P5–P8) and that
by 3 months, oligomers larger than tetramers were not observed. After 3 months of postcanning storage, levels of monomers had decreased by 10%, dimers by 16%, trimers by 45%, and tetramers by 80%.
Varietal Types and Styles of Frozen Peaches
Varietal types of peaches that are processed as frozen include:
(a) “Yellow freestone”—freestone peaches of the yellow- fleshed varieties, which may have orange or red pigments em- anating from the pit cavity; (b) “White freestone”—freestone peaches that are predominately white-fleshed; (c) “Red freestone”—freestone peaches that have substantial red col- oring in the flesh; and (d) “Yellow clingstone”—clingstone peaches of the yellow- or orange-fleshed varieties. Styles of frozen peaches include: (a) “Halved or halves”—the peaches are cut approximately in half along the suture from stem to apex, (b) “Quartered or quarters”—halved peaches cut into two approximately equal parts, (c) “Sliced or slices”—the peaches are cut into sectors smaller than quar- ters, (d) “Diced”—the peaches are cut into approximate cube- shaped units, and (e) “Mixed pieces of irregular sizes and shapes”—means peaches cut or broken into pieces of irreg- ular sizes and shapes and which do not conform to a single style of halves, quarters, or slices (USDA-AMS 1961).
Dried Peaches
As per US standards, dried peaches are the halved and pitted fruit from which the greater portion of moisture has been re- moved. Before packing, the dried fruit is processed to cleanse the fruit and may be sulfured sufficiently to retain a charac- teristic color. Federal inspection certificates shall indicate the moisture content of the finished product, which shall be not more than 25% by weight. Dried peaches may be processed from freestone or clingstone peach types (USDA- AMS 1967).
Only 1–2% of peaches produced are processed as dried halves, quarters, or slices. Fruit preparation steps of wash- ing, peeling, and cutting are the same as discussed under Section “Canned Peaches.” To minimize discoloration, cut peaches are dipped in AA or other antibrowning solution for 5–10 minutes (see more detail on antibrowning agents under Section “Fresh-Cut Peaches and Nectarines”). Prepared fruit is spread in single layers on trays, usually 3-pounds/sq. ft.
Peaches are dried to moisture content of about 25%. Germer et al. (2007) reported that fruit with low pulp (flesh) firmness presented problems during preparation steps for drying, thus resulting in low acceptance scores for appearance and color of dried product.
Generally, forced-draft tunnel dehydrators are used for dry- ing peaches. For drying in a countercurrent tunnel, temper- ature should not exceed 155◦F. Total drying time (24–30 hours) depends on the size of the product and the tempera- ture used. Blanched peaches are dried more rapidly requiring
about 18 hours to reach 25% moisture (Brekke and Nury 1964). In peach producing developing countries, sun-drying is still the most widely used method due to its low cost, how- ever, quality is not as good as those dried in controlled and sanitary environment.
Hansmann et al. (1998) investigated the drying behavior of clingstone peach halves dehydrated without sulfites and suggested that enzymatic browning reactions can be con- trolled during dehydration by selecting dehydration condi- tions favoring low superficial product temperature and lower aw. Also, peeled fruit dried faster than unpeeled one. Wang et al. (1996) dried yellow peach fruits cut into halves to a moisture content of 18% with microwaves of 2350 MHz at 0.3–0.45 m/s air velocity and hot air and far ultra-red waves.
Microwave dried peaches had better color than those dried by the other two methods.
Many researchers have shown the benefits of osmotic dry- ing before traditional dehydration. Lerici et al. (1988) re- ported that the osmotically dehydrated products had very good texture and good retention of aroma and color; the awwas reduced sufficiently to improve shelf life but further processing (e.g., freezing, drying, and pasteurization) was necessary to ensure shelf-stable products. Erba et al. (1994) described “osmodehydro-freezing” as a combined process where osmotic drying is followed by air drying and freezing to prepare reduced-moisture fruit ingredients, free of preser- vatives, with a natural flavor, color, and texture, and with functional properties suitable for different food applications.
Souti et al. (2003) studied suitability of osmotic drying as a method for predrying of peaches and showed that the use of osmotic pre-drying treatment produced dried peaches of better sensory quality than traditionally solar-dried peaches as judged by a sensory panel.
Peach Puree/Nectar
Peach puree is used extensively in baby food formulations.
Typical processing steps for production of peach and nec- tarine puree and nectar are shown in Figure 30.2. Toralles et al. (2008) investigated the degradation of AA in ‘Jade’
peach puree (12◦, 22◦, and 32◦Brix) under anaerobic condi- tions at 70–90◦C. The kinetic analysis of the data showed that the degradation was significantly represented by zero- and first-order kinetic models and that the rate of AA degra- dation in peach puree was highly temperature-dependent.
Lavelli et al. (2009) reported that carotenoid and pheno- lic contents were lower in the nectars obtained from peeled peaches (cv. ‘Elegant Lady’ and ‘Red Haven’) than in those obtained from unpeeled fruits. However, the color of peach nectars was improved by processing lye-peeled fruits at room temperature.
Attempts have been made to develop carbonated bever- ages using peach puree (Arora and Aggarwal 2009; Ag- garwal and Arora 2010). Aggarwal and Arora (2010) stud- ied the effects of processing and storage conditions on